ECKey: Factor out findRecoveryId() from signMessage().

core/src/main/java/org/bitcoinj/core/ECKey.java

@@ -865,17 +865,7 @@ public class ECKey implements EncryptableItem {
         byte[] data = formatMessageForSigning(message);
         Sha256Hash hash = Sha256Hash.twiceOf(data);
         ECDSASignature sig = sign(hash, aesKey);
-        // Now we have to work backwards to figure out the recId needed to recover the signature.
+        byte recId = findRecoveryId(hash, sig);
-        int recId = -1;
-        for (int i = 0; i < 4; i++) {
-            ECKey k = ECKey.recoverFromSignature(i, sig, hash, isCompressed());
-            if (k != null && k.pub.equals(pub)) {
-                recId = i;
-                break;
-            }
-        }
-        if (recId == -1)
-            throw new RuntimeException("Could not construct a recoverable key. This should never happen.");
         int headerByte = recId + 27 + (isCompressed() ? 4 : 0);
         byte[] sigData = new byte[65];  // 1 header + 32 bytes for R + 32 bytes for S
         sigData[0] = (byte)headerByte;
@@ -940,6 +930,26 @@ public class ECKey implements EncryptableItem {
             throw new SignatureException("Signature did not match for message");
     }
 
+    /**
+     * Returns the recovery ID, a byte with value between 0 and 3, inclusive, that specifies which of 4 possible
+     * curve points was used to sign a message. This value is also referred to as "v".
+     *
+     * @throws RuntimeException if no recovery ID can be found.
+     */
+    public byte findRecoveryId(Sha256Hash hash, ECDSASignature sig) {
+        byte recId = -1;
+        for (byte i = 0; i < 4; i++) {
+            ECKey k = ECKey.recoverFromSignature(i, sig, hash, isCompressed());
+            if (k != null && k.pub.equals(pub)) {
+                recId = i;
+                break;
+            }
+        }
+        if (recId == -1)
+            throw new RuntimeException("Could not construct a recoverable key. This should never happen.");
+        return recId;
+    }
+
     /**
      * <p>Given the components of a signature and a selector value, recover and return the public key
      * that generated the signature according to the algorithm in SEC1v2 section 4.1.6.</p>

core/src/test/java/org/bitcoinj/core/ECKeyTest.java

@@ -239,6 +239,52 @@ public class ECKeyTest {
         assertEquals(expectedAddress, gotAddress);
     }
 
+    @Test
+    public void findRecoveryId() {
+        ECKey key = new ECKey();
+        String message = "Hello World!";
+        Sha256Hash hash = Sha256Hash.of(message.getBytes());
+        ECKey.ECDSASignature sig = key.sign(hash);
+        key = ECKey.fromPublicOnly(key.getPubKeyPoint());
+
+        List<Byte> possibleRecIds = Lists.newArrayList((byte) 0, (byte) 1, (byte) 2, (byte) 3);
+        byte recId = key.findRecoveryId(hash, sig);
+        assertTrue(possibleRecIds.contains(recId));
+    }
+
+    @Test
+    public void keyRecoveryTestVector() {
+        // a test that exercises key recovery with findRecoveryId() on a test vector
+        // test vector from https://crypto.stackexchange.com/a/41339
+        ECKey key = ECKey.fromPrivate(
+                new BigInteger("ebb2c082fd7727890a28ac82f6bdf97bad8de9f5d7c9028692de1a255cad3e0f", 16));
+        String message = "Maarten Bodewes generated this test vector on 2016-11-08";
+        Sha256Hash hash = Sha256Hash.of(message.getBytes());
+        ECKey.ECDSASignature sig = key.sign(hash);
+        key = ECKey.fromPublicOnly(key.getPubKeyPoint());
+
+        byte recId = key.findRecoveryId(hash, sig);
+        byte expectedRecId = 0;
+        assertEquals(recId, expectedRecId);
+
+        ECKey pubKey = ECKey.fromPublicOnly(key.getPubKeyPoint());
+        ECKey recoveredKey = ECKey.recoverFromSignature(recId, sig, hash, true);
+        assertEquals(recoveredKey, pubKey);
+    }
+
+    @Test
+    public void keyRecoveryWithFindRecoveryId() throws Exception {
+        ECKey key = new ECKey();
+        String message = "Hello World!";
+        Sha256Hash hash = Sha256Hash.of(message.getBytes());
+        ECKey.ECDSASignature sig = key.sign(hash);
+
+        byte recId = key.findRecoveryId(hash, sig);
+        ECKey pubKey = ECKey.fromPublicOnly(key.getPubKeyPoint());
+        ECKey recoveredKey = ECKey.recoverFromSignature(recId, sig, hash, true);
+        assertEquals(recoveredKey, pubKey);
+    }
+
     @Test
     public void keyRecovery() throws Exception {
         ECKey key = new ECKey();