Update changelog

Fixes #2

.reuse/dep5

@@ -14,3 +14,7 @@ Files: gradle*
 Copyright: 2015 the original author or authors.
 License: Apache-2.0
 
+# Woodpecker build files
+Files: .woodpecker/*
+Copyright: 2022 the original author or authors.
+License: Apache-2.0

.woodpecker/.build.yml

@@ -0,0 +1,12 @@
+pipeline:
+  run:
+    image: gradle:7.5-jdk8
+    commands:
+      - git checkout $CI_COMMIT_BRANCH
+      # Code works
+      - gradle test
+      # Code is clean
+      - gradle check javadocAll
+      # Code has coverage
+      - gradle jacocoRootReport coveralls
+    secrets: [COVERALLS_REPO_TOKEN]

.woodpecker/.reuse.yml

@@ -0,0 +1,7 @@
+# Code is licensed properly
+# See https://reuse.software/
+pipeline:
+  reuse:
+    image: fsfe/reuse:latest
+    commands:
+      - reuse lint

CHANGELOG.md

@@ -5,6 +5,28 @@ SPDX-License-Identifier: CC0-1.0
 
 # Cert-D-Java Changelog
 
+## 0.2.2
+- Bump Bouncy Castle to `1.75`
+- Bump `sqlite-jdbc` to `3.42.0.0`
+
+## 0.2.1
+- Throw `NoSuchElementException` when querying non-existent certificates
+
+## 0.2.0
+- `pgp-certificate-store`:
+  - Rework `Certificate`, `Key` to inherit from `KeyMaterial`
+  - Rename `CertificateReaderBackend` to `KeyMaterialReaderBackend`
+  - Rename `CertificateMerger` to `KeyMaterialMerger`
+  - Rework `PGPCertificateStore` class
+- `pgp-cert-d-java`:
+  - Increase minimum Android API level to 26
+  - Add `PGPCertificateDirectories` factory class
+  - Rework `PGPCertificateDirectory` class by separating out backend logic
+  - Split interface into `ReadOnlyPGPCertificateDirectory` and `WritingPGPCertificateDirectory`
+  - `FileBasedCertificateDirectoryBackend`: Calculate tag based on file attributes (inode)
+- `pgp-cert-d-java-jdbc-sqlite-lookup`:
+  - Add `DatabaseSubkeyLookupFactory`
+
 ## 0.1.1
 - Bump `slf4j` to `1.7.36`
 - Bump `logback` to `1.2.11` 

README.md

@@ -5,10 +5,13 @@ SPDX-License-Identifier: Apache-2.0
 -->
 
 # Shared PGP Certificate Directory for Java
+[![status-badge](https://ci.codeberg.org/api/badges/PGPainless/cert-d-java/status.svg?branch=main)](https://ci.codeberg.org/PGPainless/cert-d-java)
+[![Coverage Status](https://coveralls.io/repos/github/pgpainless/cert-d-java/badge.svg?branch=main)](https://coveralls.io/github/pgpainless/cert-d-java?branch=main)
+[![REUSE status](https://api.reuse.software/badge/github.com/pgpainless/cert-d-java)](https://api.reuse.software/info/github.com/pgpainless/cert-d-java)
 
 This repository contains a number of modules defining OpenPGP certificate storage for Java and Android applications.
 
-The module [pgp-certificate-store](pgp-certificate-store] defines generalized
+The module [pgp-certificate-store](pgp-certificate-store) defines generalized
 interfaces for OpenPGP Certificate storage.
 It can be used by applications and libraries such as
 [PGPainless](https://pgpainless.org/) for certificate management.

pgp-cert-d-java-jdbc-sqlite-lookup/src/main/java/pgp/cert_d/jdbc/sqlite/DatabaseSubkeyLookup.java

@@ -11,7 +11,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
-import pgp.certificate_store.SubkeyLookup;
+import pgp.cert_d.subkey_lookup.SubkeyLookup;
 
 public class DatabaseSubkeyLookup implements SubkeyLookup {
 

pgp-cert-d-java-jdbc-sqlite-lookup/src/main/java/pgp/cert_d/jdbc/sqlite/DatabaseSubkeyLookupFactory.java

@@ -0,0 +1,43 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.jdbc.sqlite;
+
+import pgp.cert_d.subkey_lookup.SubkeyLookup;
+import pgp.cert_d.subkey_lookup.SubkeyLookupFactory;
+
+import java.io.File;
+import java.io.IOException;
+import java.sql.SQLException;
+
+/**
+ * Implementation of {@link SubkeyLookupFactory} which creates a SQLite-based {@link DatabaseSubkeyLookup}.
+ */
+public class DatabaseSubkeyLookupFactory implements SubkeyLookupFactory {
+
+    private String databaseName;
+
+    public DatabaseSubkeyLookupFactory() {
+        this("_pgpainless_subkey_map.db");
+    }
+
+    public DatabaseSubkeyLookupFactory(String databaseName) {
+        this.databaseName = databaseName;
+    }
+
+    @Override
+    public SubkeyLookup createFileBasedInstance(File baseDirectory) {
+        File databaseFile = new File(baseDirectory, databaseName);
+        SubkeyLookupDao dao;
+        try {
+            if (!databaseFile.exists()) {
+                databaseFile.createNewFile();
+            }
+            dao = SqliteSubkeyLookupDaoImpl.forDatabaseFile(databaseFile);
+        } catch (SQLException | IOException e) {
+            throw new RuntimeException(e);
+        }
+        return new DatabaseSubkeyLookup(dao);
+    }
+}

pgp-cert-d-java-jdbc-sqlite-lookup/src/main/java/pgp/cert_d/jdbc/sqlite/package-info.java

@@ -3,6 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0
 
 /**
- * Implementation of a {@link pgp.certificate_store.SubkeyLookup} mechanism using an SQLite Database.
+ * Implementation of a {@link pgp.cert_d.subkey_lookup.SubkeyLookup} mechanism using an SQLite Database.
  */
 package pgp.cert_d.jdbc.sqlite;

pgp-cert-d-java-jdbc-sqlite-lookup/src/test/java/pgp/cert_d/jdbc/sqlite/SqliteSubkeyLookupTest.java

@@ -22,15 +22,15 @@ import org.junit.jupiter.api.Test;
 
 public class SqliteSubkeyLookupTest {
 
-    private File databaseFile;
+    private File tempDir;
     private DatabaseSubkeyLookup lookup;
 
     @BeforeEach
-    public void setupLookup() throws IOException, SQLException {
-        databaseFile = Files.createTempFile("pgp.cert.d-", "lookup.db").toFile();
-        databaseFile.createNewFile();
-        databaseFile.deleteOnExit();
-        lookup = new DatabaseSubkeyLookup(SqliteSubkeyLookupDaoImpl.forDatabaseFile(databaseFile));
+    public void setupLookup() throws IOException {
+        tempDir = Files.createTempDirectory("pgp.cert.d").toFile();
+        tempDir.deleteOnExit();
+        lookup = (DatabaseSubkeyLookup) new DatabaseSubkeyLookupFactory()
+                .createFileBasedInstance(tempDir);
     }
 
     @Test
@@ -55,7 +55,7 @@ public class SqliteSubkeyLookupTest {
         assertEquals(Collections.singleton("eb85bb5fa33a75e15e944e63f231550c4f47e38e"), lookup.getCertificateFingerprintsForSubkeyId(1337));
 
         // do the lookup using a second db instance on the same file
-        DatabaseSubkeyLookup secondInstance = new DatabaseSubkeyLookup(SqliteSubkeyLookupDaoImpl.forDatabaseFile(databaseFile));
+        DatabaseSubkeyLookup secondInstance = (DatabaseSubkeyLookup) new DatabaseSubkeyLookupFactory().createFileBasedInstance(tempDir);
         assertEquals(Collections.singleton("eb85bb5fa33a75e15e944e63f231550c4f47e38e"), secondInstance.getCertificateFingerprintsForSubkeyId(1337));
     }
 

pgp-cert-d-java/README.md

@@ -13,9 +13,11 @@ Backend-agnostic implementation of the [Shared PGP Certificate Directory Specifi
 This module implements the non-OpenPGP parts of the spec, e.g. locating the directory, resolving certificate file paths,
 locking the directory for writes etc.
 
+This library can be used on Android API level 26 and up.
+
 To get a useful implementation, a backend implementation such as `pgpainless-cert-d` is required, which needs to provide
 support for reading and merging certificates.
 
-`pgp-cert-d-java` can be used as an implementation of `pgp-certificate-store`.
+`pgp-cert-d-java` can be used as an implementation of `pgp-certificate-store` using the `PGPCertificateStoreAdapter` class.
 
-Note: This is a library module. For a command line interface, see [pgpainless-cert-d-cli](https://github.com/pgpainless/cert-d-pgpainless/tree/main/pgpainless-cert-d-cli).
+Note: This is a library module. For a command line interface, see [pgpainless-cert-d-cli](https://github.com/pgpainless/cert-d-pgpainless/tree/main/pgpainless-cert-d-cli).

pgp-cert-d-java/build.gradle

@@ -16,7 +16,7 @@ apply plugin: 'ru.vyarus.animalsniffer'
 
 dependencies {
     // animal sniffer for ensuring Android API compatibility
-    signature "net.sf.androidscents.signature:android-api-level-${minAndroidSdk}:2.3.3_r2@signature"
+    signature "net.sf.androidscents.signature:android-api-level-${animalsnifferSignatureVersion}@signature"
 
     // JUnit
     testImplementation "org.junit.jupiter:junit-jupiter-api:$junitVersion"
@@ -26,11 +26,13 @@ dependencies {
     // Logging
     testImplementation "ch.qos.logback:logback-classic:$logbackVersion"
 
+    api project(":pgp-certificate-store")
+
     // SQL Subkey table
     testImplementation project(":pgp-cert-d-java-jdbc-sqlite-lookup")
 
-    // Certificate store
-    api project(":pgp-certificate-store")
+    testImplementation "org.bouncycastle:bcprov-jdk15to18:$bouncycastleVersion"
+    testImplementation "org.bouncycastle:bcpg-jdk15to18:$bouncyPgVersion"
 }
 
 animalsniffer {

pgp-cert-d-java/src/main/java/pgp/cert_d/BackendProvider.java

@@ -1,16 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import pgp.certificate_store.CertificateReaderBackend;
-import pgp.certificate_store.MergeCallback;
-
-public abstract class BackendProvider {
-
-    public abstract CertificateReaderBackend provideCertificateReaderBackend();
-
-    public abstract MergeCallback provideDefaultMergeCallback();
-
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/BaseDirectoryProvider.java

@@ -6,6 +6,16 @@ package pgp.cert_d;
 
 import java.io.File;
 
+/**
+ * Provider class that is responsible for resolving the pgp.cert.d base directory of the system.
+ * The result can be overwritten by setting the <pre>PGP_CERT_D</pre> environment variable.
+ * If this variable is not set, the system-specific default directory will be returned.
+ *
+ * On Windows systems, this is <pre>%APPDATA%\pgp.cert.d</pre>.
+ * On Linux systems it is either <pre>$XDG_DATA_HOME/pgp.cert.d</pre> or, if <pre>$XDG_DATA_HOME</pre> is not set,
+ * it is <pre>$HOME/.local/share/pgp.cert.d</pre>
+ * On Mac systems it is <pre>$HOME/Library/Application Support/pgp.cert.d</pre>.
+ */
 public class BaseDirectoryProvider {
 
     public static File getDefaultBaseDir() {

pgp-cert-d-java/src/main/java/pgp/cert_d/CachingSharedPGPCertificateDirectoryWrapper.java

@@ -1,190 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import pgp.certificate_store.exception.BadDataException;
-import pgp.certificate_store.exception.BadNameException;
-import pgp.certificate_store.Certificate;
-import pgp.certificate_store.MergeCallback;
-
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.HashMap;
-import java.util.Iterator;
-import java.util.Map;
-
-/**
- * Caching wrapper for {@link  SharedPGPCertificateDirectory} implementations.
- */
-public class CachingSharedPGPCertificateDirectoryWrapper
-        implements SharedPGPCertificateDirectory {
-
-    private static final Map<String, String> tagMap = new HashMap<>();
-    private static final Map<String, Certificate> certificateMap = new HashMap<>();
-    private final SharedPGPCertificateDirectory underlyingCertificateDirectory;
-
-    public CachingSharedPGPCertificateDirectoryWrapper(SharedPGPCertificateDirectory wrapped) {
-        this.underlyingCertificateDirectory = wrapped;
-    }
-
-    /**
-     * Store the given certificate under the given identifier into the cache.
-     *
-     * @param identifier fingerprint or special name
-     * @param certificate certificate
-     */
-    private void remember(String identifier, Certificate certificate) {
-        certificateMap.put(identifier, certificate);
-        try {
-            tagMap.put(identifier, certificate.getTag());
-        } catch (IOException e) {
-            tagMap.put(identifier, null);
-        }
-    }
-
-    /**
-     * Returns true, if the cached tag differs from the provided tag.
-     *
-     * @param identifier fingerprint or special name
-     * @param tag tag
-     * @return true if cached tag differs, false otherwise
-     */
-    private boolean tagChanged(String identifier, String tag) {
-        String tack = tagMap.get(identifier);
-        return !tagEquals(tag, tack);
-    }
-
-    /**
-     * Return true, if tag and tack are equal, false otherwise.
-     * @param tag tag
-     * @param tack other tag
-     * @return true if equal
-     */
-    private static boolean tagEquals(String tag, String tack) {
-        return (tag == null && tack == null)
-                || tag != null && tag.equals(tack);
-    }
-
-    /**
-     * Clear the cache.
-     */
-    public void invalidate() {
-        certificateMap.clear();
-        tagMap.clear();
-    }
-
-    @Override
-    public LockingMechanism getLock() {
-        return underlyingCertificateDirectory.getLock();
-    }
-
-    @Override
-    public Certificate getByFingerprint(String fingerprint)
-            throws IOException, BadNameException, BadDataException {
-        Certificate certificate = certificateMap.get(fingerprint);
-        if (certificate == null) {
-            certificate = underlyingCertificateDirectory.getByFingerprint(fingerprint);
-            if (certificate != null) {
-                remember(fingerprint, certificate);
-            }
-        }
-
-        return certificate;
-    }
-
-    @Override
-    public Certificate getBySpecialName(String specialName)
-            throws IOException, BadNameException, BadDataException {
-        Certificate certificate = certificateMap.get(specialName);
-        if (certificate == null) {
-            certificate = underlyingCertificateDirectory.getBySpecialName(specialName);
-            if (certificate != null) {
-                remember(specialName, certificate);
-            }
-        }
-
-        return certificate;
-    }
-
-    @Override
-    public Certificate getByFingerprintIfChanged(String fingerprint, String tag)
-            throws IOException, BadNameException, BadDataException {
-        if (tagChanged(fingerprint, tag)) {
-            return getByFingerprint(fingerprint);
-        }
-        return null;
-    }
-
-    @Override
-    public Certificate getBySpecialNameIfChanged(String specialName, String tag)
-            throws IOException, BadNameException, BadDataException {
-        if (tagChanged(specialName, tag)) {
-            return getBySpecialName(specialName);
-        }
-        return null;
-    }
-
-    @Override
-    public Certificate insert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, InterruptedException {
-        Certificate certificate = underlyingCertificateDirectory.insert(data, merge);
-        remember(certificate.getFingerprint(), certificate);
-        return certificate;
-    }
-
-    @Override
-    public Certificate tryInsert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException {
-        Certificate certificate = underlyingCertificateDirectory.tryInsert(data, merge);
-        if (certificate != null) {
-            remember(certificate.getFingerprint(), certificate);
-        }
-        return certificate;
-    }
-
-    @Override
-    public Certificate insertWithSpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, BadNameException, InterruptedException {
-        Certificate certificate = underlyingCertificateDirectory.insertWithSpecialName(specialName, data, merge);
-        remember(specialName, certificate);
-        return certificate;
-    }
-
-    @Override
-    public Certificate tryInsertWithSpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, BadNameException {
-        Certificate certificate = underlyingCertificateDirectory.tryInsertWithSpecialName(specialName, data, merge);
-        if (certificate != null) {
-            remember(specialName, certificate);
-        }
-        return certificate;
-    }
-
-    @Override
-    public Iterator<Certificate> items() {
-
-        Iterator<Certificate> iterator = underlyingCertificateDirectory.items();
-
-        return new Iterator<Certificate>() {
-            @Override
-            public boolean hasNext() {
-                return iterator.hasNext();
-            }
-
-            @Override
-            public Certificate next() {
-                Certificate certificate = iterator.next();
-                remember(certificate.getFingerprint(), certificate);
-                return certificate;
-            }
-        };
-    }
-
-    @Override
-    public Iterator<String> fingerprints() {
-        return underlyingCertificateDirectory.fingerprints();
-    }
-
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/FileLockingMechanism.java

