adhere more closely to BIP66

src/ecsignature.js

@@ -32,39 +32,63 @@ ECSignature.parseCompact = function (buffer) {
   }
 }
 
+// Strict DER - https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki
+// NOTE: SIGHASH byte ignored
 ECSignature.fromDER = function (buffer) {
-  assert.equal(buffer.readUInt8(0), 0x30, 'Not a DER sequence')
-  assert.equal(buffer.readUInt8(1), buffer.length - 2, 'Invalid sequence length')
-  assert.equal(buffer.readUInt8(2), 0x02, 'Expected a DER integer')
+  // Minimum and maximum size constraints.
+  if (buffer.length < 9) throw new Error('Invalid sequence length')
+  if (buffer.length > 73) throw new Error('Invalid sequence length')
 
-  var rLen = buffer.readUInt8(3)
-  assert(rLen > 0, 'R length is zero')
+  // A signature is of type 0x30 (compound).
+  if (buffer[0] !== 0x30) throw new Error('Not a DER sequence')
 
-  var offset = 4 + rLen
-  assert.equal(buffer.readUInt8(offset), 0x02, 'Expected a DER integer (2)')
+  // Make sure the length covers the entire signature.
+  if (buffer[1] !== buffer.length - 2) throw new Error('Invalid sequence length')
 
-  var sLen = buffer.readUInt8(offset + 1)
-  assert(sLen > 0, 'S length is zero')
+  // Check whether the R element is an integer.
+  if (buffer[2] !== 0x02) throw new Error('Expected a DER integer')
 
-  var rB = buffer.slice(4, offset)
-  var sB = buffer.slice(offset + 2)
-  offset += 2 + sLen
+  // Extract the length of the R element.
+  var lenR = buffer.readUInt8(3)
 
-  if (rLen > 1 && rB.readUInt8(0) === 0x00) {
-    assert(rB.readUInt8(1) & 0x80, 'R value excessively padded')
-  }
+  // Zero-length integers are not allowed for R.
+  if (lenR === 0) throw new Error('R length is zero')
 
-  if (sLen > 1 && sB.readUInt8(0) === 0x00) {
-    assert(sB.readUInt8(1) & 0x80, 'S value excessively padded')
-  }
+  // Make sure the length of the R element is still inside the signature.
+  if (5 + lenR >= buffer.length) throw new Error('Invalid DER encoding')
 
-  assert.equal(offset, buffer.length, 'Invalid DER encoding')
+  // Check whether the S element is an integer.
+  if (buffer[4 + lenR] !== 0x02) throw new Error('Expected a DER integer (2)')
+
+  var lenS = buffer[5 + lenR]
+
+  // Zero-length integers are not allowed for S.
+  if (lenS === 0) throw new Error('S length is zero')
+
+  // Verify that the length of the signature matches the sum of the length
+  // of the elements.
+  if ((lenR + lenS + 6) !== buffer.length) throw new Error('Invalid DER encoding (2)')
+
+  // Negative numbers are not allowed for R.
+  if (buffer[4] & 0x80) throw new Error('R value is negative')
+
+  // Null bytes at the start of R are not allowed, unless R would
+  // otherwise be interpreted as a negative number.
+  if (lenR > 1 && (buffer[4] === 0x00) && !(buffer[5] & 0x80)) throw new Error('R value excessively padded')
+
+  // Negative numbers are not allowed for S.
+  if (buffer[lenR + 6] & 0x80) throw new Error('S value is negative')
+
+  // Null bytes at the start of S are not allowed, unless S would otherwise be
+  // interpreted as a negative number.
+  if (lenS > 1 && (buffer[lenR + 6] === 0x00) && !(buffer[lenR + 7] & 0x80)) throw new Error('S value excessively padded')
+
+  // non-BIP66 - extract R, S values
+  var rB = buffer.slice(4, 4 + lenR)
+  var sB = buffer.slice(lenR + 6)
   var r = BigInteger.fromDERInteger(rB)
   var s = BigInteger.fromDERInteger(sB)
 
-  assert(r.signum() >= 0, 'R value is negative')
-  assert(s.signum() >= 0, 'S value is negative')
-
   return new ECSignature(r, s)
 }
 

test/ecsignature.js

@@ -66,7 +66,7 @@ describe('ECSignature', function () {
     })
 
     fixtures.invalid.DER.forEach(function (f) {
-      it('throws on ' + f.hex, function () {
+      it('throws "' + f.exception + '" for ' + f.hex, function () {
         var buffer = new Buffer(f.hex, 'hex')
 
         assert.throws(function () {

test/fixtures/ecsignature.json

@@ -147,7 +147,11 @@
       },
       {
         "exception": "Invalid DER encoding",
-        "hex": "300c020400ffffff020200ffffff"
+        "hex": "300c02040fffffff020200ffffff"
+      },
+      {
+        "exception": "Invalid DER encoding \\(2\\)",
+        "hex": "300c020400ffffff020800ffffff"
       },
       {
         "exception": "R length is zero",