Add sighashPreimage function

lib/transaction/sighash.js

@@ -17,16 +17,9 @@ var SIGHASH_SINGLE_BUG = '000000000000000000000000000000000000000000000000000000
 var BITS_64_ON = 'ffffffffffffffff';
 
 /**
- * Returns a buffer of length 32 bytes with the hash that needs to be signed
- * for OP_CHECKSIG.
- *
- * @name Signing.sighash
- * @param {Transaction} transaction the transaction to sign
- * @param {number} sighashType the type of the hash
- * @param {number} inputNumber the input index for the signature
- * @param {Script} subscript the script that will be signed
+ * Returns the preimage to be signed
  */
-var sighash = function sighash(transaction, sighashType, inputNumber, subscript) {
+var sighashPreimage = function sighash(transaction, sighashType, inputNumber, subscript) {
   var Transaction = require('./transaction');
   var Input = require('./input');
 
@@ -79,11 +72,31 @@ var sighash = function sighash(transaction, sighashType, inputNumber, subscript)
   if (sighashType & Signature.SIGHASH_ANYONECANPAY) {
     txcopy.inputs = [txcopy.inputs[inputNumber]];
   }
-
-  var buf = new BufferWriter()
+  return new BufferWriter()
     .write(txcopy.toBuffer())
     .writeInt32LE(sighashType)
     .toBuffer();
+};
+
+/**
+ * Returns a buffer of length 32 bytes with the hash that needs to be signed
+ * for OP_CHECKSIG.
+ *
+ * @name Signing.sighash
+ * @param {Transaction} transaction the transaction to sign
+ * @param {number} sighashType the type of the hash
+ * @param {number} inputNumber the input index for the signature
+ * @param {Script} subscript the script that will be signed
+ */
+var sighash = function sighash(transaction, sighashType, inputNumber, subscript) {
+  if ((sighashType & 31) === Signature.SIGHASH_SINGLE) {
+    // The SIGHASH_SINGLE bug.
+    // https://bitcointalk.org/index.php?topic=260595.0
+    if (inputNumber >= transaction.outputs.length) {
+      return new Buffer(SIGHASH_SINGLE_BUG, 'hex');
+    }
+  }
+  var buf = sighashPreimage(transaction, sighashType, inputNumber, subscript);
   var ret = Hash.sha256sha256(buf);
   ret = new BufferReader(ret).readReverse();
   return ret;
@@ -132,5 +145,6 @@ function verify(transaction, signature, publicKey, inputIndex, subscript) {
 module.exports = {
   sighash: sighash,
   sign: sign,
+  sighashPreimage: sighashPreimage,
   verify: verify
 };

test/transaction/sighash.js

@@ -8,6 +8,8 @@ var bitcore = require('../../');
 var Script = bitcore.Script;
 var Transaction = bitcore.Transaction;
 var sighash = Transaction.sighash;
+var BufferUtils = bitcore.util.buffer;
+var Signature = bitcore.crypto.Signature;
 
 var vectors_sighash = require('../data/sighash.json');
 
@@ -28,10 +30,27 @@ describe('sighash', function() {
       var tx = new Transaction(txbuf);
 
       //make sure transacion to/from buffer is isomorphic
-      tx.uncheckedSerialize().should.equal(txbuf.toString('hex'));
+      tx.uncheckedSerialize().should.equal(vector[0]);
 
       //sighash ought to be correct
-      sighash.sighash(tx, nhashtype, nin, subscript).toString('hex').should.equal(sighashbuf.toString('hex'));
+      BufferUtils.equal(sighash.sighash(tx, nhashtype, nin, subscript), sighashbuf).should.be.true;
     });
   });
+
+  it('sighashPreimage handles correctly the SIGHASH_SINGLE bug', function() {
+
+    // Use the first transaction from vectors_sighash as template
+    var faketx = new Transaction(vectors_sighash[1][0]);
+
+    // Clear all outputs
+    faketx.outputs = [];
+    var script = new Script();
+
+    // Calculate preimage with the SIGHASH_SINGLE bug
+    var preimage = sighash.sighashPreimage(faketx, Signature.SIGHASH_SINGLE, 0, script)
+
+    var SIGHASH_SINGLE_BUG_RESULT = '0000000000000000000000000000000000000000000000000000000000000001';
+
+    preimage.toString('hex').should.equal(SIGHASH_SINGLE_BUG_RESULT)
+  });
 });