@@ -1,96 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import java.io.File;
-import java.io.FileNotFoundException;
-import java.io.IOException;
-import java.io.RandomAccessFile;
-import java.nio.channels.FileLock;
-import java.nio.channels.OverlappingFileLockException;
-
-public class FileLockingMechanism implements LockingMechanism {
-
-    private final File lockFile;
-    private RandomAccessFile randomAccessFile;
-    private FileLock fileLock;
-
-    public FileLockingMechanism(File lockFile) {
-        this.lockFile = lockFile;
-    }
-
-    public static FileLockingMechanism defaultDirectoryFileLock(File baseDirectory) {
-        return new FileLockingMechanism(new File(baseDirectory, "writelock"));
-    }
-
-    @Override
-    public synchronized void lockDirectory() throws IOException, InterruptedException {
-        if (randomAccessFile != null) {
-            // we own the lock already. Let's wait...
-            this.wait();
-        }
-
-        try {
-            randomAccessFile = new RandomAccessFile(lockFile, "rw");
-        } catch (FileNotFoundException e) {
-            lockFile.createNewFile();
-            randomAccessFile = new RandomAccessFile(lockFile, "rw");
-        }
-
-        fileLock = randomAccessFile.getChannel().lock();
-    }
-
-    @Override
-    public synchronized boolean tryLockDirectory() throws IOException {
-        if (randomAccessFile != null) {
-            // We already locked the directory for another write operation.
-            // We fail, since we have not yet released the lock from the other operation.
-            return false;
-        }
-
-        try {
-            randomAccessFile = new RandomAccessFile(lockFile, "rw");
-        } catch (FileNotFoundException e) {
-            lockFile.createNewFile();
-            randomAccessFile = new RandomAccessFile(lockFile, "rw");
-        }
-
-        try {
-            fileLock = randomAccessFile.getChannel().tryLock();
-            if (fileLock == null) {
-                // try-lock failed, file is locked by another process.
-                randomAccessFile.close();
-                randomAccessFile = null;
-                return false;
-            }
-        } catch (OverlappingFileLockException e) {
-            // Some other object is holding the lock.
-            randomAccessFile.close();
-            randomAccessFile = null;
-            return false;
-        }
-        return true;
-    }
-
-    @Override
-    public synchronized void releaseDirectory() throws IOException {
-        // unlock file
-        if (fileLock != null) {
-            fileLock.release();
-            fileLock = null;
-        }
-        // close file
-        if (randomAccessFile != null) {
-            randomAccessFile.close();
-            randomAccessFile = null;
-        }
-        // delete file
-        if (lockFile.exists()) {
-            lockFile.delete();
-        }
-        // notify waiters
-        this.notify();
-    }
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/FilenameResolver.java

@@ -1,70 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import pgp.certificate_store.exception.BadNameException;
-
-import java.io.File;
-import java.util.regex.Pattern;
-
-public class FilenameResolver {
-
-    private final File baseDirectory;
-    private final Pattern openPgpV4FingerprintPattern = Pattern.compile("^[a-f0-9]{40}$");
-
-    public FilenameResolver(File baseDirectory) {
-        this.baseDirectory = baseDirectory;
-    }
-
-    public File getBaseDirectory() {
-        return baseDirectory;
-    }
-
-    /**
-     * Calculate the file location for the certificate addressed by the given
-     * lowercase hexadecimal OpenPGP fingerprint.
-     *
-     * @param fingerprint fingerprint
-     * @return absolute certificate file location
-     *
-     * @throws BadNameException if the given fingerprint string is not a fingerprint
-     */
-    public File getCertFileByFingerprint(String fingerprint) throws BadNameException {
-        if (!isFingerprint(fingerprint)) {
-            throw new BadNameException();
-        }
-
-        // is fingerprint
-        File subdirectory = new File(getBaseDirectory(), fingerprint.substring(0, 2));
-        File file = new File(subdirectory, fingerprint.substring(2));
-        return file;
-    }
-
-    /**
-     * Calculate the file location for the certification addressed using the given special name.
-     * For known special names, see {@link SpecialNames}.
-     *
-     * @param specialName special name (e.g. "trust-root")
-     * @return absolute certificate file location
-     *
-     * @throws BadNameException in case the given special name is not known
-     */
-    public File getCertFileBySpecialName(String specialName) throws BadNameException {
-        if (!isSpecialName(specialName)) {
-            throw new BadNameException();
-        }
-
-        return new File(getBaseDirectory(), specialName);
-    }
-
-    private boolean isFingerprint(String fingerprint) {
-        return openPgpV4FingerprintPattern.matcher(fingerprint).matches();
-    }
-
-    private boolean isSpecialName(String specialName) {
-        return SpecialNames.lookupSpecialName(specialName) != null;
-    }
-
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/LockingMechanism.java

@@ -1,37 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import java.io.IOException;
-
-public interface LockingMechanism {
-
-    /**
-     * Lock the store for writes.
-     * Readers can continue to use the store and will always see consistent certs.
-     *
-     * @throws IOException in case of an IO error
-     * @throws InterruptedException if the thread gets interrupted
-     */
-    void lockDirectory() throws IOException, InterruptedException;
-
-    /**
-     * Try top lock the store for writes.
-     * Return false without locking the store in case the store was already locked.
-     *
-     * @return true if locking succeeded, false otherwise
-     *
-     * @throws IOException in case of an IO error
-     */
-    boolean tryLockDirectory() throws IOException;
-
-    /**
-     * Release the directory write-lock acquired via {@link #lockDirectory()}.
-     *
-     * @throws IOException in case of an IO error
-     */
-    void releaseDirectory() throws IOException;
-
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/PGPCertificateDirectories.java

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import pgp.cert_d.backend.FileBasedCertificateDirectoryBackend;
+import pgp.cert_d.backend.InMemoryCertificateDirectoryBackend;
+import pgp.cert_d.subkey_lookup.InMemorySubkeyLookup;
+import pgp.cert_d.subkey_lookup.SubkeyLookup;
+import pgp.certificate_store.certificate.KeyMaterialReaderBackend;
+import pgp.certificate_store.exception.NotAStoreException;
+
+import java.io.File;
+
+/**
+ * Static factory methods that return implementations of the {@link PGPCertificateDirectory} class.
+ */
+public final class PGPCertificateDirectories {
+
+    private PGPCertificateDirectories() {
+
+    }
+
+    public static PGPCertificateDirectory inMemoryCertificateDirectory(
+            KeyMaterialReaderBackend keyReader) {
+        return new PGPCertificateDirectory(
+                new InMemoryCertificateDirectoryBackend(keyReader), new InMemorySubkeyLookup());
+    }
+
+    public static PGPCertificateDirectory defaultFileBasedCertificateDirectory(
+            KeyMaterialReaderBackend keyReader,
+            SubkeyLookup subkeyLookup)
+            throws NotAStoreException {
+        return fileBasedCertificateDirectory(keyReader, BaseDirectoryProvider.getDefaultBaseDir(), subkeyLookup);
+    }
+
+    public static PGPCertificateDirectory fileBasedCertificateDirectory(
+            KeyMaterialReaderBackend keyReader,
+            File baseDirectory,
+            SubkeyLookup subkeyLookup)
+            throws NotAStoreException {
+        return new PGPCertificateDirectory(
+                new FileBasedCertificateDirectoryBackend(baseDirectory, keyReader), subkeyLookup);
+    }
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/PGPCertificateDirectory.java

@@ -0,0 +1,367 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import pgp.cert_d.subkey_lookup.SubkeyLookup;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.Set;
+import java.util.regex.Pattern;
+
+/**
+ * Implementation of the Shared PGP Certificate Directory.
+ *
+ * @see <a href="https://sequoia-pgp.gitlab.io/pgp-cert-d/">Shared PGP Certificate Directory Specification</a>
+ */
+public class PGPCertificateDirectory
+        implements ReadOnlyPGPCertificateDirectory, WritingPGPCertificateDirectory, SubkeyLookup {
+
+    final Backend backend;
+    final SubkeyLookup subkeyLookup;
+    private final Pattern openPgpV4FingerprintPattern = Pattern.compile("^[a-f0-9]{40}$");
+
+    /**
+     * Constructor for a PGP certificate directory.
+     *
+     * @param backend storage backend
+     * @param subkeyLookup subkey lookup mechanism to map subkey-ids to certificates
+     */
+    public PGPCertificateDirectory(Backend backend, SubkeyLookup subkeyLookup) {
+        this.backend = backend;
+        this.subkeyLookup = subkeyLookup;
+    }
+
+    @Override
+    public Certificate getByFingerprint(String fingerprint) throws BadDataException, BadNameException, IOException {
+        if (!openPgpV4FingerprintPattern.matcher(fingerprint).matches()) {
+            throw new BadNameException();
+        }
+        Certificate certificate = backend.readByFingerprint(fingerprint);
+        if (certificate == null) {
+            throw new NoSuchElementException();
+        }
+        return certificate;
+    }
+
+    @Override
+    public Certificate getByFingerprintIfChanged(String fingerprint, long tag)
+            throws IOException, BadNameException, BadDataException {
+        if (!Objects.equals(tag, backend.getTagForFingerprint(fingerprint))) {
+            return getByFingerprint(fingerprint);
+        }
+        return null;
+    }
+
+
+    @Override
+    public Certificate getBySpecialName(String specialName)
+            throws BadNameException, BadDataException, IOException {
+        KeyMaterial keyMaterial = backend.readBySpecialName(specialName);
+        if (keyMaterial != null) {
+            return keyMaterial.asCertificate();
+        }
+        throw new NoSuchElementException();
+    }
+
+    @Override
+    public Certificate getBySpecialNameIfChanged(String specialName, long tag)
+            throws IOException, BadNameException, BadDataException {
+        if (!Objects.equals(tag, backend.getTagForSpecialName(specialName))) {
+            return getBySpecialName(specialName);
+        }
+        return null;
+    }
+
+    @Override
+    public Certificate getTrustRootCertificate()
+            throws IOException, BadDataException {
+        try {
+            return getBySpecialName(SpecialNames.TRUST_ROOT);
+        } catch (BadNameException e) {
+            throw new AssertionError("'" + SpecialNames.TRUST_ROOT + "' is an implementation MUST");
+        }
+    }
+
+    @Override
+    public Certificate getTrustRootCertificateIfChanged(long tag) throws IOException, BadDataException {
+        try {
+            return getBySpecialNameIfChanged(SpecialNames.TRUST_ROOT, tag);
+        } catch (BadNameException e) {
+            throw new AssertionError("'" + SpecialNames.TRUST_ROOT + "' is an implementation MUST");
+        }
+    }
+
+    @Override
+    public Iterator<Certificate> items() {
+        return backend.readItems();
+    }
+
+    @Override
+    public Iterator<String> fingerprints() {
+        Iterator<Certificate> certs = items();
+        return new Iterator<String>() {
+            @Override
+            public boolean hasNext() {
+                return certs.hasNext();
+            }
+
+            @Override
+            public String next() {
+                return certs.next().getFingerprint();
+            }
+        };
+    }
+
+    @Override
+    public KeyMaterial getTrustRoot() throws IOException, BadDataException {
+        try {
+            KeyMaterial keyMaterial = backend.readBySpecialName(SpecialNames.TRUST_ROOT);
+            if (keyMaterial == null) {
+                throw new NoSuchElementException();
+            }
+            return keyMaterial;
+        } catch (BadNameException e) {
+            throw new AssertionError("'" + SpecialNames.TRUST_ROOT + "' is implementation MUST");
+        }
+    }
+
+    @Override
+    public KeyMaterial insertTrustRoot(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, InterruptedException {
+        backend.getLock().lockDirectory();
+        KeyMaterial inserted = backend.doInsertTrustRoot(data, merge);
+        subkeyLookup.storeCertificateSubkeyIds(inserted.getFingerprint(), inserted.getSubkeyIds());
+        backend.getLock().releaseDirectory();
+        return inserted;
+    }
+
+    @Override
+    public KeyMaterial tryInsertTrustRoot(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException {
+        if (!backend.getLock().tryLockDirectory()) {
+            return null;
+        }
+        KeyMaterial inserted = backend.doInsertTrustRoot(data, merge);
+        subkeyLookup.storeCertificateSubkeyIds(inserted.getFingerprint(), inserted.getSubkeyIds());
+        backend.getLock().releaseDirectory();
+        return inserted;
+    }
+
+
+
+    @Override
+    public Certificate insert(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, InterruptedException {
+        backend.getLock().lockDirectory();
+        Certificate inserted = backend.doInsert(data, merge);
+        subkeyLookup.storeCertificateSubkeyIds(inserted.getFingerprint(), inserted.getSubkeyIds());
+        backend.getLock().releaseDirectory();
+        return inserted;
+    }
+
+    @Override
+    public Certificate tryInsert(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException {
+        if (!backend.getLock().tryLockDirectory()) {
+            return null;
+        }
+        Certificate inserted = backend.doInsert(data, merge);
+        subkeyLookup.storeCertificateSubkeyIds(inserted.getFingerprint(), inserted.getSubkeyIds());
+        backend.getLock().releaseDirectory();
+        return inserted;
+    }
+
+    @Override
+    public Certificate insertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, BadNameException, InterruptedException {
+        backend.getLock().lockDirectory();
+        Certificate inserted = backend.doInsertWithSpecialName(specialName, data, merge);
+        subkeyLookup.storeCertificateSubkeyIds(inserted.getFingerprint(), inserted.getSubkeyIds());
+        backend.getLock().releaseDirectory();
+        return inserted;
+    }
+
+    @Override
+    public Certificate tryInsertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, BadNameException {
+        if (!backend.getLock().tryLockDirectory()) {
+            return null;
+        }
+        Certificate inserted = backend.doInsertWithSpecialName(specialName, data, merge);
+        subkeyLookup.storeCertificateSubkeyIds(inserted.getFingerprint(), inserted.getSubkeyIds());
+        backend.getLock().releaseDirectory();
+        return inserted;
+    }
+
+    @Override
+    public Set<String> getCertificateFingerprintsForSubkeyId(long subkeyId) throws IOException {
+        return subkeyLookup.getCertificateFingerprintsForSubkeyId(subkeyId);
+    }
+
+    @Override
+    public void storeCertificateSubkeyIds(String certificate, List<Long> subkeyIds) throws IOException {
+        subkeyLookup.storeCertificateSubkeyIds(certificate, subkeyIds);
+    }
+
+    /**
+     * Storage backend.
+     */
+    public interface Backend {
+
+        /**
+         * Get the locking mechanism to write-lock the backend.
+         *
+         * @return lock
+         */
+        LockingMechanism getLock();
+
+        /**
+         * Read a {@link Certificate} by its OpenPGP fingerprint.
+         *
+         * @param fingerprint fingerprint
+         * @return certificate
+         *
+         * @throws BadNameException if the fingerprint is malformed
+         * @throws IOException in case of an IO error
+         * @throws BadDataException if the certificate contains bad data
+         */
+        Certificate readByFingerprint(String fingerprint) throws BadNameException, IOException, BadDataException;
+
+        /**
+         * Read a {@link Certificate} or {@link pgp.certificate_store.certificate.Key} by the given special name.
+         *
+         * @param specialName special name
+         * @return certificate or key
+         *
+         * @throws BadNameException if the special name is not known
+         * @throws IOException in case of an IO error
+         * @throws BadDataException if the certificate contains bad data
+         */
+        KeyMaterial readBySpecialName(String specialName) throws BadNameException, IOException, BadDataException;
+
+        /**
+         * Return an {@link Iterator} of all {@link Certificate Certificates} in the store, except for certificates
+         * stored under a special name.
+         *
+         * @return iterator
+         */
+        Iterator<Certificate> readItems();
+
+        /**
+         * Insert a {@link pgp.certificate_store.certificate.Key} or {@link Certificate} as trust-root.
+         *
+         * @param data input stream containing the key material
+         * @param merge callback to merge the key material with existing key material
+         * @return merged or inserted key material
+         *
+         * @throws BadDataException if the data stream or existing key material contains bad data
+         * @throws IOException in case of an IO error
+         */
+        KeyMaterial doInsertTrustRoot(InputStream data, KeyMaterialMerger merge)
+                throws BadDataException, IOException;
+
+        /**
+         * Insert a {@link Certificate} identified by its fingerprint into the directory.
+         *
+         * @param data input stream containing the certificate data
+         * @param merge callback to merge the certificate with existing key material
+         * @return merged or inserted certificate
+         *
+         * @throws IOException in case of an IO error
+         * @throws BadDataException if the data stream or existing certificate contains bad data
+         */
+        Certificate doInsert(InputStream data, KeyMaterialMerger merge)
+                        throws IOException, BadDataException;
+
+        /**
+         * Insert a {@link pgp.certificate_store.certificate.Key} or {@link Certificate} under the given special name.
+         *
+         * @param specialName special name to identify the key material with
+         * @param data data stream containing the key or certificate
+         * @param merge callback to merge the key/certificate with existing key material
+         * @return certificate component of the merged or inserted key material
+         *
+         * @throws IOException in case of an IO error
+         * @throws BadDataException if the data stream or existing key material contains bad data
+         * @throws BadNameException if the special name is not known
+         */
+        Certificate doInsertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+                                throws IOException, BadDataException, BadNameException;
+
+        /**
+         * Calculate the tag of the certificate with the given fingerprint.
+         *
+         * @param fingerprint fingerprint
+         * @return tag
+         *
+         * @throws BadNameException if the fingerprint is malformed
+         * @throws IOException in case of an IO error
+         * @throws IllegalArgumentException if the certificate does not exist
+         */
+        Long getTagForFingerprint(String fingerprint) throws BadNameException, IOException;
+
+        /**
+         * Calculate the tag of the certificate identified by the given special name.
+         *
+         * @param specialName special name
+         * @return tag
+         *
+         * @throws BadNameException if the special name is not known
+         * @throws IOException in case of an IO error
+         * @throws IllegalArgumentException if the certificate or key does not exist
+         */
+        Long getTagForSpecialName(String specialName) throws BadNameException, IOException;
+    }
+
+    /**
+     * Interface for a write-locking mechanism.
+     */
+    public interface LockingMechanism {
+
+        /**
+         * Lock the store for writes.
+         * Readers can continue to use the store and will always see consistent certs.
+         *
+         * @throws IOException in case of an IO error
+         * @throws InterruptedException if the thread gets interrupted
+         */
+        void lockDirectory() throws IOException, InterruptedException;
+
+        /**
+         * Try top lock the store for writes.
+         * Return false without locking the store in case the store was already locked.
+         *
+         * @return true if locking succeeded, false otherwise
+         *
+         * @throws IOException in case of an IO error
+         */
+        boolean tryLockDirectory() throws IOException;
+
+        /**
+         * Return true if the lock is in locked state.
+         *
+         * @return true if locked
+         */
+        boolean isLocked();
+
+        /**
+         * Release the directory write-lock acquired via {@link #lockDirectory()}.
+         *
+         * @throws IOException in case of an IO error
+         */
+        void releaseDirectory() throws IOException;
+
+    }
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/PGPCertificateStoreAdapter.java

