Allow the user to specify a purpose for encryption

pgpainless-core/src/main/java/org/pgpainless/PGPainless.java

@@ -66,12 +66,26 @@ public class PGPainless {
 
     /**
      * Create an {@link EncryptionStream}, which can be used to encrypt and/or sign data using OpenPGP.
+     * This method assumes that the stream will be used to encrypt data for communication purposes.
+     * If you instead want to encrypt data that will be saved on disk (eg. a backup), use
+     * {@link #encryptAndOrSign(EncryptionStream.Purpose)} and chose an appropriate purpose.
+     *
      * @return builder
      */
     public static EncryptionBuilder encryptAndOrSign() {
         return new EncryptionBuilder();
     }
 
+    /**
+     * Create an {@link EncryptionStream}, that can be used to encrypt and/or sign data using OpenPGP.
+     *
+     * @param purpose how will the data be used?
+     * @return builder
+     */
+    public static EncryptionBuilder encryptAndOrSign(EncryptionStream.Purpose purpose) {
+        return new EncryptionBuilder(purpose);
+    }
+
     /**
      * Create a {@link DecryptionStream}, which can be used to decrypt and/or verify data using OpenPGP.
      *

pgpainless-core/src/main/java/org/pgpainless/encryption_signing/EncryptionBuilder.java

@@ -37,6 +37,7 @@ import org.bouncycastle.openpgp.PGPSecretKeyRingCollection;
 import org.bouncycastle.openpgp.operator.PBESecretKeyDecryptor;
 import org.pgpainless.algorithm.CompressionAlgorithm;
 import org.pgpainless.algorithm.HashAlgorithm;
+import org.pgpainless.algorithm.KeyFlag;
 import org.pgpainless.algorithm.SignatureType;
 import org.pgpainless.algorithm.SymmetricKeyAlgorithm;
 import org.pgpainless.exception.SecretKeyNotFoundException;
@@ -55,6 +56,7 @@ import org.pgpainless.util.Passphrase;
 
 public class EncryptionBuilder implements EncryptionBuilderInterface {
 
+    private final EncryptionStream.Purpose purpose;
     private OutputStream outputStream;
     private final Set<PGPPublicKey> encryptionKeys = new HashSet<>();
     private final Set<Passphrase> encryptionPassphrases = new HashSet<>();
@@ -67,6 +69,14 @@ public class EncryptionBuilder implements EncryptionBuilderInterface {
     private CompressionAlgorithm compressionAlgorithm = CompressionAlgorithm.UNCOMPRESSED;
     private boolean asciiArmor = false;
 
+    public EncryptionBuilder() {
+        this.purpose = EncryptionStream.Purpose.COMMUNICATIONS;
+    }
+
+    public EncryptionBuilder(@Nonnull EncryptionStream.Purpose purpose) {
+        this.purpose = purpose;
+    }
+
     @Override
     public ToRecipients onOutputStream(@Nonnull OutputStream outputStream) {
         this.outputStream = outputStream;
@@ -425,9 +435,28 @@ public class EncryptionBuilder implements EncryptionBuilderInterface {
     }
 
     PublicKeySelectionStrategy encryptionKeySelector() {
+        KeyFlag[] flags = mapPurposeToKeyFlags(purpose);
         return new And.PubKeySelectionStrategy(
                 new NoRevocation.PubKeySelectionStrategy(),
-                new EncryptionKeySelectionStrategy());
+                new EncryptionKeySelectionStrategy(flags));
+    }
+
+    private static KeyFlag[] mapPurposeToKeyFlags(EncryptionStream.Purpose purpose) {
+        KeyFlag[] flags;
+        switch (purpose) {
+            case COMMUNICATIONS:
+                flags = new KeyFlag[] {KeyFlag.ENCRYPT_COMMS};
+                break;
+            case STORAGE:
+                flags = new KeyFlag[] {KeyFlag.ENCRYPT_STORAGE};
+                break;
+            case STORAGE_AND_COMMUNICATIONS:
+                flags = new KeyFlag[] {KeyFlag.ENCRYPT_COMMS, KeyFlag.ENCRYPT_STORAGE};
+                break;
+            default:
+                throw new AssertionError("Illegal purpose enum value encountered.");
+        }
+        return flags;
     }
 
     SecretKeySelectionStrategy signingKeySelector() {

pgpainless-core/src/main/java/org/pgpainless/encryption_signing/EncryptionStream.java

@@ -59,6 +59,23 @@ import org.pgpainless.util.Passphrase;
  */
 public final class EncryptionStream extends OutputStream {
 
+    public enum Purpose {
+        /**
+         * The stream will encrypt communication that goes over the wire.
+         * Eg. EMail, Chat...
+         */
+        COMMUNICATIONS,
+        /**
+         * The stream will encrypt data that is stored on disk.
+         * Eg. Encrypted backup...
+         */
+        STORAGE,
+        /**
+         * The stream will use keys with either flags to encrypt the data.
+         */
+        STORAGE_AND_COMMUNICATIONS
+    }
+
     private static final Logger LOGGER = Logger.getLogger(EncryptionStream.class.getName());
     private static final Level LEVEL = Level.FINE;
 

pgpainless-core/src/main/java/org/pgpainless/key/selection/key/impl/EncryptionKeySelectionStrategy.java

@@ -26,11 +26,14 @@ import org.pgpainless.key.selection.key.PublicKeySelectionStrategy;
  */
 public class EncryptionKeySelectionStrategy extends PublicKeySelectionStrategy {
 
-    private static final HasKeyFlagSelectionStrategy.PublicKey HAS_ENCRYPT_COMMS_FLAG =
+    private final HasKeyFlagSelectionStrategy.PublicKey keyFlagSelector;
-            new HasKeyFlagSelectionStrategy.PublicKey(KeyFlag.ENCRYPT_COMMS);
+
+    public EncryptionKeySelectionStrategy(KeyFlag... flags) {
+        this.keyFlagSelector = new HasKeyFlagSelectionStrategy.PublicKey(flags);
+    }
 
     @Override
     public boolean accept(@Nonnull PGPPublicKey key) {
-        return key.isEncryptionKey() && HAS_ENCRYPT_COMMS_FLAG.accept(key);
+        return key.isEncryptionKey() && keyFlagSelector.accept(key);
     }
 }