diff --git a/pgp-cert-d-java/src/main/java/pgp/cert_d/backend/FileBasedCertificateDirectoryBackend.java b/pgp-cert-d-java/src/main/java/pgp/cert_d/backend/FileBasedCertificateDirectoryBackend.java
index e5750e8..def57e6 100644
--- a/pgp-cert-d-java/src/main/java/pgp/cert_d/backend/FileBasedCertificateDirectoryBackend.java
+++ b/pgp-cert-d-java/src/main/java/pgp/cert_d/backend/FileBasedCertificateDirectoryBackend.java
@@ -35,12 +35,20 @@ import java.util.Iterator;
 import java.util.List;
 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;
@@ -364,6 +372,9 @@ public class FileBasedCertificateDirectoryBackend implements PGPCertificateDirec
         return getTag(certFile);
     }
 
+    /**
+     * Class to resolve file names from certificate fingerprints / special names.
+     */
     public static class FilenameResolver {
 
         private final File baseDirectory;
diff --git a/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/InMemorySubkeyLookupFactory.java b/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/InMemorySubkeyLookupFactory.java
index a224c64..98e006c 100644
--- a/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/InMemorySubkeyLookupFactory.java
+++ b/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/InMemorySubkeyLookupFactory.java
@@ -6,6 +6,9 @@ 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) {
diff --git a/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/SubkeyLookupFactory.java b/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/SubkeyLookupFactory.java
index 442e9ef..a958b84 100644
--- a/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/SubkeyLookupFactory.java
+++ b/pgp-cert-d-java/src/main/java/pgp/cert_d/subkey_lookup/SubkeyLookupFactory.java
@@ -6,6 +6,9 @@ package pgp.cert_d.subkey_lookup;
 
 import java.io.File;
 
+/**
+ * Factory class to instantiate different {@link SubkeyLookup} implementations.
+ */
 public interface SubkeyLookupFactory {
 
     /**