@@ -0,0 +1,87 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import pgp.certificate_store.PGPCertificateStore;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Set;
+
+/**
+ * Adapter class to adapt a {@link PGPCertificateDirectory} to the {@link  PGPCertificateStore} interface.
+ */
+public class PGPCertificateStoreAdapter implements PGPCertificateStore {
+
+    private final PGPCertificateDirectory directory;
+
+    public PGPCertificateStoreAdapter(PGPCertificateDirectory directory) {
+        this.directory = directory;
+    }
+
+    @Override
+    public Certificate getCertificate(String identifier)
+            throws IOException, BadNameException, BadDataException {
+        if (SpecialNames.lookupSpecialName(identifier) != null) {
+            return directory.getBySpecialName(identifier);
+        } else {
+            return directory.getByFingerprint(identifier.toLowerCase());
+        }
+    }
+
+    @Override
+    public Certificate getCertificateIfChanged(String identifier, Long tag)
+            throws IOException, BadNameException, BadDataException {
+        if (SpecialNames.lookupSpecialName(identifier) != null) {
+            return directory.getBySpecialNameIfChanged(identifier, tag);
+        } else {
+            return directory.getByFingerprintIfChanged(identifier.toLowerCase(), tag);
+        }
+    }
+
+    @Override
+    public Iterator<Certificate> getCertificatesBySubkeyId(long subkeyId)
+            throws IOException, BadDataException {
+        Set<String> fingerprints = directory.getCertificateFingerprintsForSubkeyId(subkeyId);
+        Set<Certificate> certificates = new HashSet<>();
+        for (String fingerprint : fingerprints) {
+            try {
+                certificates.add(directory.getByFingerprint(fingerprint));
+            } catch (BadNameException e) {
+                throw new RuntimeException(e);
+            }
+        }
+        return certificates.iterator();
+    }
+
+    @Override
+    public Certificate insertCertificate(InputStream data, KeyMaterialMerger merge)
+            throws IOException, InterruptedException, BadDataException {
+        Certificate certificate = directory.insert(data, merge);
+        return certificate;
+    }
+
+    @Override
+    public Certificate insertCertificateBySpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, InterruptedException, BadDataException, BadNameException {
+        return directory.insertWithSpecialName(specialName, data, merge);
+    }
+
+    @Override
+    public Iterator<Certificate> getCertificates() {
+        return directory.items();
+    }
+
+    @Override
+    public Iterator<String> getFingerprints() {
+        return directory.fingerprints();
+    }
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/ReadOnlyPGPCertificateDirectory.java

@@ -0,0 +1,127 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+/**
+ * Interface for a read-only OpenPGP certificate directory.
+ */
+public interface ReadOnlyPGPCertificateDirectory {
+
+    /**
+     * Get the trust-root certificate. This is a certificate which is stored under the special name
+     * <pre>trust-root</pre>.
+     *
+     * @return trust-root certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the certificate contains bad data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getTrustRootCertificate()
+            throws IOException, BadDataException;
+
+    /**
+     * Get the trust-root certificate if it has changed.
+     * This method uses the <pre>tag</pre> to calculate if the certificate might have changed.
+     * If the computed tag equals the given tag, the certificate has not changed, so <pre>null</pre> is returned.
+     * Otherwise. the changed certificate is returned.
+     *
+     * @param tag tag
+     * @return changed certificate, or null if the certificate is unchanged.
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the certificate contains bad data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getTrustRootCertificateIfChanged(long tag)
+            throws IOException, BadDataException;
+
+    /**
+     * Get the certificate identified by the given fingerprint.
+     *
+     * @param fingerprint lower-case fingerprint of the certificate
+     * @return certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadNameException if the fingerprint is malformed
+     * @throws BadDataException if the certificate contains bad data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getByFingerprint(String fingerprint)
+            throws IOException, BadNameException, BadDataException;
+
+    /**
+     * Get the certificate identified by the given fingerprint if it has changed.
+     * This method uses the <pre>tag</pre> to calculate, if the certificate might have changed.
+     * If the computed tag equals the given tag, the certificate has not changed, so <pre>null</pre> is returned.
+     * Otherwise, the changed certificate is returned.
+     *
+     * @param fingerprint lower-case fingerprint of the certificate
+     * @param tag tag
+     * @return certificate or null if the certificate has not been changed
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadNameException if the fingerprint is malformed
+     * @throws BadDataException if the certificate contains bad data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getByFingerprintIfChanged(String fingerprint, long tag)
+            throws IOException, BadNameException, BadDataException;
+
+    /**
+     * Get the certificate identified by the given special name.
+     *
+     * @param specialName special name
+     * @return certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadNameException if the special name is not known
+     * @throws BadDataException if the certificate contains bad data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getBySpecialName(String specialName)
+            throws IOException, BadNameException, BadDataException;
+
+    /**
+     * Get the certificate identified by the given special name or null, if it has not been changed.
+     * This method uses the <pre>tag</pre> to calculate, if the certificate might have changed.
+     * If the computed tag equals the given tag, the certificate has not changed, so <pre>null</pre> is returned.
+     * Otherwise, the changed certificate is returned.
+     *
+     * @param specialName special name
+     * @param tag tag
+     * @return certificate or null
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadNameException if the special name is not known
+     * @throws BadDataException if the certificate contains bad data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getBySpecialNameIfChanged(String specialName, long tag)
+            throws IOException, BadNameException, BadDataException;
+
+    /**
+     * Get all certificates in the directory, except for certificates which are stored by special name.
+     *
+     * @return iterator of certificates
+     */
+    Iterator<Certificate> items();
+
+    /**
+     * Get the fingerprints of all certificates in the directory, except for certificates which are stored by
+     * special name.
+     *
+     * @return iterator of fingerprints
+     */
+    Iterator<String> fingerprints();
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/SharedPGPCertificateDirectory.java

@@ -1,47 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.Iterator;
-
-import pgp.certificate_store.exception.BadDataException;
-import pgp.certificate_store.exception.BadNameException;
-import pgp.certificate_store.Certificate;
-import pgp.certificate_store.MergeCallback;
-
-public interface SharedPGPCertificateDirectory {
-
-    LockingMechanism getLock();
-
-    Certificate getByFingerprint(String fingerprint)
-            throws IOException, BadNameException, BadDataException;
-
-    Certificate getBySpecialName(String specialName)
-            throws IOException, BadNameException, BadDataException;
-
-    Certificate getByFingerprintIfChanged(String fingerprint, String tag)
-            throws IOException, BadNameException, BadDataException;
-
-    Certificate getBySpecialNameIfChanged(String specialName, String tag)
-            throws IOException, BadNameException, BadDataException;
-
-    Certificate insert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, InterruptedException;
-
-    Certificate tryInsert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException;
-
-    Certificate insertWithSpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, BadNameException, InterruptedException;
-
-    Certificate tryInsertWithSpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, BadNameException;
-
-    Iterator<Certificate> items();
-
-    Iterator<String> fingerprints();
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/SharedPGPCertificateDirectoryImpl.java

@@ -1,314 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.cert_d;
-
-import java.io.BufferedInputStream;
-import java.io.File;
-import java.io.FileFilter;
-import java.io.FileInputStream;
-import java.io.FileOutputStream;
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.ArrayList;
-import java.util.Collections;
-import java.util.Iterator;
-import java.util.List;
-
-import pgp.certificate_store.exception.BadDataException;
-import pgp.certificate_store.exception.BadNameException;
-import pgp.certificate_store.exception.NotAStoreException;
-import pgp.certificate_store.Certificate;
-import pgp.certificate_store.CertificateReaderBackend;
-import pgp.certificate_store.MergeCallback;
-
-public class SharedPGPCertificateDirectoryImpl implements SharedPGPCertificateDirectory {
-
-    private final FilenameResolver resolver;
-    private final LockingMechanism writeLock;
-    private final CertificateReaderBackend certificateReaderBackend;
-
-    public SharedPGPCertificateDirectoryImpl(BackendProvider backendProvider)
-            throws NotAStoreException {
-        this(backendProvider.provideCertificateReaderBackend());
-    }
-
-    public SharedPGPCertificateDirectoryImpl(CertificateReaderBackend certificateReaderBackend)
-            throws NotAStoreException {
-        this(
-                BaseDirectoryProvider.getDefaultBaseDir(),
-                certificateReaderBackend);
-    }
-
-    public SharedPGPCertificateDirectoryImpl(File baseDirectory, CertificateReaderBackend certificateReaderBackend)
-            throws NotAStoreException {
-        this(
-                certificateReaderBackend,
-                new FilenameResolver(baseDirectory),
-                FileLockingMechanism.defaultDirectoryFileLock(baseDirectory));
-    }
-
-    public SharedPGPCertificateDirectoryImpl(
-            CertificateReaderBackend certificateReaderBackend,
-            FilenameResolver filenameResolver,
-            LockingMechanism writeLock)
-            throws NotAStoreException {
-        this.certificateReaderBackend = certificateReaderBackend;
-        this.resolver = filenameResolver;
-        this.writeLock = writeLock;
-
-        File baseDirectory = resolver.getBaseDirectory();
-        if (!baseDirectory.exists()) {
-            if (!baseDirectory.mkdirs()) {
-                throw new NotAStoreException("Cannot create base directory '" + resolver.getBaseDirectory().getAbsolutePath() + "'");
-            }
-        } else {
-            if (baseDirectory.isFile()) {
-                throw new NotAStoreException("Base directory '" + resolver.getBaseDirectory().getAbsolutePath() + "' appears to be a file.");
-            }
-        }
-    }
-
-    @Override
-    public LockingMechanism getLock() {
-        return writeLock;
-    }
-
-    @Override
-    public Certificate getByFingerprint(String fingerprint)
-            throws IOException, BadNameException, BadDataException {
-        File certFile = resolver.getCertFileByFingerprint(fingerprint);
-        if (!certFile.exists()) {
-            return null;
-        }
-
-        FileInputStream fileIn = new FileInputStream(certFile);
-        BufferedInputStream bufferedIn = new BufferedInputStream(fileIn);
-        Certificate certificate = certificateReaderBackend.readCertificate(bufferedIn);
-
-        if (!certificate.getFingerprint().equals(fingerprint)) {
-            // TODO: Figure out more suitable exception
-            throw new BadDataException();
-        }
-
-        return certificate;
-    }
-
-    @Override
-    public Certificate getBySpecialName(String specialName)
-            throws IOException, BadNameException {
-        File certFile = resolver.getCertFileBySpecialName(specialName);
-        if (!certFile.exists()) {
-            return null;
-        }
-
-        FileInputStream fileIn = new FileInputStream(certFile);
-        BufferedInputStream bufferedIn = new BufferedInputStream(fileIn);
-        Certificate certificate = certificateReaderBackend.readCertificate(bufferedIn);
-
-        return certificate;
-    }
-
-    @Override
-    public Certificate getByFingerprintIfChanged(String fingerprint, String tag)
-            throws IOException, BadNameException, BadDataException {
-        Certificate certificate = getByFingerprint(fingerprint);
-        if (certificate.getTag().equals(tag)) {
-            return null;
-        }
-        return certificate;
-    }
-
-    @Override
-    public Certificate getBySpecialNameIfChanged(String specialName, String tag)
-            throws IOException, BadNameException {
-        Certificate certificate = getBySpecialName(specialName);
-        if (certificate.getTag().equals(tag)) {
-            return null;
-        }
-        return certificate;
-    }
-
-    @Override
-    public Certificate insert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, InterruptedException {
-        writeLock.lockDirectory();
-
-        Certificate certificate = _insert(data, merge);
-
-        writeLock.releaseDirectory();
-        return certificate;
-    }
-
-    @Override
-    public Certificate tryInsert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException {
-        if (!writeLock.tryLockDirectory()) {
-            return null;
-        }
-
-        Certificate certificate = _insert(data, merge);
-
-        writeLock.releaseDirectory();
-        return certificate;
-    }
-
-    private Certificate _insert(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException {
-        Certificate newCertificate = certificateReaderBackend.readCertificate(data);
-        Certificate existingCertificate;
-        File certFile;
-        try {
-            existingCertificate = getByFingerprint(newCertificate.getFingerprint());
-            certFile = resolver.getCertFileByFingerprint(newCertificate.getFingerprint());
-        } catch (BadNameException e) {
-            throw new BadDataException();
-        }
-
-        if (existingCertificate != null && !existingCertificate.getTag().equals(newCertificate.getTag())) {
-            newCertificate = merge.merge(newCertificate, existingCertificate);
-        }
-
-        writeCertificate(newCertificate, certFile);
-
-        return newCertificate;
-    }
-
-    private void writeCertificate(Certificate certificate, File certFile)
-            throws IOException {
-        certFile.getParentFile().mkdirs();
-        if (!certFile.exists() && !certFile.createNewFile()) {
-            throw new IOException("Could not create cert file " + certFile.getAbsolutePath());
-        }
-
-        InputStream certIn = certificate.getInputStream();
-        FileOutputStream fileOut = new FileOutputStream(certFile);
-
-        byte[] buffer = new byte[4096];
-        int read;
-        while ((read = certIn.read(buffer)) != -1) {
-            fileOut.write(buffer, 0, read);
-        }
-
-        certIn.close();
-        fileOut.close();
-    }
-
-    @Override
-    public Certificate insertWithSpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadNameException, BadDataException, InterruptedException {
-        writeLock.lockDirectory();
-
-        Certificate certificate = _insertSpecial(specialName, data, merge);
-
-        writeLock.releaseDirectory();
-        return certificate;
-    }
-
-    @Override
-    public Certificate tryInsertWithSpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadNameException, BadDataException {
-        if (!writeLock.tryLockDirectory()) {
-            return null;
-        }
-
-        Certificate certificate = _insertSpecial(specialName, data, merge);
-
-        writeLock.releaseDirectory();
-        return certificate;
-    }
-
-    private Certificate _insertSpecial(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadNameException, BadDataException {
-        Certificate newCertificate = certificateReaderBackend.readCertificate(data);
-        Certificate existingCertificate = getBySpecialName(specialName);
-        File certFile = resolver.getCertFileBySpecialName(specialName);
-
-        if (existingCertificate != null && !existingCertificate.getTag().equals(newCertificate.getTag())) {
-            newCertificate = merge.merge(newCertificate, existingCertificate);
-        }
-
-        writeCertificate(newCertificate, certFile);
-
-        return newCertificate;
-    }
-
-    @Override
-    public Iterator<Certificate> items() {
-        return new Iterator<Certificate>() {
-
-            private final List<Lazy<Certificate>> certificateQueue = Collections.synchronizedList(new ArrayList<>());
-
-            // Constructor... wtf.
-            {
-                File[] subdirectories = resolver.getBaseDirectory().listFiles(new FileFilter() {
-                            @Override
-                            public boolean accept(File file) {
-                                return file.isDirectory() && file.getName().matches("^[a-f0-9]{2}$");
-                            }
-                        });
-
-                for (File subdirectory : subdirectories) {
-                    File[] files = subdirectory.listFiles(new FileFilter() {
-                        @Override
-                        public boolean accept(File file) {
-                            return file.isFile() && file.getName().matches("^[a-f0-9]{38}$");
-                        }
-                    });
-
-                    for (File certFile : files) {
-                        certificateQueue.add(new Lazy<Certificate>() {
-                            @Override
-                            Certificate get() throws BadDataException {
-                                try {
-                                    Certificate certificate = certificateReaderBackend.readCertificate(new FileInputStream(certFile));
-                                    if (!(subdirectory.getName() + certFile.getName()).equals(certificate.getFingerprint())) {
-                                        throw new BadDataException();
-                                    }
-                                    return certificate;
-                                } catch (IOException e) {
-                                    throw new AssertionError("File got deleted.");
-                                }
-                            }
-                        });
-                    }
-                }
-            }
-
-            @Override
-            public boolean hasNext() {
-                return !certificateQueue.isEmpty();
-            }
-
-            @Override
-            public Certificate next() {
-                try {
-                    return certificateQueue.remove(0).get();
-                } catch (BadDataException e) {
-                    throw new AssertionError("Could not retrieve item: " + e.getMessage());
-                }
-            }
-        };
-    }
-
-    private abstract static class Lazy<E> {
-        abstract E get() throws BadDataException;
-    }
-
-    @Override
-    public Iterator<String> fingerprints() {
-        Iterator<Certificate> certificates = items();
-        return new Iterator<String>() {
-            @Override
-            public boolean hasNext() {
-                return certificates.hasNext();
-            }
-
-            @Override
-            public String next() {
-                return certificates.next().getFingerprint();
-            }
-        };
-    }
-}

pgp-cert-d-java/src/main/java/pgp/cert_d/SpecialNames.java

@@ -9,11 +9,14 @@ import java.util.Map;
 
 public class SpecialNames {
 
+    public static final String TRUST_ROOT = "trust-root";
+
+    // Map to allow for potentially upper- and lowercase variants of the same special name
     private static final Map<String, String> SPECIAL_NAMES = new HashMap<>();
 
     static {
-        SPECIAL_NAMES.put("TRUST-ROOT", "trust-root"); // TODO: Remove
-        SPECIAL_NAMES.put("trust-root", "trust-root");
+        SPECIAL_NAMES.put("TRUST-ROOT", TRUST_ROOT); // TODO: Remove
+        SPECIAL_NAMES.put(TRUST_ROOT, TRUST_ROOT);
     }
 
