#1844 Improve comparison logic during signature joining

Author: gefeili

pg/src/main/java/org/bouncycastle/openpgp/PGPPublicKey.java

@@ -1216,7 +1216,8 @@ private static void joinPgpSignatureList(List<PGPSignature> source,
             for (int i = 0; isNotNull && i < rlt.size(); i++)
             {
                 PGPSignature existingSubSig = (PGPSignature)rlt.get(i);
-                if (PGPSignature.isSignatureEncodingEqual(existingSubSig, copySubSig))
+                if (existingSubSig.getVersion() == copySubSig.getVersion() &&
+                        PGPSignature.isSignatureEncodingEqual(existingSubSig, copySubSig))
                 {
                     found = true;
                     // join existing sig with copy to apply modifications in unhashed subpackets

pg/src/main/java/org/bouncycastle/openpgp/PGPSignature.java

@@ -923,7 +923,13 @@ public static boolean isSignatureEncodingEqual(PGPSignature sig1, PGPSignature s
     public static PGPSignature join(PGPSignature sig1, PGPSignature sig2)
         throws PGPException
     {
-        if (!isSignatureEncodingEqual(sig1, sig2))
+        if (sig1.getVersion() < SignaturePacket.VERSION_4) {
+            // Version 2/3 signatures have no subpackets, so don't need to get merged.
+            return sig1;
+        }
+
+        if (sig1.getVersion() != sig2.getVersion() ||
+                !isSignatureEncodingEqual(sig1, sig2))
         {
             throw new IllegalArgumentException("These are different signatures.");
         }