There's something wrong with the V5 broadcast signature. I'm not quite shure what it is, so I made a test.

domain/src/main/java/ch/dissem/bitmessage/entity/Plaintext.java

@@ -67,14 +67,15 @@ public class Plaintext implements Streamable {
     }
 
     public static Plaintext.Builder readWithoutSignature(Type type, InputStream in) throws IOException {
+        long version = Decode.varInt(in);
         return new Builder(type)
-                .addressVersion(Decode.varInt(in))
+                .addressVersion(version)
                 .stream(Decode.varInt(in))
                 .behaviorBitfield(Decode.int32(in))
                 .publicSigningKey(Decode.bytes(in, 64))
                 .publicEncryptionKey(Decode.bytes(in, 64))
-                .nonceTrialsPerByte(Decode.varInt(in))
-                .extraBytes(Decode.varInt(in))
+                .nonceTrialsPerByte(version >= 3 ? Decode.varInt(in) : 0)
+                .extraBytes(version >= 3 ? Decode.varInt(in) : 0)
                 .destinationRipe(type == Type.MSG ? Decode.bytes(in, 20) : null)
                 .encoding(Decode.varInt(in))
                 .message(Decode.varBytes(in))
@@ -136,8 +137,10 @@ public class Plaintext implements Streamable {
         Encode.int32(from.getPubkey().getBehaviorBitfield(), out);
         out.write(from.getPubkey().getSigningKey(), 1, 64);
         out.write(from.getPubkey().getEncryptionKey(), 1, 64);
+        if (from.getVersion() >= 3) {
             Encode.varInt(from.getPubkey().getNonceTrialsPerByte(), out);
             Encode.varInt(from.getPubkey().getExtraBytes(), out);
+        }
         if (type == Type.MSG) {
             out.write(to.getRipe());
         }

domain/src/main/java/ch/dissem/bitmessage/entity/payload/Broadcast.java

@@ -47,6 +47,21 @@ public abstract class Broadcast extends ObjectPayload implements Encrypted, Plai
         return address.getVersion() < 4 ? 4 : 5;
     }
 
+    @Override
+    public boolean isSigned() {
+        return true;
+    }
+
+    @Override
+    public byte[] getSignature() {
+        return plaintext.getSignature();
+    }
+
+    @Override
+    public void setSignature(byte[] signature) {
+        plaintext.setSignature(signature);
+    }
+
     @Override
     public long getStream() {
         return stream;

domain/src/main/java/ch/dissem/bitmessage/entity/payload/V4Broadcast.java

@@ -52,16 +52,6 @@ public class V4Broadcast extends Broadcast {
         plaintext.write(out, false);
     }
 
-    @Override
-    public byte[] getSignature() {
-        return plaintext.getSignature();
-    }
-
-    @Override
-    public void setSignature(byte[] signature) {
-        plaintext.setSignature(signature);
-    }
-
     @Override
     public void write(OutputStream out) throws IOException {
         encrypted.write(out);

domain/src/test/java/ch/dissem/bitmessage/DecryptionTest.java

@@ -27,21 +27,28 @@ import org.junit.Test;
 import java.io.IOException;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
 
 public class DecryptionTest {
     @Test
     public void ensureV4BroadcastIsDecryptedCorrectly() throws IOException, DecryptionFailedException {
+        BitmessageAddress address = new BitmessageAddress("BM-2D9Vc5rFxxR5vTi53T9gkLfemViHRMVLQZ");
+        TestUtils.loadPubkey(address);
         ObjectMessage objectMessage = TestUtils.loadObjectMessage(5, "V4Broadcast.payload");
         V4Broadcast broadcast = (V4Broadcast) objectMessage.getPayload();
-        broadcast.decrypt(new BitmessageAddress("BM-2D9Vc5rFxxR5vTi53T9gkLfemViHRMVLQZ"));
+        broadcast.decrypt(address);
         assertEquals("Test-Broadcast", broadcast.getPlaintext().getSubject());
+        assertTrue(objectMessage.isSignatureValid(address.getPubkey()));
     }
 
     @Test
     public void ensureV5BroadcastIsDecryptedCorrectly() throws IOException, DecryptionFailedException {
+        BitmessageAddress address = new BitmessageAddress("BM-2cXxfcSetKnbHJX2Y85rSkaVpsdNUZ5q9h");
+        TestUtils.loadPubkey(address);
         ObjectMessage objectMessage = TestUtils.loadObjectMessage(5, "V5Broadcast.payload");
         V5Broadcast broadcast = (V5Broadcast) objectMessage.getPayload();
-        broadcast.decrypt(new BitmessageAddress("BM-2cXxfcSetKnbHJX2Y85rSkaVpsdNUZ5q9h"));
+        broadcast.decrypt(address);
         assertEquals("Test-Broadcast", broadcast.getPlaintext().getSubject());
+        assertTrue(objectMessage.isSignatureValid(address.getPubkey()));
     }
 }

domain/src/test/java/ch/dissem/bitmessage/utils/TestUtils.java

@@ -18,6 +18,7 @@ package ch.dissem.bitmessage.utils;
 
 import ch.dissem.bitmessage.entity.BitmessageAddress;
 import ch.dissem.bitmessage.entity.ObjectMessage;
+import ch.dissem.bitmessage.entity.payload.Pubkey;
 import ch.dissem.bitmessage.entity.payload.V4Pubkey;
 import ch.dissem.bitmessage.entity.valueobject.PrivateKey;
 import ch.dissem.bitmessage.exception.DecryptionFailedException;
@@ -76,4 +77,10 @@ public class TestUtils {
         address.setPubkey((V4Pubkey) object.getPayload());
         return address;
     }
+
+    public static void loadPubkey(BitmessageAddress address) throws IOException {
+        byte[] bytes = getBytes(address.getAddress() + ".pubkey");
+        Pubkey pubkey = Factory.readPubkey(address.getVersion(), address.getStream(), new ByteArrayInputStream(bytes), bytes.length, false);
+        address.setPubkey(pubkey);
+    }
 }