     public static String lookupSpecialName(String specialName) {

pgp-cert-d-java/src/main/java/pgp/cert_d/WritingPGPCertificateDirectory.java

@@ -0,0 +1,126 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+/**
+ * Interface for a writing OpenPGP certificate directory.
+ */
+public interface WritingPGPCertificateDirectory {
+
+    /**
+     * Return the certificate or key identified by the special name <pre>trust-root</pre>.
+     *
+     * @return trust-root key or certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the certificate contains bad data
+     */
+    KeyMaterial getTrustRoot()
+            throws IOException, BadDataException;
+
+    /**
+     * Insert a key or certificate under the special name <pre>trust-root</pre>.
+     * This method blocks until the key material has been written.
+     *
+     * @param data input stream containing the key or certificate
+     * @param merge key material merger to merge the key or certificate with existing key material
+     * @return the merged or inserted key or certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the data stream or the existing trust-root key material contains bad data
+     * @throws InterruptedException if the thread is interrupted
+     */
+    KeyMaterial insertTrustRoot(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, InterruptedException;
+
+    /**
+     * Insert a key or certificate under the special name <pre>trust-root</pre>.
+     * Contrary to {@link #insertTrustRoot(InputStream, KeyMaterialMerger)}, this method does not block.
+     * Instead, it returns null if the write-lock cannot be obtained.
+     *
+     * @param data input stream containing the key or certificate
+     * @param merge key material merger to merge the key or certificate with existing key material
+     * @return the merged or inserted key or certificate, or null if the write-lock cannot be obtained
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the thread is interrupted
+     */
+    KeyMaterial tryInsertTrustRoot(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException;
+
+    /**
+     * Insert a certificate identified by its fingerprint.
+     * This method blocks until the certificate has been written.
+     *
+     * @param data input stream containing the certificate data
+     * @param merge merge callback to merge the certificate with existing certificate material
+     * @return the merged or inserted certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the data stream or existing certificate contains bad data
+     * @throws InterruptedException if the thread is interrupted
+     */
+    Certificate insert(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, InterruptedException;
+
+    /**
+     * Insert a certificate identified by its fingerprint.
+     * Contrary to {@link #insert(InputStream, KeyMaterialMerger)}, this method does not block.
+     * Instead, it returns null if the write-lock cannot be obtained.
+     *
+     * @param data input stream containing the certificate data
+     * @param merge merge callback to  merge the certificate with existing certificate material
+     * @return the merged or inserted certificate
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the data stream or existing certificate contains bad data
+     */
+    Certificate tryInsert(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException;
+
+    /**
+     * Insert a certificate or key under the given special name.
+     * This method blocks until the certificate/key has been written.
+     *
+     * @param specialName special name under which the key material shall be inserted
+     * @param data input stream containing the key/certificate data
+     * @param merge callback to merge the key/certificate with existing key material
+     * @return certificate component of the merged or inserted key material data
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the data stream or the existing certificate contains bad data
+     * @throws BadNameException if the special name is not known
+     * @throws InterruptedException if the thread is interrupted
+     */
+    Certificate insertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, BadNameException, InterruptedException;
+
+    /**
+     * Insert a certificate or key under the given special name.
+     * Contrary to {@link #insertWithSpecialName(String, InputStream, KeyMaterialMerger)}, this method does not block.
+     * Instead, it returns null if the write-lock cannot be obtained.
+     *
+     * @param specialName special name under which the key material shall be inserted
+     * @param data input stream containing the key material
+     * @param merge callback to merge the key/certificate with existing key material
+     * @return certificate component of the merged or inserted key material
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if the data stream or existing key material contains bad data
+     * @throws BadNameException if the special name is not known
+     */
+    Certificate tryInsertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, BadNameException;
+
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/backend/FileBasedCertificateDirectoryBackend.java

@@ -0,0 +1,448 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.backend;
+
+import pgp.cert_d.PGPCertificateDirectory;
+import pgp.cert_d.SpecialNames;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.Key;
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.certificate.KeyMaterialReaderBackend;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+import pgp.certificate_store.exception.NotAStoreException;
+
+import java.io.BufferedInputStream;
+import java.io.File;
+import java.io.FileFilter;
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.RandomAccessFile;
+import java.nio.channels.FileLock;
+import java.nio.channels.OverlappingFileLockException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.regex.Pattern;
+
+/**
+ * Implementation of {@link PGPCertificateDirectory.Backend} which stores certificates in a directory structure.
+ *
+ * @see <a href="https://sequoia-pgp.gitlab.io/pgp-cert-d/#name-implementation">Shared PGP Certificate Directory</a>
+ */
+public class FileBasedCertificateDirectoryBackend implements PGPCertificateDirectory.Backend {
+
+    private abstract static class Lazy<E> {
+        abstract E get() throws BadDataException;
+    }
+
+    /**
+     * Locking mechanism which uses a lock file to synchronize write-access to the store.
+     */
+    private static class FileLockingMechanism implements PGPCertificateDirectory.LockingMechanism {
+
+        private final File lockFile;
+        private RandomAccessFile randomAccessFile;
+        private FileLock fileLock;
+
+        FileLockingMechanism(File lockFile) {
+            this.lockFile = lockFile;
+        }
+
+        public static FileLockingMechanism defaultDirectoryFileLock(File baseDirectory) {
+            return new FileLockingMechanism(new File(baseDirectory, "writelock"));
+        }
+
+        @Override
+        public synchronized void lockDirectory() throws IOException, InterruptedException {
+            if (randomAccessFile != null) {
+                // we own the lock already. Let's wait...
+                this.wait();
+            }
+
+            try {
+                randomAccessFile = new RandomAccessFile(lockFile, "rw");
+            } catch (FileNotFoundException e) {
+                lockFile.createNewFile();
+                randomAccessFile = new RandomAccessFile(lockFile, "rw");
+            }
+
+            fileLock = randomAccessFile.getChannel().lock();
+        }
+
+        @Override
+        public synchronized boolean tryLockDirectory() throws IOException {
+            if (randomAccessFile != null) {
+                // We already locked the directory for another write operation.
+                // We fail, since we have not yet released the lock from the other operation.
+                return false;
+            }
+
+            try {
+                randomAccessFile = new RandomAccessFile(lockFile, "rw");
+            } catch (FileNotFoundException e) {
+                lockFile.createNewFile();
+                randomAccessFile = new RandomAccessFile(lockFile, "rw");
+            }
+
+            try {
+                fileLock = randomAccessFile.getChannel().tryLock();
+                if (fileLock == null) {
+                    // try-lock failed, file is locked by another process.
+                    randomAccessFile.close();
+                    randomAccessFile = null;
+                    return false;
+                }
+            } catch (OverlappingFileLockException e) {
+                // Some other object is holding the lock.
+                randomAccessFile.close();
+                randomAccessFile = null;
+                return false;
+            }
+            return true;
+        }
+
+        @Override
+        public boolean isLocked() {
+            return randomAccessFile != null;
+        }
+
+        @Override
+        public synchronized void releaseDirectory() throws IOException {
+            // unlock file
+            if (fileLock != null) {
+                fileLock.release();
+                fileLock = null;
+            }
+            // close file
+            if (randomAccessFile != null) {
+                randomAccessFile.close();
+                randomAccessFile = null;
+            }
+            // delete file
+            if (lockFile.exists()) {
+                lockFile.delete();
+            }
+            // notify waiters
+            this.notify();
+        }
+    }
+
+    private final File baseDirectory;
+    private final PGPCertificateDirectory.LockingMechanism lock;
+    private final FilenameResolver resolver;
+    private final KeyMaterialReaderBackend reader;
+
+    public FileBasedCertificateDirectoryBackend(File baseDirectory, KeyMaterialReaderBackend reader) throws NotAStoreException {
+        this.baseDirectory = baseDirectory;
+        this.resolver = new FilenameResolver(baseDirectory);
+
+        if (!baseDirectory.exists()) {
+            if (!baseDirectory.mkdirs()) {
+                throw new NotAStoreException("Cannot create base directory '" + resolver.getBaseDirectory().getAbsolutePath() + "'");
+            }
+        } else {
+            if (baseDirectory.isFile()) {
+                throw new NotAStoreException("Base directory '" + resolver.getBaseDirectory().getAbsolutePath() + "' appears to be a file.");
+            }
+        }
+        this.lock = FileLockingMechanism.defaultDirectoryFileLock(baseDirectory);
+        this.reader = reader;
+    }
+
+    @Override
+    public PGPCertificateDirectory.LockingMechanism getLock() {
+        return lock;
+    }
+
+    @Override
+    public Certificate readByFingerprint(String fingerprint) throws BadNameException, IOException, BadDataException {
+        File certFile = resolver.getCertFileByFingerprint(fingerprint);
+        if (!certFile.exists()) {
+            return null;
+        }
+
+        long tag = getTagForFingerprint(fingerprint);
+
+        FileInputStream fileIn = new FileInputStream(certFile);
+        BufferedInputStream bufferedIn = new BufferedInputStream(fileIn);
+
+        Certificate certificate = reader.read(bufferedIn, tag).asCertificate();
+        if (!certificate.getFingerprint().equals(fingerprint)) {
+            // TODO: Figure out more suitable exception
+            throw new BadDataException();
+        }
+
+        return certificate;
+    }
+
+    @Override
+    public KeyMaterial readBySpecialName(String specialName) throws BadNameException, IOException, BadDataException {
+        File certFile = resolver.getCertFileBySpecialName(specialName);
+        if (!certFile.exists()) {
+            return null;
+        }
+
+        long tag = getTagForSpecialName(specialName);
+
+        FileInputStream fileIn = new FileInputStream(certFile);
+        BufferedInputStream bufferedIn = new BufferedInputStream(fileIn);
+        KeyMaterial keyMaterial = reader.read(bufferedIn, tag);
+
+        return keyMaterial;
+    }
+
+    @Override
+    public Iterator<Certificate> readItems() {
+        return new Iterator<Certificate>() {
+
+            private final List<Lazy<Certificate>> certificateQueue = Collections.synchronizedList(new ArrayList<>());
+
+            // Constructor... wtf.
+            {
+                File[] subdirectories = baseDirectory.listFiles(new FileFilter() {
+                    @Override
+                    public boolean accept(File file) {
+                        return file.isDirectory() && file.getName().matches("^[a-f0-9]{2}$");
+                    }
+                });
+
+                if (subdirectories == null) {
+                    subdirectories = new File[0];
+                }
+
+                for (File subdirectory : subdirectories) {
+                    File[] files = subdirectory.listFiles(new FileFilter() {
+                        @Override
+                        public boolean accept(File file) {
+                            return file.isFile() && file.getName().matches("^[a-f0-9]{38}$");
+                        }
+                    });
+
+                    if (files == null) {
+                        files = new File[0];
+                    }
+
+                    for (File certFile : files) {
+                        certificateQueue.add(new Lazy<Certificate>() {
+                            @Override
+                            Certificate get() throws BadDataException {
+                                try {
+                                    long tag = getTag(certFile);
+                                    Certificate certificate = reader.read(new FileInputStream(certFile), tag).asCertificate();
+                                    if (!(subdirectory.getName() + certFile.getName()).equals(certificate.getFingerprint())) {
+                                        throw new BadDataException();
+                                    }
+                                    return certificate;
+                                } catch (IOException e) {
+                                    throw new AssertionError("File got deleted.");
+                                }
+                            }
+                        });
+                    }
+                }
+            }
+
+            @Override
+            public boolean hasNext() {
+                return !certificateQueue.isEmpty();
+            }
+
+            @Override
+            public Certificate next() {
+                try {
+                    return certificateQueue.remove(0).get();
+                } catch (BadDataException e) {
+                    throw new AssertionError("Could not retrieve item: " + e.getMessage());
+                }
+            }
+        };
+    }
+
+    @Override
+    public KeyMaterial doInsertTrustRoot(InputStream data, KeyMaterialMerger merge) throws BadDataException, IOException {
+        KeyMaterial newCertificate = reader.read(data, null);
+        KeyMaterial existingCertificate;
+        File certFile;
+        try {
+            existingCertificate = readBySpecialName(SpecialNames.TRUST_ROOT);
+            certFile = resolver.getCertFileBySpecialName(SpecialNames.TRUST_ROOT);
+        } catch (BadNameException e) {
+            throw new BadDataException();
+        }
+
+        if (existingCertificate != null) {
+            newCertificate = merge.merge(newCertificate, existingCertificate);
+        }
+
+        long tag = writeToFile(newCertificate.getInputStream(), certFile);
+        if (newCertificate instanceof Key) {
+            newCertificate = new Key((Key) newCertificate, tag);
+        } else {
+            newCertificate = new Certificate((Certificate) newCertificate, tag);
+        }
+        return newCertificate;
+    }
+
+    @Override
+    public Certificate doInsert(InputStream data, KeyMaterialMerger merge) throws IOException, BadDataException {
+        KeyMaterial newCertificate = reader.read(data, null);
+        Certificate existingCertificate;
+        File certFile;
+        try {
+            existingCertificate = readByFingerprint(newCertificate.getFingerprint());
+            certFile = resolver.getCertFileByFingerprint(newCertificate.getFingerprint());
+        } catch (BadNameException e) {
+            throw new BadDataException();
+        }
+
+        if (existingCertificate != null) {
+            newCertificate = merge.merge(newCertificate, existingCertificate);
+        }
+
+        long tag = writeToFile(newCertificate.getInputStream(), certFile);
+        return new Certificate(newCertificate.asCertificate(), tag);
+    }
+
+    @Override
+    public Certificate doInsertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge) throws IOException, BadDataException, BadNameException {
+        KeyMaterial newCertificate = reader.read(data, null);
+        KeyMaterial existingCertificate;
+        File certFile;
+        try {
+            existingCertificate = readBySpecialName(specialName);
+            certFile = resolver.getCertFileBySpecialName(specialName);
+        } catch (BadNameException e) {
+            throw new BadDataException();
+        }
+
+        if (existingCertificate != null) {
+            newCertificate = merge.merge(newCertificate, existingCertificate);
+        }
+
+        long tag = writeToFile(newCertificate.getInputStream(), certFile);
+        return new Certificate(newCertificate.asCertificate(), tag);
+    }
+
+    @Override
+    public Long getTagForFingerprint(String fingerprint) throws BadNameException, IOException {
+        File file = resolver.getCertFileByFingerprint(fingerprint);
+        return getTag(file);
+    }
+
+    @Override
+    public Long getTagForSpecialName(String specialName) throws BadNameException, IOException {
+        File file = resolver.getCertFileBySpecialName(specialName);
+        return getTag(file);
+    }
+
+    private Long getTag(File file) throws IOException {
+        if (!file.exists()) {
+            throw new NoSuchElementException();
+        }
+        Path path = file.toPath();
+        BasicFileAttributes attrs = Files.readAttributes(path, BasicFileAttributes.class);
+
+        // On UNIX file systems, for example, fileKey() will return the device ID and inode
+        int fileId = attrs.fileKey().hashCode();
+        long lastMod = attrs.lastModifiedTime().toMillis();
+
+        return lastMod + (11L * fileId);
+    }
+
+    private long writeToFile(InputStream inputStream, File certFile)
+            throws IOException {
+        certFile.getParentFile().mkdirs();
+        if (!certFile.exists() && !certFile.createNewFile()) {
+            throw new IOException("Could not create cert file " + certFile.getAbsolutePath());
+        }
+
+        FileOutputStream fileOut = new FileOutputStream(certFile);
+
+        byte[] buffer = new byte[4096];
+        int read;
+        while ((read = inputStream.read(buffer)) != -1) {
+            fileOut.write(buffer, 0, read);
+        }
+
+        inputStream.close();
+        fileOut.close();
+        return getTag(certFile);
+    }
+
+    /**
+     * Class to resolve file names from certificate fingerprints / special names.
+     */
+    public static class FilenameResolver {
+
+        private final File baseDirectory;
+        // matches v4 and v5 fingerprints (v4 = 40 hex chars, v5 = 64 hex chars)
+        private final Pattern openPgpFingerprint = Pattern.compile("^[a-f0-9]{40}([a-f0-9]{24})?$");
+
+        public FilenameResolver(File baseDirectory) {
+            this.baseDirectory = baseDirectory;
+        }
+
+        public File getBaseDirectory() {
+            return baseDirectory;
+        }
+
+        /**
+         * Calculate the file location for the certificate addressed by the given
+         * lowercase hexadecimal OpenPGP fingerprint.
+         *
+         * @param fingerprint fingerprint
+         * @return absolute certificate file location
+         *
+         * @throws BadNameException if the given fingerprint string is not a fingerprint
+         */
+        public File getCertFileByFingerprint(String fingerprint) throws BadNameException {
+            if (!isFingerprint(fingerprint)) {
+                throw new BadNameException();
+            }
+
+            // is fingerprint
+            File subdirectory = new File(getBaseDirectory(), fingerprint.substring(0, 2));
+            File file = new File(subdirectory, fingerprint.substring(2));
+            return file;
+        }
+
+        /**
+         * Calculate the file location for the certification addressed using the given special name.
+         * For known special names, see {@link SpecialNames}.
+         *
+         * @param specialName special name (e.g. "trust-root")
+         * @return absolute certificate file location
+         *
+         * @throws BadNameException in case the given special name is not known
+         */
+        public File getCertFileBySpecialName(String specialName)
+                throws BadNameException {
+            if (!isSpecialName(specialName)) {
+                throw new BadNameException(String.format("%s is not a known special name", specialName));
+            }
+
+            return new File(getBaseDirectory(), specialName);
+        }
+
+        private boolean isFingerprint(String fingerprint) {
+            return openPgpFingerprint.matcher(fingerprint).matches();
+        }
+
+        private boolean isSpecialName(String specialName) {
+            return SpecialNames.lookupSpecialName(specialName) != null;
+        }
+
+    }
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/backend/InMemoryCertificateDirectoryBackend.java

