elements: Added small helper to compute the right signature hash

Depending on the network we end up with different signature hash algorithms,
so we just collect that decision in one place.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

bitcoin/signature.c

@@ -88,6 +88,32 @@ void sign_hash(const struct privkey *privkey,
 	assert(ok);
 }
 
+static void bitcoin_tx_hash_for_sig(const struct bitcoin_tx *tx, unsigned int in,
+				    const u8 *script,
+				    enum sighash_type sighash_type,
+				    struct sha256_double *dest)
+{
+	int ret;
+	u8 value[9];
+	u64 satoshis = tx->input_amounts[in]->satoshis /* Raw: sig-helper */;
+	int flags = WALLY_TX_FLAG_USE_WITNESS;
+
+	if (is_elements) {
+		ret = wally_tx_confidential_value_from_satoshi(satoshis, value, sizeof(value));
+		assert(ret == WALLY_OK);
+		ret = wally_tx_get_elements_signature_hash(
+		    tx->wtx, in, script, tal_bytelen(script), value,
+		    sizeof(value), sighash_type, flags, dest->sha.u.u8,
+		    sizeof(*dest));
+		assert(ret == WALLY_OK);
+	} else {
+		ret = wally_tx_get_btc_signature_hash(
+		    tx->wtx, in, script, tal_bytelen(script), satoshis,
+		    sighash_type, flags, dest->sha.u.u8, sizeof(*dest));
+		assert(ret == WALLY_OK);
+	}
+}
+
 void sign_tx_input(const struct bitcoin_tx *tx,
 		   unsigned int in,
 		   const u8 *subscript,
@@ -98,16 +124,12 @@ void sign_tx_input(const struct bitcoin_tx *tx,
 {
 	struct sha256_double hash;
 	bool use_segwit = witness_script != NULL;
-	int flags = use_segwit ? WALLY_TX_FLAG_USE_WITNESS : 0;
 	const u8 *script = use_segwit ? witness_script : subscript;
 
 	assert(sighash_type_valid(sighash_type));
 
 	sig->sighash_type = sighash_type;
-	wally_tx_get_btc_signature_hash(
-	    tx->wtx, in, script, tal_bytelen(script),
-	    tx->input_amounts[in]->satoshis /* Raw: low-level helper */,
-	    sighash_type, flags, hash.sha.u.u8, sizeof(hash));
+	bitcoin_tx_hash_for_sig(tx, in, script, sighash_type, &hash);
 
 	dump_tx("Signing", tx, in, subscript, key, &hash);
 	sign_hash(privkey, &hash, &sig->s);
@@ -133,7 +155,6 @@ bool check_tx_sig(const struct bitcoin_tx *tx, size_t input_num,
 {
 	struct sha256_double hash;
 	bool use_segwit = witness_script != NULL;
-	int flags = use_segwit ? WALLY_TX_FLAG_USE_WITNESS : 0;
 	const u8 *script = use_segwit ? witness_script : redeemscript;
 	bool ret;
 
@@ -146,10 +167,8 @@ bool check_tx_sig(const struct bitcoin_tx *tx, size_t input_num,
 	}
 	assert(input_num < tx->wtx->num_inputs);
 
-	wally_tx_get_btc_signature_hash(
-	    tx->wtx, input_num, script, tal_bytelen(script),
-	    tx->input_amounts[input_num]->satoshis /* Raw: low-level helper */,
-	    sig->sighash_type, flags, hash.sha.u.u8, sizeof(hash));
+	dump_tx("check_tx_sig", tx, input_num, script, key, &hash);
+	bitcoin_tx_hash_for_sig(tx, input_num, script, sig->sighash_type, &hash);
 
 	ret = check_signed_hash(&hash, &sig->s, key);
 	if (!ret)