watchtower/multi: use new blob.BreachKey for enc/dec of justice kits

watchtower/lookout/lookout.go

@@ -202,13 +202,16 @@ func (l *Lookout) processEpoch(epoch *chainntnfs.BlockEpoch,
 
 		// The decryption key for the state update should be the full
 		// txid of the breaching commitment transaction.
-		commitTxID := commitTx.TxHash()
+		// The decryption key for the state update should be computed as
+		//   key = SHA256(txid).
+		breachTxID := commitTx.TxHash()
+		breachKey := blob.NewBreachKeyFromHash(&breachTxID)
 
 		// Now, decrypt the blob of justice that we received in the
 		// state update. This will contain all information required to
 		// sweep the breached commitment outputs.
 		justiceKit, err := blob.Decrypt(
-			commitTxID[:], match.EncryptedBlob,
+			breachKey, match.EncryptedBlob,
 			match.SessionInfo.Policy.BlobType,
 		)
 		if err != nil {

watchtower/lookout/lookout_test.go

@@ -148,14 +148,17 @@ func TestLookoutBreachMatching(t *testing.T) {
 		CommitToLocalSig: makeArray64(2),
 	}
 
-	// Encrypt the first justice kit under the txid of the first txn.
-	encBlob1, err := blob1.Encrypt(hash1[:], blob.FlagCommitOutputs.Type())
+	key1 := blob.NewBreachKeyFromHash(&hash1)
+	key2 := blob.NewBreachKeyFromHash(&hash2)
+
+	// Encrypt the first justice kit under breach key one.
+	encBlob1, err := blob1.Encrypt(key1, blob.FlagCommitOutputs.Type())
 	if err != nil {
 		t.Fatalf("unable to encrypt sweep detail 1: %v", err)
 	}
 
-	// Encrypt the second justice kit under the txid of the second txn.
-	encBlob2, err := blob2.Encrypt(hash2[:], blob.FlagCommitOutputs.Type())
+	// Encrypt the second justice kit under breach key two.
+	encBlob2, err := blob2.Encrypt(key2, blob.FlagCommitOutputs.Type())
 	if err != nil {
 		t.Fatalf("unable to encrypt sweep detail 2: %v", err)
 	}