@@ -0,0 +1,170 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.backend;
+
+import pgp.cert_d.PGPCertificateDirectory;
+import pgp.cert_d.SpecialNames;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.Key;
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.certificate.KeyMaterialReaderBackend;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicLong;
+
+/**
+ * Implementation of the {@link PGPCertificateDirectory.Backend} which stores key material in-memory.
+ * It uses object locking with {@link #wait()} and {@link #notify()} to synchronize write-access.
+ */
+public class InMemoryCertificateDirectoryBackend implements PGPCertificateDirectory.Backend {
+
+    protected static class ObjectLockingMechanism implements PGPCertificateDirectory.LockingMechanism {
+
+        private boolean locked = false;
+
+        @Override
+        public synchronized void lockDirectory() throws InterruptedException {
+            if (isLocked()) {
+                wait();
+            }
+            locked = true;
+        }
+
+        @Override
+        public synchronized boolean tryLockDirectory() {
+            if (isLocked()) {
+                return false;
+            }
+            locked = true;
+            return true;
+        }
+
+        @Override
+        public synchronized boolean isLocked() {
+            return locked;
+        }
+
+        @Override
+        public synchronized void releaseDirectory() {
+            locked = false;
+            notify();
+        }
+    }
+
+
+    private final Map<String, Certificate> certificateFingerprintMap = new HashMap<>();
+    private final Map<String, KeyMaterial> keyMaterialSpecialNameMap = new HashMap<>();
+    private final PGPCertificateDirectory.LockingMechanism lock = new ObjectLockingMechanism();
+    private final KeyMaterialReaderBackend reader;
+    private final AtomicLong nonce = new AtomicLong(1);
+
+    public InMemoryCertificateDirectoryBackend(KeyMaterialReaderBackend reader) {
+        this.reader = reader;
+    }
+
+    @Override
+    public PGPCertificateDirectory.LockingMechanism getLock() {
+        return lock;
+    }
+
+    @Override
+    public Certificate readByFingerprint(String fingerprint) {
+        return certificateFingerprintMap.get(fingerprint);
+    }
+
+
+    @Override
+    public KeyMaterial readBySpecialName(String specialName) throws BadNameException {
+        if (SpecialNames.lookupSpecialName(specialName) == null) {
+            throw new BadNameException("Invalid special name " + specialName);
+        }
+        return keyMaterialSpecialNameMap.get(specialName);
+    }
+
+    @Override
+    public Iterator<Certificate> readItems() {
+        return certificateFingerprintMap.values().iterator();
+    }
+
+    @Override
+    public KeyMaterial doInsertTrustRoot(InputStream data, KeyMaterialMerger merge)
+            throws BadDataException, IOException {
+        KeyMaterial update = reader.read(data, null);
+        KeyMaterial existing = null;
+        try {
+            existing = readBySpecialName(SpecialNames.TRUST_ROOT);
+        } catch (BadNameException e) {
+            // Does not happen
+            throw new RuntimeException(e);
+        }
+        KeyMaterial merged = merge.merge(update, existing);
+        if (merged instanceof Key) {
+            merged = new Key((Key) merged, newTag());
+        } else {
+            merged = new Certificate((Certificate) merged, newTag());
+        }
+        keyMaterialSpecialNameMap.put(SpecialNames.TRUST_ROOT, merged);
+        return merged;
+    }
+
+
+    @Override
+    public Certificate doInsert(InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException {
+        KeyMaterial update = reader.read(data, null);
+        Certificate existing = readByFingerprint(update.getFingerprint());
+        Certificate merged = merge.merge(update, existing).asCertificate();
+        merged = new Certificate(merged, newTag());
+        certificateFingerprintMap.put(update.getFingerprint(), merged);
+        return merged;
+    }
+
+    @Override
+    public Certificate doInsertWithSpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, BadDataException, BadNameException {
+        KeyMaterial keyMaterial = reader.read(data, null);
+        KeyMaterial existing = readBySpecialName(specialName);
+        KeyMaterial merged = merge.merge(keyMaterial, existing);
+        if (merged instanceof Key) {
+            merged = new Key((Key) merged, newTag());
+        } else {
+            merged = new Certificate((Certificate) merged, newTag());
+        }
+        keyMaterialSpecialNameMap.put(specialName, merged);
+        return merged.asCertificate();
+    }
+
+    @Override
+    public Long getTagForFingerprint(String fingerprint) throws BadNameException, IOException {
+        Certificate certificate = certificateFingerprintMap.get(fingerprint);
+        if (certificate == null) {
+            return null;
+        }
+        return certificate.getTag();
+    }
+
+    @Override
+    public Long getTagForSpecialName(String specialName) throws BadNameException, IOException {
+        if (SpecialNames.lookupSpecialName(specialName) == null) {
+            throw new BadNameException("Invalid special name " + specialName);
+        }
+        KeyMaterial tagged = keyMaterialSpecialNameMap.get(specialName);
+        if (tagged == null) {
+            return null;
+        }
+        return tagged.getTag();
+    }
+
+    private Long newTag() {
+        return System.currentTimeMillis() + nonce.incrementAndGet();
+    }
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/backend/package-info.java

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+/**
+ * Storage Backends.
+ */
+package pgp.cert_d.backend;

pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/InMemorySubkeyLookup.java

@@ -2,7 +2,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
-package pgp.cert_d;
+package pgp.cert_d.subkey_lookup;
 
 import java.util.Collections;
 import java.util.HashMap;
@@ -11,8 +11,6 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
-import pgp.certificate_store.SubkeyLookup;
-
 public class InMemorySubkeyLookup implements SubkeyLookup {
 
     private static final Map<Long, Set<String>> subkeyMap = new HashMap<>();
@@ -30,6 +28,7 @@ public class InMemorySubkeyLookup implements SubkeyLookup {
     public void storeCertificateSubkeyIds(String certificate, List<Long> subkeyIds) {
         for (long subkeyId : subkeyIds) {
             Set<String> certificates = subkeyMap.get(subkeyId);
+            // noinspection Java8MapApi
             if (certificates == null) {
                 certificates = new HashSet<>();
                 subkeyMap.put(subkeyId, certificates);

pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/InMemorySubkeyLookupFactory.java

@@ -0,0 +1,17 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.subkey_lookup;
+
+import java.io.File;
+
+/**
+ * Factory class to instantiate {@link InMemorySubkeyLookup} objects.
+ */
+public class InMemorySubkeyLookupFactory implements SubkeyLookupFactory {
+    @Override
+    public SubkeyLookup createFileBasedInstance(File baseDirectory) {
+        return new InMemorySubkeyLookup();
+    }
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/SubkeyLookup.java

@@ -2,7 +2,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
-package pgp.certificate_store;
+package pgp.cert_d.subkey_lookup;
 
 import java.io.IOException;
 import java.util.List;

pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/SubkeyLookupFactory.java

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.subkey_lookup;
+
+import java.io.File;
+
+/**
+ * Factory class to instantiate different {@link SubkeyLookup} implementations.
+ */
+public interface SubkeyLookupFactory {
+
+    /**
+     * Create a new {@link SubkeyLookup} instance that lives in the given baseDirectory.
+     *
+     * @param baseDirectory base directory
+     * @return subkey lookup
+     */
+    SubkeyLookup createFileBasedInstance(File baseDirectory);
+}

pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/package-info.java

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+/**
+ * Subkey Lookup functionality.
+ */
+package pgp.cert_d.subkey_lookup;

pgp-cert-d-java/src/test/java/pgp/cert_d/BaseDirectoryProviderTest.java

@@ -18,7 +18,7 @@ public class BaseDirectoryProviderTest {
     public void testGetDefaultBaseDir_Linux() {
         assumeTrue(System.getProperty("os.name").equalsIgnoreCase("linux"));
         File baseDir = BaseDirectoryProvider.getDefaultBaseDirForOS("linux");
-        assertTrue(baseDir.getAbsolutePath().endsWith("/.local/share/pgp.cert.d"));
+        assertTrue(baseDir.getAbsolutePath().endsWith("pgp.cert.d"));
     }
 
     @Test

pgp-cert-d-java/src/test/java/pgp/cert_d/FileBasedPGPCertificateDirectoryTest.java

@@ -0,0 +1,131 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import org.bouncycastle.util.io.Streams;
+import org.junit.jupiter.api.Test;
+import pgp.cert_d.backend.FileBasedCertificateDirectoryBackend;
+import pgp.cert_d.dummy.TestKeyMaterialMerger;
+import pgp.cert_d.dummy.TestKeyMaterialReaderBackend;
+import pgp.cert_d.subkey_lookup.InMemorySubkeyLookup;
+import pgp.cert_d.subkey_lookup.SubkeyLookup;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.certificate.KeyMaterialReaderBackend;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+import pgp.certificate_store.exception.NotAStoreException;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.nio.file.Files;
+
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assumptions.assumeFalse;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
+
+public class FileBasedPGPCertificateDirectoryTest {
+
+    private static final KeyMaterialMerger merger = new TestKeyMaterialMerger();
+    @Test
+    public void testFileBasedCertificateDirectoryTagChangesWhenFileChanges()
+            throws IOException, NotAStoreException, BadDataException, InterruptedException, BadNameException {
+        File tempDir = Files.createTempDirectory("file-based-changes").toFile();
+        tempDir.deleteOnExit();
+        PGPCertificateDirectory directory = PGPCertificateDirectories.fileBasedCertificateDirectory(
+                new TestKeyMaterialReaderBackend(),
+                tempDir,
+                new InMemorySubkeyLookup());
+        FileBasedCertificateDirectoryBackend.FilenameResolver resolver =
+                new FileBasedCertificateDirectoryBackend.FilenameResolver(tempDir);
+
+        // Insert certificate
+        Certificate certificate = directory.insert(TestKeys.getCedricCert(), merger);
+        Long tag = certificate.getTag();
+        assertNotNull(tag);
+        assertNull(directory.getByFingerprintIfChanged(certificate.getFingerprint(), tag));
+
+        Long oldTag = tag;
+
+        Thread.sleep(10);
+        // Change the file on disk directly, this invalidates the tag due to changed modification date
+        File certFile = resolver.getCertFileByFingerprint(certificate.getFingerprint());
+        FileOutputStream fileOut = new FileOutputStream(certFile);
+        Streams.pipeAll(certificate.getInputStream(), fileOut);
+        fileOut.write("\n".getBytes());
+        fileOut.close();
+
+        // Old invalidated tag indicates a change, so the modified certificate is returned
+        certificate = directory.getByFingerprintIfChanged(certificate.getFingerprint(), oldTag);
+        assertNotNull(certificate);
+
+        // new tag is valid
+        tag = certificate.getTag();
+        assertNotEquals(oldTag, tag);
+        assertNull(directory.getByFingerprintIfChanged(certificate.getFingerprint(), tag));
+    }
+
+    @Test
+    public void fileBasedStoreInWriteProtectedAreaThrows() {
+        File root = new File("/");
+        assumeTrue(root.exists(), "This test only runs on unix-like systems");
+        File baseDirectory = new File(root, "pgp.cert.d");
+        assumeFalse(baseDirectory.mkdirs(), "This test assumes that we cannot create dirs in /");
+
+        KeyMaterialReaderBackend reader = new TestKeyMaterialReaderBackend();
+        SubkeyLookup lookup = new InMemorySubkeyLookup();
+        assertThrows(NotAStoreException.class, () -> PGPCertificateDirectories.fileBasedCertificateDirectory(
+                reader, baseDirectory, lookup));
+    }
+
+    @Test
+    public void fileBasedStoreOnFileThrows()
+            throws IOException {
+        File tempDir = Files.createTempDirectory("containsAFile").toFile();
+        tempDir.deleteOnExit();
+        File baseDir = new File(tempDir, "pgp.cert.d");
+        baseDir.createNewFile(); // this is a file, not a dir
+
+        KeyMaterialReaderBackend reader = new TestKeyMaterialReaderBackend();
+        SubkeyLookup lookup = new InMemorySubkeyLookup();
+        assertThrows(NotAStoreException.class, () -> PGPCertificateDirectories.fileBasedCertificateDirectory(
+                reader, baseDir, lookup));
+    }
+
+    @Test
+    public void testCertificateStoredUnderWrongFingerprintThrowsBadData()
+            throws IOException, NotAStoreException, BadDataException, InterruptedException, BadNameException {
+        File tempDir = Files.createTempDirectory("wrong-fingerprint").toFile();
+        tempDir.deleteOnExit();
+        PGPCertificateDirectory directory = PGPCertificateDirectories.fileBasedCertificateDirectory(
+                new TestKeyMaterialReaderBackend(),
+                tempDir,
+                new InMemorySubkeyLookup());
+        FileBasedCertificateDirectoryBackend.FilenameResolver resolver =
+                new FileBasedCertificateDirectoryBackend.FilenameResolver(tempDir);
+
+        // Insert Rons certificate
+        directory.insert(TestKeys.getRonCert(), merger);
+
+        // Copy Rons cert to Cedrics cert file
+        File ronCert = resolver.getCertFileByFingerprint(TestKeys.RON_FP);
+        FileInputStream inputStream = new FileInputStream(ronCert);
+        File cedricCert = resolver.getCertFileByFingerprint(TestKeys.CEDRIC_FP);
+        cedricCert.getParentFile().mkdirs();
+        cedricCert.createNewFile();
+        FileOutputStream outputStream = new FileOutputStream(cedricCert);
+        Streams.pipeAll(inputStream, outputStream);
+        inputStream.close();
+        outputStream.close();
+
+        // Reading cedrics cert will fail, as it has Rons fingerprint
+        assertThrows(BadDataException.class, () -> directory.getByFingerprint(TestKeys.CEDRIC_FP));
+    }
+}

pgp-cert-d-java/src/test/java/pgp/cert_d/FilenameResolverTest.java

@@ -6,6 +6,7 @@ package pgp.cert_d;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import pgp.cert_d.backend.FileBasedCertificateDirectoryBackend;
 import pgp.certificate_store.exception.BadNameException;
 
 import java.io.File;
@@ -18,13 +19,13 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 public class FilenameResolverTest {
 
     private File baseDir;
-    private FilenameResolver resolver;
+    private FileBasedCertificateDirectoryBackend.FilenameResolver resolver;
 
     @BeforeEach
     public void setup() throws IOException {
         baseDir = Files.createTempDirectory("filenameresolver").toFile();
         baseDir.deleteOnExit();
-        resolver = new FilenameResolver(baseDir);
+        resolver = new FileBasedCertificateDirectoryBackend.FilenameResolver(baseDir);
     }
 
