Add LibrePGPPreferredEncryptionModes signature subpacket

pg/src/main/java/org/bouncycastle/bcpg/SignatureSubpacketInputStream.java

@@ -165,6 +165,7 @@ else if (l == 255)
         case PREFERRED_COMP_ALGS:
         case PREFERRED_HASH_ALGS:
         case PREFERRED_SYM_ALGS:
+        case LIBREPGP_PREFERRED_ENCRYPTION_MODES:
             return new PreferredAlgorithms(type, isCritical, isLongLength, data);
         case PREFERRED_AEAD_ALGORITHMS:
             return new PreferredAEADCiphersuites(isCritical, isLongLength, data);

pg/src/main/java/org/bouncycastle/bcpg/sig/LibrePGPPreferredEncryptionModes.java

@@ -0,0 +1,23 @@
+package org.bouncycastle.bcpg.sig;
+
+import org.bouncycastle.bcpg.SignatureSubpacketTags;
+
+/**
+ * This is a deprecated LibrePGP signature subpacket with encryption mode numbers to indicate which modes
+ * the key holder prefers to use with OCB Encrypted Data Packets ({@link org.bouncycastle.bcpg.AEADEncDataPacket}).
+ *  Implementations SHOULD ignore this subpacket and assume {@link org.bouncycastle.bcpg.AEADAlgorithmTags#OCB}.
+ */
+public class LibrePGPPreferredEncryptionModes
+        extends PreferredAlgorithms
+{
+
+    public LibrePGPPreferredEncryptionModes(boolean isCritical, int[] encryptionModes)
+    {
+        this(isCritical, false, intToByteArray(encryptionModes));
+    }
+
+    protected LibrePGPPreferredEncryptionModes(boolean critical, boolean isLongLength, byte[] data)
+    {
+        super(SignatureSubpacketTags.LIBREPGP_PREFERRED_ENCRYPTION_MODES, critical, isLongLength, data);
+    }
+}