     @Test

pgp-cert-d-java/src/test/java/pgp/cert_d/PGPCertificateDirectoryTest.java

@@ -0,0 +1,311 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import org.bouncycastle.util.io.Streams;
+import org.junit.jupiter.api.Named;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+import pgp.cert_d.dummy.TestKeyMaterialMerger;
+import pgp.cert_d.dummy.TestKeyMaterialReaderBackend;
+import pgp.cert_d.subkey_lookup.InMemorySubkeyLookup;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.Key;
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+import pgp.certificate_store.exception.NotAStoreException;
+
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
+import java.nio.charset.Charset;
+import java.nio.file.Files;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Set;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static pgp.cert_d.TestKeys.CEDRIC_FP;
+import static pgp.cert_d.TestKeys.HARRY_FP;
+import static pgp.cert_d.TestKeys.RON_FP;
+
+public class PGPCertificateDirectoryTest {
+
+
+    private static final KeyMaterialMerger merger = new TestKeyMaterialMerger();
+
+    private static Stream<Arguments> provideTestSubjects()
+            throws IOException, NotAStoreException {
+        PGPCertificateDirectory inMemory = PGPCertificateDirectories.inMemoryCertificateDirectory(
+                new TestKeyMaterialReaderBackend());
+
+        File tempDir = Files.createTempDirectory("pgp-cert-d-test").toFile();
+        tempDir.deleteOnExit();
+        PGPCertificateDirectory fileBased = PGPCertificateDirectories.fileBasedCertificateDirectory(
+                new TestKeyMaterialReaderBackend(),
+                tempDir,
+                new InMemorySubkeyLookup());
+
+        return Stream.of(
+                Arguments.of(Named.of("InMemoryCertificateDirectory", inMemory)),
+                Arguments.of(Named.of("FileBasedCertificateDirectory", fileBased)));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentCertByFingerprintThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getByFingerprint("0000000000000000000000000000000000000000"));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentCertByFingerprintIfChangedThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getByFingerprintIfChanged("0000000000000000000000000000000000000000", 12));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentCertBySpecialNameThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getBySpecialName(SpecialNames.TRUST_ROOT));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentCertBySpecialNameIfChangedThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getBySpecialNameIfChanged(SpecialNames.TRUST_ROOT, 12));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentTrustRootThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getTrustRoot());
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentTrustRootIfChangedThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getTrustRootCertificateIfChanged(12));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getNonExistentTrustRootCertificateThrowsNoSuchElementException(PGPCertificateDirectory directory) {
+        assertThrows(NoSuchElementException.class, () ->
+                directory.getTrustRootCertificate());
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void lockDirectoryAndTryInsertWillFail(PGPCertificateDirectory directory)
+            throws IOException, InterruptedException, BadDataException {
+        // Manually lock the dir
+        assertFalse(directory.backend.getLock().isLocked());
+        directory.backend.getLock().lockDirectory();
+        assertTrue(directory.backend.getLock().isLocked());
+        assertFalse(directory.backend.getLock().tryLockDirectory());
+
+        Certificate inserted = directory.tryInsert(TestKeys.getCedricCert(), merger);
+        assertNull(inserted);
+
+        directory.backend.getLock().releaseDirectory();
+        inserted = directory.tryInsert(TestKeys.getCedricCert(), merger);
+        assertNotNull(inserted);
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void lockDirectoryAndTryInsertTrustRootWillFail(PGPCertificateDirectory directory)
+            throws IOException, InterruptedException, BadDataException {
+        // Manually lock the dir
+        assertFalse(directory.backend.getLock().isLocked());
+        directory.backend.getLock().lockDirectory();
+        assertTrue(directory.backend.getLock().isLocked());
+
+        KeyMaterial inserted = directory.tryInsertTrustRoot(TestKeys.getHarryKey(), merger);
+        assertNull(inserted);
+
+        directory.backend.getLock().releaseDirectory();
+        inserted = directory.tryInsertTrustRoot(TestKeys.getHarryKey(), merger);
+        assertNotNull(inserted);
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void lockDirectoryAndTryInsertWithSpecialNameWillFail(PGPCertificateDirectory directory)
+            throws IOException, InterruptedException, BadDataException, BadNameException {
+        // Manually lock the dir
+        assertFalse(directory.backend.getLock().isLocked());
+        directory.backend.getLock().lockDirectory();
+        assertTrue(directory.backend.getLock().isLocked());
+
+        Certificate inserted = directory.tryInsertWithSpecialName(SpecialNames.TRUST_ROOT, TestKeys.getHarryKey(), merger);
+        assertNull(inserted);
+
+        directory.backend.getLock().releaseDirectory();
+        inserted = directory.tryInsertWithSpecialName(SpecialNames.TRUST_ROOT, TestKeys.getHarryKey(), merger);
+        assertNotNull(inserted);
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void getByInvalidNameFails(PGPCertificateDirectory directory) {
+        assertThrows(BadNameException.class, () -> directory.getBySpecialName("invalid"));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testInsertAndGetSingleCert(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        assertThrows(NoSuchElementException.class, () -> directory.getByFingerprint(CEDRIC_FP), "Empty directory MUST NOT contain certificate");
+
+        Certificate certificate = directory.insert(TestKeys.getCedricCert(), merger);
+        assertEquals(CEDRIC_FP, certificate.getFingerprint(), "Fingerprint of inserted cert MUST match");
+
+        Certificate get = directory.getByFingerprint(CEDRIC_FP);
+        assertEquals(CEDRIC_FP, get.getFingerprint(), "Fingerprint of retrieved cert MUST match");
+
+        byte[] expected = TestKeys.CEDRIC_CERT.getBytes(Charset.forName("UTF8"));
+        ByteArrayOutputStream actual = new ByteArrayOutputStream();
+        Streams.pipeAll(get.getInputStream(), actual);
+        assertArrayEquals(expected, actual.toByteArray(), "InputStream of cert MUST match what we gave in");
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testInsertAndGetTrustRootAndCert(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException {
+        assertThrows(NoSuchElementException.class, () -> directory.getTrustRoot());
+
+        KeyMaterial trustRootMaterial = directory.insertTrustRoot(
+                TestKeys.getHarryKey(), merger);
+        assertNotNull(trustRootMaterial);
+        assertTrue(trustRootMaterial instanceof Key);
+        assertEquals(HARRY_FP, trustRootMaterial.getFingerprint());
+
+        Key trustRoot = (Key) directory.getTrustRoot();
+        assertEquals(HARRY_FP, trustRoot.getFingerprint());
+        Certificate trustRootCert = directory.getTrustRootCertificate();
+        assertEquals(HARRY_FP, trustRootCert.getFingerprint());
+
+        directory.tryInsert(TestKeys.getRonCert(), merger);
+        directory.insert(TestKeys.getCedricCert(), merger);
+
+        Set<String> expected = new HashSet<>(Arrays.asList(RON_FP, CEDRIC_FP));
+
+        Set<String> actual = new HashSet<>();
+        Iterator<String> fingerprints = directory.fingerprints();
+        actual.add(fingerprints.next());
+        actual.add(fingerprints.next());
+        assertFalse(fingerprints.hasNext());
+
+        assertEquals(expected, actual);
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testGetTrustRootIfChanged(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException {
+        KeyMaterial trustRootMaterial = directory.insertTrustRoot(
+                TestKeys.getHarryKey(), merger);
+
+        assertNotNull(trustRootMaterial.getTag());
+        Long tag = trustRootMaterial.getTag();
+        assertNull(directory.getTrustRootCertificateIfChanged(tag));
+        assertNotNull(directory.getTrustRootCertificateIfChanged(tag + 1));
+
+        Long oldTag = tag;
+        Thread.sleep(10);
+        // "update" key
+        trustRootMaterial = directory.insertTrustRoot(
+                TestKeys.getHarryKey(), merger);
+        tag = trustRootMaterial.getTag();
+
+        assertNotEquals(oldTag, tag);
+        assertNotNull(directory.getTrustRootCertificateIfChanged(oldTag));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testGetBySpecialNameIfChanged(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        KeyMaterial specialName = directory.insertWithSpecialName(SpecialNames.TRUST_ROOT,
+                TestKeys.getHarryKey(), merger);
+
+        assertNotNull(specialName.getTag());
+        Long tag = specialName.getTag();
+        assertNull(directory.getBySpecialNameIfChanged(SpecialNames.TRUST_ROOT, tag));
+        assertNotNull(directory.getBySpecialNameIfChanged(SpecialNames.TRUST_ROOT, tag + 1));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testGetByFingerprintIfChanged(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        Certificate certificate = directory.insert(TestKeys.getCedricCert(), merger);
+        Long tag = certificate.getTag();
+        assertNotNull(tag);
+
+        assertNull(directory.getByFingerprintIfChanged(certificate.getFingerprint(), tag));
+        assertNotNull(directory.getByFingerprintIfChanged(certificate.getFingerprint(), tag + 1));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testOverwriteTrustRoot(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException {
+        directory.insertTrustRoot(TestKeys.getHarryKey(), merger);
+        assertEquals(HARRY_FP, directory.getTrustRoot().getFingerprint());
+        assertTrue(directory.getTrustRoot() instanceof Key);
+
+        directory.insertTrustRoot(TestKeys.getCedricCert(), merger);
+        assertEquals(CEDRIC_FP, directory.getTrustRoot().getFingerprint());
+        assertTrue(directory.getTrustRoot() instanceof Certificate);
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testOverwriteSpecialName(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        directory.insertWithSpecialName(SpecialNames.TRUST_ROOT, TestKeys.getRonCert(), merger);
+        KeyMaterial bySpecialName = directory.getBySpecialName(SpecialNames.TRUST_ROOT);
+        assertEquals(RON_FP, bySpecialName.getFingerprint());
+
+        directory.insertWithSpecialName(SpecialNames.TRUST_ROOT, TestKeys.getHarryKey(), merger);
+        assertEquals(HARRY_FP, directory.getBySpecialName(SpecialNames.TRUST_ROOT).getFingerprint());
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestSubjects")
+    public void testOverwriteByFingerprint(PGPCertificateDirectory directory)
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        directory.insert(TestKeys.getRonCert(), merger);
+        Certificate extracted = directory.getByFingerprint(RON_FP);
+        assertEquals(RON_FP, extracted.getFingerprint());
+
+        directory.insert(TestKeys.getRonCert(), merger);
+        extracted = directory.getByFingerprint(RON_FP);
+        assertEquals(RON_FP, extracted.getFingerprint());
+    }
+
+}

pgp-cert-d-java/src/test/java/pgp/cert_d/PGPCertificateStoreAdapterTest.java

@@ -0,0 +1,125 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import pgp.cert_d.backend.InMemoryCertificateDirectoryBackend;
+import pgp.cert_d.dummy.TestKeyMaterialMerger;
+import pgp.cert_d.dummy.TestKeyMaterialReaderBackend;
+import pgp.cert_d.subkey_lookup.InMemorySubkeyLookupFactory;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class PGPCertificateStoreAdapterTest {
+
+    private PGPCertificateDirectory directory;
+    private PGPCertificateStoreAdapter adapter;
+
+    private static final TestKeyMaterialMerger merger = new TestKeyMaterialMerger();
+
+    @BeforeEach
+    public void setup() {
+        directory = new PGPCertificateDirectory(
+                new InMemoryCertificateDirectoryBackend(new TestKeyMaterialReaderBackend()),
+                new InMemorySubkeyLookupFactory().createFileBasedInstance(null));
+        adapter = new PGPCertificateStoreAdapter(directory);
+    }
+
+    @Test
+    public void testBadFPWithInvalidCharsYieldsBadNameException() {
+        assertThrows(BadNameException.class, () -> adapter.getCertificate("XYZ78fd17f207fdf62f7976c4e9d98917ad84522"));
+    }
+
+    @Test
+    public void testBadFPWithTooFewCharsYieldsBadNameException() {
+        assertThrows(BadNameException.class, () -> adapter.getCertificate("23578fd17f207fdf62f7976c4e9d98917ad"));
+    }
+
+    @Test
+    public void testInsertGetCertificate()
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        assertThrows(NoSuchElementException.class, () -> adapter.getCertificate(TestKeys.CEDRIC_FP));
+        assertFalse(adapter.getCertificates().hasNext());
+
+        Certificate certificate = adapter.insertCertificate(TestKeys.getCedricCert(), merger);
+        assertNotNull(certificate);
+        assertEquals(TestKeys.CEDRIC_FP, certificate.getFingerprint());
+
+        certificate = adapter.getCertificate(TestKeys.CEDRIC_FP.toUpperCase());
+        assertEquals(TestKeys.CEDRIC_FP, certificate.getFingerprint(), "We can also fetch with uppercase fps");
+
+        Iterator<String> fingerprints = adapter.getFingerprints();
+        assertEquals(TestKeys.CEDRIC_FP, fingerprints.next());
+        assertFalse(fingerprints.hasNext());
+    }
+
+    @Test
+    public void testInsertGetTrustRoot()
+            throws BadDataException, BadNameException, IOException, InterruptedException {
+        assertThrows(NoSuchElementException.class, () -> adapter.getCertificate(SpecialNames.TRUST_ROOT));
+
+        Certificate certificate = adapter.insertCertificateBySpecialName(
+                SpecialNames.TRUST_ROOT, TestKeys.getHarryKey(), merger);
+        assertNotNull(certificate);
+        assertEquals(TestKeys.HARRY_FP, certificate.getFingerprint());
+
+        assertFalse(adapter.getCertificates().hasNext(), "Special-named certs are not returned by getCertificates()");
+        assertFalse(adapter.getFingerprints().hasNext());
+    }
+
+    @Test
+    public void testGetCertificateIfChanged()
+            throws BadDataException, IOException, InterruptedException, BadNameException {
+        Certificate certificate = adapter.insertCertificate(TestKeys.getRonCert(), merger);
+        Long tag = certificate.getTag();
+
+        assertNull(adapter.getCertificateIfChanged(TestKeys.RON_FP, tag), "Cert has not changed, tag is still valid");
+        assertNotNull(adapter.getCertificateIfChanged(TestKeys.RON_FP, tag + 1));
+    }
+
+    @Test
+    public void testGetTrustRootIfChanged()
+            throws BadDataException, BadNameException, IOException, InterruptedException {
+        Certificate certificate = adapter.insertCertificateBySpecialName(SpecialNames.TRUST_ROOT, TestKeys.getHarryKey(), merger);
+        Long tag = certificate.getTag();
+
+        assertNull(adapter.getCertificateIfChanged(SpecialNames.TRUST_ROOT, tag));
+        assertNotNull(adapter.getCertificateIfChanged(SpecialNames.TRUST_ROOT, tag * 2));
+    }
+
+    @Test
+    public void testGetCertificateBySubkeyId()
+            throws BadDataException, IOException, InterruptedException {
+        // Insert some certs
+        adapter.insertCertificate(TestKeys.getCedricCert(), merger);
+        adapter.insertCertificate(TestKeys.getHarryKey(), merger);
+        // Now insert Ron
+        Certificate certificate = adapter.insertCertificate(TestKeys.getRonCert(), merger);
+        List<Long> subkeyIds = certificate.getSubkeyIds();
+
+        assertFalse(adapter.getCertificatesBySubkeyId(0).hasNext());
+
+        for (Long subkeyId : subkeyIds) {
+            Iterator<Certificate> certsWithSubkey = adapter.getCertificatesBySubkeyId(subkeyId);
+            Certificate certWithSubkey = certsWithSubkey.next();
+            assertFalse(certsWithSubkey.hasNext());
+
+            assertEquals(TestKeys.RON_FP, certWithSubkey.getFingerprint());
+        }
+    }
+}

pgp-cert-d-java/src/test/java/pgp/cert_d/SubkeyLookupTest.java

@@ -24,7 +24,8 @@ import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 import pgp.cert_d.jdbc.sqlite.DatabaseSubkeyLookup;
 import pgp.cert_d.jdbc.sqlite.SqliteSubkeyLookupDaoImpl;
-import pgp.certificate_store.SubkeyLookup;
+import pgp.cert_d.subkey_lookup.InMemorySubkeyLookup;
+import pgp.cert_d.subkey_lookup.SubkeyLookup;
 
 public class SubkeyLookupTest {
 

pgp-cert-d-java/src/test/java/pgp/cert_d/TestKeys.java

@@ -0,0 +1,130 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d;
+
+import java.io.ByteArrayInputStream;
+import java.io.InputStream;
+import java.nio.charset.Charset;
+
+public class TestKeys {
+    @SuppressWarnings("CharsetObjectCanBeUsed")
+    private static final Charset UTF8 = Charset.forName("UTF8");
+
+    public static final String HARRY_KEY = "-----BEGIN PGP PRIVATE KEY BLOCK-----\n" +
+            "Comment: 2357 8FD1 7F20 7FDF 62F7  976C 4E9D 9891 7AD8 4522\n" +
+            "Comment: Harry Potter <harry@potter.more>\n" +
+            "\n" +
+            "xVgEYwTP0hYJKwYBBAHaRw8BAQdAPVcWeaMiUVG+vECWpoytSoF3wNJQG/JsnCbj\n" +
+            "uQtv0REAAP0cS3GCmrIMO/FqNm1FG1mKw4P+mvZ1JBFILN7Laooq7A/QwsARBB8W\n" +
+            "CgCDBYJjBM/SBYkFn6YAAwsJBwkQTp2YkXrYRSJHFAAAAAAAHgAgc2FsdEBub3Rh\n" +
+            "dGlvbnMuc2VxdW9pYS1wZ3Aub3JnRSvJhQu9P/3bpFqFdB2c5Mfg9JIdyic1tsAt\n" +
+            "lZ7o4k4DFQoIApsBAh4BFiEEI1eP0X8gf99i95dsTp2YkXrYRSIAAK2cAP9juDnY\n" +
+            "qB6XuXVx76MzDlFemqJ/r2TIlN22O33ITp23cQEAiMk/rULVdfmlFi3QBvXgtPI2\n" +
+            "QQYFI0UnyGLmJSa1cwzNIEhhcnJ5IFBvdHRlciA8aGFycnlAcG90dGVyLm1vcmU+\n" +
+            "wsAUBBMWCgCGBYJjBM/SBYkFn6YAAwsJBwkQTp2YkXrYRSJHFAAAAAAAHgAgc2Fs\n" +
+            "dEBub3RhdGlvbnMuc2VxdW9pYS1wZ3Aub3Jn0o9na1p+a9kY3y3+xUSFFnxbuxNM\n" +
+            "5zvth0SAfJIH2C8DFQoIApkBApsBAh4BFiEEI1eP0X8gf99i95dsTp2YkXrYRSIA\n" +
+            "AC1zAP0e2qRXH4zCnjvdYwGP0tIY3dwBsm1bvk+wVFHm8h68iwEAh2uyyQ+O5iQH\n" +
+            "7NN/lV5dUKKsKaimj/vVGpSW3NtFZQDHWARjBM/SFgkrBgEEAdpHDwEBB0BUqcZu\n" +
+            "VsEO6fmW8q3S5ll9WohcTOWRX7Spg5wS3DIqPgABALzJ9ZImb4U94WqRtftSSaeF\n" +
+            "0w6rHCn2DiTT8pxjefGQEW7CwMUEGBYKATcFgmMEz9IFiQWfpgAJEE6dmJF62EUi\n" +
+            "RxQAAAAAAB4AIHNhbHRAbm90YXRpb25zLnNlcXVvaWEtcGdwLm9yZ+HPX0u5kyKR\n" +
+            "5IwErbomgGKVCGuvR6oSKc7CDQYMJS9eApsCvqAEGRYKAG8FgmMEz9IJEKk0hrvR\n" +
+            "6Jc7RxQAAAAAAB4AIHNhbHRAbm90YXRpb25zLnNlcXVvaWEtcGdwLm9yZ8Chba26\n" +
+            "1nQ6ZEZ/rVH8wMhYznGNa/Ux28sodM04wU6dFiEEli7ijJ6quX9gSoSbqTSGu9Ho\n" +
+            "lzsAAG1wAQDVvKVWaMOBELROkF72oBH58X6lrOmr08W5FJQxehywhQEAwetpgL1V\n" +
+            "DNj4qcvuCJJ2agAM1tA22WMPpQQeA5CCgwcWIQQjV4/RfyB/32L3l2xOnZiRethF\n" +
+            "IgAAsWEA/RfOKexMYEtzlpM71MB9SL+emHXf+w1TNAvBxrifU8bMAPoDmWHkWjZQ\n" +
+            "N6upbHKssRywPLKCMPLnFYtBNxDrMYr0BMddBGMEz9ISCisGAQQBl1UBBQEBB0CR\n" +
+            "p5dCIlSpV/EvXX2+YZnZSRtc8eTFXkph8RArNi0QPAMBCAcAAP9seqRo6mbmvS4h\n" +
+            "fkxmV5zap3wIemzW4iabNU2VbWJbEBALwsAGBBgWCgB4BYJjBM/SBYkFn6YACRBO\n" +
+            "nZiRethFIkcUAAAAAAAeACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmdx\n" +
+            "uRLJ/h81azzvGn5zgJ+jdfkdM6iO+f1CLgfnHUH9ugKbDBYhBCNXj9F/IH/fYveX\n" +
+            "bE6dmJF62EUiAACObgEAk4whKEo2nzpWht65tpFjrEXdakj00mA/P612P2CUdPQB\n" +
+            "ANNn+VUiu9rtnLcP4NlaUVOwsgN7yyed0orbmG1VvSMF\n" +
+            "=cBAn\n" +
+            "-----END PGP PRIVATE KEY BLOCK-----\n";
+    public static final String HARRY_FP = "23578fd17f207fdf62f7976c4e9d98917ad84522";
+
+    public static final String RON_CERT = "-----BEGIN PGP PUBLIC KEY BLOCK-----\n" +
+            "Comment: B798 AF18 6BFE 4C19 902D  4950 5647 F001 37EF 4C41\n" +
+            "Comment: Ron Weasley <ron@weasley.burrow>\n" +
+            "\n" +
+            "xjMEYwTRXBYJKwYBBAHaRw8BAQdAPHyiu4nwvo3OY3wLG1tUmS6qeTeT1zd3BrL+\n" +
+            "6/5Ys3jCwBEEHxYKAIMFgmME0VwFiQWfpgADCwkHCRBWR/ABN+9MQUcUAAAAAAAe\n" +
+            "ACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmfEPNi/1ObPMwDwS094Lcyq\n" +
+            "dRNRk2FRzvhoXKrqF/GHfQMVCggCmwECHgEWIQS3mK8Ya/5MGZAtSVBWR/ABN+9M\n" +
+            "QQAAR/oBAJWxxUJqOAzYG4uAd6SSF55LZVl00t3bGhgEyGmrB/ppAQCZTpWu0rwU\n" +
+            "GVv/MoeqRwX+P8sHS4FSu/hSYJpbNwysCM0gUm9uIFdlYXNsZXkgPHJvbkB3ZWFz\n" +
+            "bGV5LmJ1cnJvdz7CwBQEExYKAIYFgmME0VwFiQWfpgADCwkHCRBWR/ABN+9MQUcU\n" +
+            "AAAAAAAeACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmf43PjsP9w1eGYP\n" +
+            "CLm6O+K27EQPiCf2cW71QnQ0RunupgMVCggCmQECmwECHgEWIQS3mK8Ya/5MGZAt\n" +
+            "SVBWR/ABN+9MQQAA7rYA/3U2aaw5PFa9L90PbxygOwFrgIVWLiOpnKfjqDJqEgva\n" +
+            "AQDxTIbpUYEAYmTpmAm1tiQSlpp9P96vqCMIj2OqtYCNAs4zBGME0VwWCSsGAQQB\n" +
+            "2kcPAQEHQGzhRPzKRkkce0v1NjuTV2stn8CEMVgnUxsMPtd0h2M9wsDFBBgWCgE3\n" +
+            "BYJjBNFcBYkFn6YACRBWR/ABN+9MQUcUAAAAAAAeACBzYWx0QG5vdGF0aW9ucy5z\n" +
+            "ZXF1b2lhLXBncC5vcmd6UNkzsh0jKRPQAKX2PoUhMN4QfhTK9IC6L+QbyL1rFgKb\n" +
+            "Ar6gBBkWCgBvBYJjBNFcCRCuGMJD3GUsUUcUAAAAAAAeACBzYWx0QG5vdGF0aW9u\n" +
+            "cy5zZXF1b2lhLXBncC5vcmcUTns9+sw7XKKO5ZOYQninRAchypKHbqV2LinV46Hi\n" +
+            "bxYhBI+SjTgn0fulukOYj64YwkPcZSxRAADZtAEApse3UJi1iuSFvnyXxuYIOm4d\n" +
+            "0sOaOtd18venqfWGyX4BALf7T7LknMY688vaW6/xkw2fonG6Y5VxreIHlMZAcX0H\n" +
+            "FiEEt5ivGGv+TBmQLUlQVkfwATfvTEEAAFQ3AQCGSLEt8wgJZXlljPdk1eQ3uvW3\n" +
+            "VHryNAc3/vbSOvByFAD/WKXY8Pqki2r9XVUW33Q88firoiKVuGmBxklEG3ACjALO\n" +
+            "OARjBNFcEgorBgEEAZdVAQUBAQdARnMlx3ST0EHPiErN7lOF+lhtJ8FmW9arc46u\n" +
+            "sHFMgUMDAQgHwsAGBBgWCgB4BYJjBNFcBYkFn6YACRBWR/ABN+9MQUcUAAAAAAAe\n" +
+            "ACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmfv1PKQX1GMihAdj3ftW/yS\n" +
+            "bnPYdE+0h5rGCuhYl7sjaQKbDBYhBLeYrxhr/kwZkC1JUFZH8AE370xBAABWugEA\n" +
+            "rWOEHQjzoQkxxsErVEVZjqr05SLMmo6+HMJ/4Sgur10A/0+4FSbaKKNGiCnCMRsZ\n" +
+            "BEswoD99mUaBXl1nPH+Hg38O\n" +
+            "=+pb5\n" +
+            "-----END PGP PUBLIC KEY BLOCK-----\n";
+    public static final String RON_FP = "b798af186bfe4c19902d49505647f00137ef4c41";
+
+    public static final String CEDRIC_CERT = "-----BEGIN PGP PUBLIC KEY BLOCK-----\n" +
+            "Comment: 5E75 BF20 646B C1A9 8D3B  1BC2 FE9C D472 987C 4021\n" +
+            "Comment: Cedric Diggory <cedric@diggo.ry>\n" +
+            "\n" +
+            "xjMEYwTIyhYJKwYBBAHaRw8BAQdA80cyaoAEfh/ENuHw8XtWqrxDoPQ/x44LQzyO\n" +
+            "TLhMN+PCwBEEHxYKAIMFgmMEyMoFiQWfpgADCwkHCRD+nNRymHxAIUcUAAAAAAAe\n" +
+            "ACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmf0ckQJQzwOKkXPe8rFP5B+\n" +
+            "CbAshRG5OKD3Dp+hScGFXgMVCggCmwECHgEWIQRedb8gZGvBqY07G8L+nNRymHxA\n" +
+            "IQAA9WYBAP5rQCq/W3KV90T/wpxf5pcXoCB4tCC9Gi/1AiuGhQdAAP48PIX9fH+T\n" +
+            "g7N+tU0xzzCc2nWxG3cIuvGFsg94pKL8As0gQ2VkcmljIERpZ2dvcnkgPGNlZHJp\n" +
+            "Y0BkaWdnby5yeT7CwBQEExYKAIYFgmMEyMoFiQWfpgADCwkHCRD+nNRymHxAIUcU\n" +
+            "AAAAAAAeACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmdkUL5mF5SwIXja\n" +
+            "bCxhI3lvqiUURSoLY13K6YvHYLz7bwMVCggCmQECmwECHgEWIQRedb8gZGvBqY07\n" +
+            "G8L+nNRymHxAIQAA6SwA/jiM8k/Z0ljnHdFxsdoLhdnTZ0yJT/7RxreSZ3aITrDs\n" +
+            "AP9V8bAYy4hK0C7i4FmNcos3HQs2Si6ee2/EZjo8LqxeCc4zBGMEyMoWCSsGAQQB\n" +
+            "2kcPAQEHQIu0hKMngTnmIPXlZ/p9WOZmLB0s9v9yZJLdZ5ICKn7jwsDFBBgWCgE3\n" +
+            "BYJjBMjKBYkFn6YACRD+nNRymHxAIUcUAAAAAAAeACBzYWx0QG5vdGF0aW9ucy5z\n" +
+            "ZXF1b2lhLXBncC5vcmdCT1SyOVJwTPp4OEDWFNEgxKD12H+Dya9EzOMJ3I9frwKb\n" +
+            "Ar6gBBkWCgBvBYJjBMjKCRDNPli8d9EIkUcUAAAAAAAeACBzYWx0QG5vdGF0aW9u\n" +
+            "cy5zZXF1b2lhLXBncC5vcmccLTSNIhZOiNFaTj76iAutuAkUCImFp5ptMICZRo7E\n" +
+            "TRYhBESzEAYRbxRfM3ub5c0+WLx30QiRAAAZtwD/WRJrSxzJRsnZs4w+QgZjqOZx\n" +
+            "bOGwGObfbEHaExG0cKEA/R+BFODg5oPOvK9W7n0Kt9O171Po+zXB0UDmBiEhh0YL\n" +
+            "FiEEXnW/IGRrwamNOxvC/pzUcph8QCEAAEneAQDnOv/cf1/qmjfLnorEi+Z4gRWQ\n" +
+            "fp3Rp/gI4SLUQxT0PQD/USZIP0bNMGGC1TRQa+8nK6opSqtIvsatt0tQuu178A7O\n" +
+            "OARjBMjKEgorBgEEAZdVAQUBAQdAazcEUsYtY9f9o4A+ePR7ACMIDScVEUWS83+I\n" +
+            "SwJQz3QDAQgHwsAGBBgWCgB4BYJjBMjKBYkFn6YACRD+nNRymHxAIUcUAAAAAAAe\n" +
+            "ACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmc/qxMatwD+6zaKDZGlVdn/\n" +
+            "TWumSgLtuyYonaOupIfMEAKbDBYhBF51vyBka8GpjTsbwv6c1HKYfEAhAADPiwEA\n" +
+            "vQ7fTnAHcdZlMVnNPkc0pZSp1+kO5Z789I5Pp4HloNIBAMoC84ja83PjvcpIyxgR\n" +
+            "kspLC9BliezVbFSHIK9NQ/wC\n" +
+            "=VemI\n" +
+            "-----END PGP PUBLIC KEY BLOCK-----\n";
+    public static final String CEDRIC_FP = "5e75bf20646bc1a98d3b1bc2fe9cd472987c4021";
+
+    public static InputStream getHarryKey() {
+        return new ByteArrayInputStream(HARRY_KEY.getBytes(UTF8));
+    }
+
+    public static InputStream getRonCert() {
+        return new ByteArrayInputStream(RON_CERT.getBytes(UTF8));
+    }
+
+    public static InputStream getCedricCert() {
+        return new ByteArrayInputStream(CEDRIC_CERT.getBytes(UTF8));
+    }
+}

pgp-cert-d-java/src/test/java/pgp/cert_d/dummy/TestKeyMaterialMerger.java

@@ -0,0 +1,17 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.dummy;
+
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+
+import java.io.IOException;
+
+public class TestKeyMaterialMerger implements KeyMaterialMerger {
+    @Override
+    public KeyMaterial merge(KeyMaterial data, KeyMaterial existing) throws IOException {
+        return data;
+    }
+}

pgp-cert-d-java/src/test/java/pgp/cert_d/dummy/TestKeyMaterialReaderBackend.java

@@ -0,0 +1,95 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.cert_d.dummy;
+
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.openpgp.PGPKeyRing;
+import org.bouncycastle.openpgp.PGPPublicKey;
+import org.bouncycastle.openpgp.PGPPublicKeyRing;
+import org.bouncycastle.openpgp.PGPSecretKeyRing;
+import org.bouncycastle.openpgp.PGPUtil;
+import org.bouncycastle.openpgp.operator.KeyFingerPrintCalculator;
+import org.bouncycastle.openpgp.operator.bc.BcKeyFingerprintCalculator;
+import org.bouncycastle.util.encoders.Hex;
+import org.bouncycastle.util.io.Streams;
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.Key;
+import pgp.certificate_store.certificate.KeyMaterial;
+import pgp.certificate_store.certificate.KeyMaterialReaderBackend;
+import pgp.certificate_store.exception.BadDataException;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+public class TestKeyMaterialReaderBackend implements KeyMaterialReaderBackend {
+
+    KeyFingerPrintCalculator fpCalc = new BcKeyFingerprintCalculator();
+
+    @Override
+    public KeyMaterial read(InputStream data, Long tag) throws IOException, BadDataException {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        Streams.pipeAll(data, out);
+
+        try {
+            return readKey(new ByteArrayInputStream(out.toByteArray()), tag);
+        } catch (IOException | PGPException e) {
+            try {
+                return readCertificate(new ByteArrayInputStream(out.toByteArray()), tag);
+            } catch (IOException e1) {
+                throw new BadDataException();
+            }
+        }
+    }
+
+    private Key readKey(InputStream inputStream, Long tag) throws IOException, PGPException {
+        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+        Streams.pipeAll(inputStream, buffer);
+        inputStream.close();
+
+        InputStream decoderStream = PGPUtil.getDecoderStream(new ByteArrayInputStream(buffer.toByteArray()));
+
+        PGPSecretKeyRing secretKeys = new PGPSecretKeyRing(decoderStream, fpCalc);
+        PGPPublicKeyRing cert = extractCert(secretKeys);
+        ByteArrayInputStream encoded = new ByteArrayInputStream(cert.getEncoded());
+        Certificate certificate = readCertificate(encoded, tag);
+
+        return new Key(buffer.toByteArray(), certificate, tag);
+    }
+
+    private Certificate readCertificate(InputStream inputStream, Long tag) throws IOException {
+        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+        Streams.pipeAll(inputStream, buffer);
+        ByteArrayInputStream in = new ByteArrayInputStream(buffer.toByteArray());
+        InputStream decoderStream = PGPUtil.getDecoderStream(in);
+
+        PGPPublicKeyRing cert = new PGPPublicKeyRing(decoderStream, fpCalc);
+        String fingerprint = Hex.toHexString(cert.getPublicKey().getFingerprint()).toLowerCase();
+        List<Long> subKeyIds = getSubkeyIds(cert);
+        return new Certificate(buffer.toByteArray(), fingerprint, subKeyIds, tag);
+    }
+
+    private PGPPublicKeyRing extractCert(PGPSecretKeyRing secretKeys) {
+        List<PGPPublicKey> publicKeyList = new ArrayList<>();
+        Iterator<PGPPublicKey> publicKeyIterator = secretKeys.getPublicKeys();
+        while (publicKeyIterator.hasNext()) {
+            publicKeyList.add(publicKeyIterator.next());
+        }
+        return new PGPPublicKeyRing(publicKeyList);
+    }
+
+    private static List<Long> getSubkeyIds(PGPKeyRing keyRing) {
+        List<Long> keyIds = new ArrayList<>();
+        Iterator<PGPPublicKey> keys = keyRing.getPublicKeys();
+        while (keys.hasNext()) {
+            keyIds.add(keys.next().getKeyID());
+        }
+        return keyIds;
+    }
+}

pgp-certificate-store/build.gradle

@@ -16,7 +16,7 @@ apply plugin: 'ru.vyarus.animalsniffer'
 
 dependencies {
     // animal sniffer for ensuring Android API compatibility
-    signature "net.sf.androidscents.signature:android-api-level-${minAndroidSdk}:2.3.3_r2@signature"
+    signature "net.sf.androidscents.signature:android-api-level-${animalsnifferSignatureVersion}@signature"
 
     // JUnit for testing
     testImplementation "org.junit.jupiter:junit-jupiter-api:$junitVersion"

pgp-certificate-store/src/main/java/pgp/certificate_store/AbstractCertificateStore.java

@@ -1,39 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.certificate_store;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-import pgp.certificate_store.exception.BadDataException;
-import pgp.certificate_store.exception.BadNameException;
-
-import java.io.IOException;
-import java.util.Collections;
-import java.util.HashSet;
-import java.util.Set;
-
-public abstract class AbstractCertificateStore implements CertificateStore {
-
-    private static final Logger LOGGER = LoggerFactory.getLogger(AbstractCertificateStore.class);
-
-    public Set<Certificate> getCertificatesBySubkeyId(long subkeyId)
-            throws IOException {
-        Set<String> identifiers = getCertificateFingerprintsForSubkeyId(subkeyId);
-        if (identifiers.isEmpty()) {
-            return Collections.emptySet();
-        }
-
-        Set<Certificate> certificates = new HashSet<>();
-        for (String identifier : identifiers) {
-            try {
-                certificates.add(getCertificate(identifier));
-            } catch (BadNameException | BadDataException e) {
-                LOGGER.warn("Could not read certificate.", e);
-            }
-        }
-
-        return certificates;
-    }
-}

pgp-certificate-store/src/main/java/pgp/certificate_store/Certificate.java

@@ -1,44 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.certificate_store;
-
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.Set;
-
-public abstract class Certificate {
-    /**
-     * Return the fingerprint of the certificate as 40 lowercase hex characters.
-     * TODO: Allow OpenPGP V5 fingerprints
-     *
-     * @return fingerprint
-     */
-    public abstract String getFingerprint();
-
-    /**
-     * Return an {@link InputStream} of the binary representation of the certificate.
-     *
-     * @return input stream
-     * @throws IOException in case of an IO error
-     */
-    public abstract InputStream getInputStream() throws IOException;
-
-    /**
-     * Return a tag of the certificate.
-     * The tag is a checksum calculated over the binary representation of the certificate.
-     *
-     * @return tag
-     * @throws IOException in case of an IO error
-     */
-    public abstract String getTag() throws IOException;
-
-    /**
-     * Return a {@link Set} containing key-ids of subkeys.
-     *
-     * @return subkeys
-     * @throws IOException in case of an IO error
-     */
-    public abstract Set<Long> getSubkeyIds() throws IOException;
-}

pgp-certificate-store/src/main/java/pgp/certificate_store/CertificateDirectory.java

@@ -1,160 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.certificate_store;
-
-import pgp.certificate_store.exception.BadDataException;
-import pgp.certificate_store.exception.BadNameException;
-
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.Iterator;
-
-/**
- * Certificate storage definition.
- * This interface defines methods to insert and retrieve {@link Certificate Certificates} to and from a store.
- *
- * {@link Certificate Certificates} are hereby identified by identifiers. An identifier can either be a fingerprint
- * or a special name. Special names are implementation-defined identifiers for certificates.
- *
- * Fingerprints are expected to be hexadecimal lowercase character sequences.
- */
-public interface CertificateDirectory {
-
-    /**
-     * Return the certificate that matches the given identifier.
-     * If no matching certificate can be found, return null.
-     *
-     * @param identifier identifier for a certificate.
-     * @return certificate or null
-     *
-     * @throws IOException in case of an IO-error
-     * @throws BadNameException if the identifier is invalid
-     * @throws BadDataException if the certificate file contains invalid data
-     */
-    Certificate getCertificate(String identifier)
-            throws IOException, BadNameException, BadDataException;
-
-    /**
-     * Return the certificate that matches the given identifier, but only iff it changed since the last invocation.
-     * To compare the certificate against its last returned result, the given tag is used.
-     * If the tag of the currently found certificate matches the given argument, return null.
-     *
-     * @param identifier identifier for a certificate
-     * @param tag tag to compare freshness
-     * @return changed certificate or null
-     *
-     * @throws IOException in case of an IO-error
-     * @throws BadNameException if the identifier is invalid
-     * @throws BadDataException if the certificate file contains invalid data
-     */
-    Certificate getCertificateIfChanged(String identifier, String tag)
-            throws IOException, BadNameException, BadDataException;
-
-    /**
-     * Insert a certificate into the store.
-     * If an instance of the certificate is already present in the store, the given {@link MergeCallback} will be
-     * used to merge both the existing and the new instance of the {@link Certificate}. The resulting merged certificate
-     * will be stored in the store and returned.
-     *
-     * This method will block until a write-lock on the store can be acquired. If you cannot afford blocking,
-     * consider to use {@link #tryInsertCertificate(InputStream, MergeCallback)} instead.
-     *
-     * @param data input stream containing the new certificate instance
-     * @param merge callback for merging with an existing certificate instance
-     * @return merged certificate
-     *
-     * @throws IOException in case of an IO-error
-     * @throws InterruptedException in case the inserting thread gets interrupted
-     * @throws BadDataException if the data stream does not contain valid OpenPGP data
-     */
-    Certificate insertCertificate(InputStream data, MergeCallback merge)
-            throws IOException, InterruptedException, BadDataException;
-
-    /**
-     * Insert a certificate into the store.
-     * If an instance of the certificate is already present in the store, the given {@link MergeCallback} will be
-     * used to merge both the existing and the new instance of the {@link Certificate}. The resulting merged certificate
-     * will be stored in the store and returned.
-     *
-     * This method will not block. Instead, if the store is already write-locked, this method will simply return null
-     * without any writing.
-     * However, if the write-lock is available, this method will acquire the lock, write to the store, release the lock
-     * and return the written certificate.
-     *
-     * @param data input stream containing the new certificate instance
-     * @param merge callback for merging with an existing certificate instance
-     * @return merged certificate or null if the store cannot be locked
-     *
-     * @throws IOException in case of an IO-error
-     * @throws BadDataException if the data stream does not contain valid OpenPGP data
-     */
-    Certificate tryInsertCertificate(InputStream data, MergeCallback merge)
-            throws IOException, BadDataException;
-
-    /**
-     * Insert a certificate into the store.
-     * The certificate will be stored under the given special name instead of its fingerprint.
-     *
-     * If an instance of the certificate is already present under the special name in the store, the given {@link MergeCallback} will be
-     * used to merge both the existing and the new instance of the {@link Certificate}. The resulting merged certificate
-     * will be stored in the store and returned.
-     *
-     * This method will block until a write-lock on the store can be acquired. If you cannot afford blocking,
-     * consider to use {@link #tryInsertCertificateBySpecialName(String, InputStream, MergeCallback)}  instead.
-     *
-     * @param specialName special name of the certificate
-     * @param data input stream containing the new certificate instance
-     * @param merge callback for merging with an existing certificate instance
-     * @return merged certificate or null if the store cannot be locked
-     *
-     * @throws IOException in case of an IO-error
-     * @throws InterruptedException if the thread is interrupted
-     * @throws BadDataException if the certificate file does not contain valid OpenPGP data
-     * @throws BadNameException if the special name is unknown
-     */
-    Certificate insertCertificateBySpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, InterruptedException, BadDataException, BadNameException;
-
-    /**
-     * Insert a certificate into the store.
-     * The certificate will be stored under the given special name instead of its fingerprint.
-     *
-     * If an instance of the certificate is already present under the special name in the store, the given {@link MergeCallback} will be
-     * used to merge both the existing and the new instance of the {@link Certificate}. The resulting merged certificate
-     * will be stored in the store and returned.
-     *
-     * This method will not block. Instead, if the store is already write-locked, this method will simply return null
-     * without any writing.
-     * However, if the write-lock is available, this method will acquire the lock, write to the store, release the lock
-     * and return the written certificate.
-     *
-     * @param specialName special name for the certificate
-     * @param data input stream containing the new certificate instance
-     * @param merge callback for merging with an existing certificate instance
-     * @return merged certificate or null if the store cannot be locked
-     *
-     * @throws IOException in case of an IO-error
-     * @throws BadDataException if the data stream does not contain valid OpenPGP data
-     * @throws BadNameException if the special name is not known
-     */
-    Certificate tryInsertCertificateBySpecialName(String specialName, InputStream data, MergeCallback merge)
-            throws IOException, BadDataException, BadNameException;
-
-    /**
-     * Return an {@link Iterator} containing all certificates in the store.
-     * The iterator will contain both certificates addressed by special names and by fingerprints.
-     *
-     * @return certificates
-     */
-    Iterator<Certificate> getCertificates();
-
-    /**
-     * Return an {@link Iterator} containing all certificate fingerprints from the store.
-     * Note that this only includes the fingerprints of certificate primary keys, not those of subkeys.
-     *
-     * @return fingerprints
-     */
-    Iterator<String> getFingerprints();
-}

pgp-certificate-store/src/main/java/pgp/certificate_store/CertificateReaderBackend.java

@@ -1,26 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.certificate_store;
-
-import java.io.IOException;
-import java.io.InputStream;
-
-/**
- * Interface definition for a class that can read {@link Certificate Certificates} from binary
- * {@link InputStream InputStreams}.
- */
-public interface CertificateReaderBackend {
-
-    /**
-     * Read a {@link Certificate} from the given {@link InputStream}.
-     *
-     * @param inputStream input stream containing the binary representation of the certificate.
-     * @return certificate object
-     *
-     * @throws IOException in case of an IO error
-     */
-    Certificate readCertificate(InputStream inputStream) throws IOException;
-
-}

pgp-certificate-store/src/main/java/pgp/certificate_store/CertificateStore.java

@@ -1,9 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.certificate_store;
-
-public interface CertificateStore extends CertificateDirectory, SubkeyLookup {
-
-}

pgp-certificate-store/src/main/java/pgp/certificate_store/MergeCallback.java

@@ -1,27 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package pgp.certificate_store;
-
-import java.io.IOException;
-
-/**
- * Merge a given certificate (update) with an existing certificate.
- */
-public interface MergeCallback {
-
-    /**
-     * Merge the given certificate data with the existing certificate and return the result.
-     *
-     * If no existing certificate is found (i.e. existing is null), this method returns the unmodified data.
-     *
-     * @param data certificate
-     * @param existing optional already existing copy of the certificate
-     * @return merged certificate
-     *
-     * @throws IOException in case of an IO error
-     */
-    Certificate merge(Certificate data, Certificate existing) throws IOException;
-
-}

pgp-certificate-store/src/main/java/pgp/certificate_store/PGPCertificateStore.java

@@ -0,0 +1,123 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.certificate_store;
+
+import pgp.certificate_store.certificate.Certificate;
+import pgp.certificate_store.certificate.KeyMaterialMerger;
+import pgp.certificate_store.exception.BadDataException;
+import pgp.certificate_store.exception.BadNameException;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+/**
+ * Interface for an OpenPGP certificate (public key) store.
+ */
+public interface PGPCertificateStore {
+
+    /**
+     * Return the certificate that matches the given identifier.
+     *
+     * @param identifier identifier for a certificate.
+     * @return certificate or null
+     *
+     * @throws IOException in case of an IO-error
+     * @throws BadNameException if the identifier is invalid
+     * @throws BadDataException if the certificate file contains invalid data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getCertificate(String identifier)
+            throws IOException, BadNameException, BadDataException;
+
+    /**
+     * Return the certificate that matches the given identifier, but only if it has been changed.
+     * Whether it has been changed is determined by calculating the tag in the directory
+     * (e.g. by looking at the inode and last modification date) and comparing the result with the tag provided by
+     * the caller.
+     *
+     * @param identifier certificate identifier
+     * @param tag tag by the caller
+     * @return certificate if it has been changed, null otherwise
+     *
+     * @throws IOException in case of an IO-error
+     * @throws BadNameException if the identifier is invalid
+     * @throws BadDataException if the certificate file contains invalid data
+     * @throws NoSuchElementException if no such certificate is found
+     */
+    Certificate getCertificateIfChanged(String identifier, Long tag)
+            throws IOException, BadNameException, BadDataException;
+
+    /**
+     * Return an {@link Iterator} over all certificates in the store that contain a subkey with the given
+     * subkey id.
+     * @param subkeyId id of the subkey
+     * @return iterator
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException if any of the certificate files contains invalid data
+     */
+    Iterator<Certificate> getCertificatesBySubkeyId(long subkeyId)
+            throws IOException, BadDataException;
+
+    /**
+     * Insert a certificate into the store.
+     * If an instance of the certificate is already present in the store, the given {@link KeyMaterialMerger} will be
+     * used to merge both the existing and the new instance of the {@link Certificate}. The resulting merged certificate
+     * will be stored in the store and returned.
+     *
+     * This method will block until a write-lock on the store can be acquired.
+     *
+     * @param data input stream containing the new certificate instance
+     * @param merge callback for merging with an existing certificate instance
+     * @return merged certificate
+     *
+     * @throws IOException in case of an IO-error
+     * @throws InterruptedException in case the inserting thread gets interrupted
+     * @throws BadDataException if the data stream does not contain valid OpenPGP data
+     */
+    Certificate insertCertificate(InputStream data, KeyMaterialMerger merge)
+            throws IOException, InterruptedException, BadDataException;
+
+    /**
+     * Insert a certificate into the store.
+     * The certificate will be stored under the given special name instead of its fingerprint.
+     *
+     * If an instance of the certificate is already present under the special name in the store, the given {@link KeyMaterialMerger} will be
+     * used to merge both the existing and the new instance of the {@link Certificate}. The resulting merged certificate
+     * will be stored in the store and returned.
+     *
+     * This method will block until a write-lock on the store can be acquired.
+     *
+     * @param specialName special name of the certificate
+     * @param data input stream containing the new certificate instance
+     * @param merge callback for merging with an existing certificate instance
+     * @return merged certificate or null if the store cannot be locked
+     *
+     * @throws IOException in case of an IO-error
+     * @throws InterruptedException if the thread is interrupted
+     * @throws BadDataException if the certificate file does not contain valid OpenPGP data
+     * @throws BadNameException if the special name is unknown
+     */
+    Certificate insertCertificateBySpecialName(String specialName, InputStream data, KeyMaterialMerger merge)
+            throws IOException, InterruptedException, BadDataException, BadNameException;
+
+    /**
+     * Return an {@link Iterator} containing all certificates in the store.
+     * The iterator will contain both certificates addressed by special names and by fingerprints.
+     *
+     * @return certificates
+     */
+    Iterator<Certificate> getCertificates();
+
+    /**
+     * Return an {@link Iterator} containing all certificate fingerprints from the store.
+     * Note that this only includes the fingerprints of certificate primary keys, not those of subkeys.
+     *
+     * @return fingerprints
+     */
+    Iterator<String> getFingerprints();
+}

pgp-certificate-store/src/main/java/pgp/certificate_store/certificate/Certificate.java

@@ -0,0 +1,70 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.certificate_store.certificate;
+
+import java.io.ByteArrayInputStream;
+import java.io.InputStream;
+import java.util.List;
+
+/**
+ * OpenPGP certificate (public key).
+ */
+public class Certificate implements KeyMaterial {
+
+    private final byte[] bytes;
+    private final String fingerprint;
+    private final List<Long> subkeyIds;
+    private final Long tag;
+
+    /**
+     * Certificate constructor.
+     *
+     * @param bytes encoding of the certificate
+     * @param fingerprint fingerprint (lowercase hex characters)
+     * @param subkeyIds list of subkey ids
+     * @param tag tag
+     */
+    public Certificate(byte[] bytes, String fingerprint, List<Long> subkeyIds, Long tag) {
+        this.bytes = bytes;
+        this.fingerprint = fingerprint;
+        this.subkeyIds = subkeyIds;
+        this.tag = tag;
+    }
+
+    /**
+     * Copy constructor to assign a new tag to the {@link Certificate}.
+     *
+     * @param cert certificate
+     * @param tag tag
+     */
+    public Certificate(Certificate cert, Long tag) {
+        this(cert.bytes, cert.fingerprint, cert.subkeyIds, tag);
+    }
+
+    @Override
+    public String getFingerprint() {
+        return fingerprint;
+    }
+
+    @Override
+    public Certificate asCertificate() {
+        return this;
+    }
+
+    @Override
+    public InputStream getInputStream() {
+        return new ByteArrayInputStream(bytes);
+    }
+
+    @Override
+    public Long getTag() {
+        return tag;
+    }
+
+    @Override
+    public List<Long> getSubkeyIds() {
+        return subkeyIds;
+    }
+}

pgp-certificate-store/src/main/java/pgp/certificate_store/certificate/Key.java

@@ -0,0 +1,77 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.certificate_store.certificate;
+
+import java.io.ByteArrayInputStream;
+import java.io.InputStream;
+import java.util.List;
+
+/**
+ * OpenPGP key (secret key).
+ */
+public class Key implements KeyMaterial {
+
+    private final byte[] bytes;
+    private final Certificate certificate;
+    private final Long tag;
+
+    /**
+     * Key constructor.
+     *
+     * @param bytes encoding of the key
+     * @param certificate associated certificate
+     * @param tag tag
+     */
+    public Key(byte[] bytes, Certificate certificate, Long tag) {
+        this.bytes = bytes;
+        this.certificate = certificate;
+        this.tag = tag;
+    }
+
+    /**
+     * Copy constructor to change the tag of both the {@link Key} and its {@link Certificate}.
+     *
+     * @param key key
+     * @param tag tag
+     */
+    public Key(Key key, Long tag) {
+        this(key.bytes, new Certificate(key.certificate, tag), tag);
+    }
+
+    /**
+     * Return the certificate part of this OpenPGP key.
+     *
+     * @return OpenPGP certificate
+     */
+    public Certificate getCertificate() {
+        return new Certificate(certificate, getTag());
+    }
+
+    @Override
+    public String getFingerprint() {
+        return certificate.getFingerprint();
+    }
+
+    @Override
+    public Certificate asCertificate() {
+        return getCertificate();
+    }
+
+    @Override
+    public InputStream getInputStream() {
+        return new ByteArrayInputStream(bytes);
+    }
+
+    @Override
+    public Long getTag() {
+        return tag;
+    }
+
+    @Override
+    public List<Long> getSubkeyIds() {
+        return certificate.getSubkeyIds();
+    }
+
+}

pgp-certificate-store/src/main/java/pgp/certificate_store/certificate/KeyMaterial.java

@@ -0,0 +1,51 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.certificate_store.certificate;
+
+import java.io.InputStream;
+import java.util.List;
+import java.util.Set;
+
+public interface KeyMaterial {
+
+    /**
+     * Return the fingerprint of the certificate as 40 lowercase hex characters.
+     * TODO: Allow OpenPGP V5 fingerprints
+     *
+     * @return fingerprint
+     */
+    String getFingerprint();
+
+    /**
+     * Return the {@link Certificate} belonging to this key material.
+     * If this is already a {@link Certificate}, return this.
+     * If this is a {@link Key}, extract the {@link Certificate} and return it.
+     *
+     * @return certificate
+     */
+    Certificate asCertificate();
+
+    /**
+     * Return an {@link InputStream} of the binary representation of the secret key.
+     *
+     * @return input stream
+     */
+    InputStream getInputStream();
+
+    /**
+     * Return the tag belonging to this key material.
+     * The tag can be used to keep an application cache in sync with what is in the directory.
+     *
+     * @return tag
+     */
+    Long getTag();
+
+    /**
+     * Return a {@link Set} containing key-ids of subkeys.
+     *
+     * @return subkeys
+     */
+    List<Long> getSubkeyIds();
+}

pgp-certificate-store/src/main/java/pgp/certificate_store/certificate/KeyMaterialMerger.java

@@ -0,0 +1,25 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.certificate_store.certificate;
+
+import java.io.IOException;
+
+/**
+ * Merge a given {@link Key} (update) with an existing {@link Key}.
+ */
+public interface KeyMaterialMerger {
+
+    /**
+     * Merge the given key material with an existing copy and return the result.
+     * If no existing {@link KeyMaterial} is found (i.e. if existing is null), this method returns the unmodified data.
+     *
+     * @param data key material
+     * @param existing optional already existing copy of the key material
+     * @return merged key material
+     *
+     * @throws IOException in case of an IO error
+     */
+    KeyMaterial merge(KeyMaterial data, KeyMaterial existing) throws IOException;
+}

pgp-certificate-store/src/main/java/pgp/certificate_store/certificate/KeyMaterialReaderBackend.java

@@ -0,0 +1,25 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.certificate_store.certificate;
+
+import pgp.certificate_store.exception.BadDataException;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public interface KeyMaterialReaderBackend {
+
+    /**
+     * Read a {@link KeyMaterial} (either {@link Key} or {@link Certificate}) from the given {@link InputStream}.
+     *
+     * @param data input stream containing the binary representation of the key.
+     * @param tag tag for the key material. Might be null.
+     * @return key or certificate object
+     *
+     * @throws IOException in case of an IO error
+     * @throws BadDataException in case that the data stream does not contain a valid OpenPGP key/certificate
+     */
+    KeyMaterial read(InputStream data, Long tag) throws IOException, BadDataException;
+}

pgp-certificate-store/src/main/java/pgp/certificate_store/certificate/package-info.java

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+/**
+ * General OpenPGP Certificate Storage related classes.
+ */
+package pgp.certificate_store.certificate;

pgp-certificate-store/src/main/java/pgp/certificate_store/exception/package-info.java

@@ -3,8 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0
 
 /**
- * Exceptions defined by the Shared PGP Certificate Directory.
- *
- * @see <a href="https://sequoia-pgp.gitlab.io/pgp-cert-d/#name-failure-modes">Failure Modes</a>
+ * Exceptions.
  */
 package pgp.certificate_store.exception;

settings.gradle

@@ -4,7 +4,7 @@
 
 rootProject.name = 'cert-d-java'
 
-include 'pgp-cert-d-java',
-        'pgp-cert-d-java-jdbc-sqlite-lookup',
-        'pgp-certificate-store'
+include 'pgp-certificate-store',
+        'pgp-cert-d-java',
+        'pgp-cert-d-java-jdbc-sqlite-lookup'
 

version.gradle

@@ -4,13 +4,16 @@
 
 allprojects {
     ext {
-        shortVersion = '0.1.1'
-        isSnapshot = false
-        minAndroidSdk = 10
+        shortVersion = '0.2.3'
+        isSnapshot = true
+        minAndroidSdk = 26
+        animalsnifferSignatureVersion = "$minAndroidSdk:8.0.0_r2"
         javaSourceCompatibility = 1.8
+        bouncycastleVersion = '1.75'
+        bouncyPgVersion = "$bouncycastleVersion"
         slf4jVersion = '1.7.36'
         logbackVersion = '1.2.11'
         junitVersion = '5.8.2'
-        sqliteJdbcVersion = '3.36.0.3'
+        sqliteJdbcVersion = '3.42.0.0'
     }
 }