Fixed DER-encoding - expects signed integers.

Thanks to Ben Reeves for the report and Tomas Pomin for the solution.

See http://crypto.stackexchange.com/questions/1795/converting-a-der-ecdsa-signature-to-asn-1

src/ecdsa.js

@@ -305,8 +305,8 @@ Bitcoin.ECDSA = (function () {
      * Takes two BigIntegers representing r and s and returns a byte array.
      */
     serializeSig: function (r, s) {
-      var rBa = r.toByteArrayUnsigned();
+      var rBa = r.toByteArraySigned();
-      var sBa = s.toByteArrayUnsigned();
+      var sBa = s.toByteArraySigned();
 
       var sequence = [];
       sequence.push(0x02); // INTEGER
@@ -327,7 +327,7 @@ Bitcoin.ECDSA = (function () {
      * Parses a byte array containing a DER-encoded signature.
      *
      * This function will return an object of the form:
-     * 
+     *
      * {
      *   r: BigInteger,
      *   s: BigInteger

src/util.js

@@ -1,7 +1,14 @@
 // BigInteger monkey patching
 BigInteger.valueOf = nbv;
+
+/**
+ * Returns a byte array representation of the big integer.
+ *
+ * This returns the absolute of the contained value in big endian
+ * form. A value of zero results in an empty array.
+ */
 BigInteger.prototype.toByteArrayUnsigned = function () {
-  var ba = this.toByteArray();
+  var ba = this.abs().toByteArray();
   if (ba.length) {
     if (ba[0] == 0) {
       ba = ba.slice(1);
@@ -14,6 +21,13 @@ BigInteger.prototype.toByteArrayUnsigned = function () {
     return ba;
   }
 };
+
+/**
+ * Turns a byte array into a big integer.
+ *
+ * This function will interpret a byte array as a big integer in big
+ * endian notation and ignore leading zeros.
+ */
 BigInteger.fromByteArrayUnsigned = function (ba) {
   if (!ba.length) {
     return ba.valueOf(0);
@@ -26,6 +40,66 @@ BigInteger.fromByteArrayUnsigned = function (ba) {
   }
 };
 
+/**
+ * Converts big integer to signed byte representation.
+ *
+ * The format for this value uses a the most significant bit as a sign
+ * bit. If the most significant bit is already occupied by the
+ * absolute value, an extra byte is prepended and the sign bit is set
+ * there.
+ *
+ * Examples:
+ *
+ *      0 =>     0x00
+ *      1 =>     0x01
+ *     -1 =>     0x81
+ *    127 =>     0x7f
+ *   -127 =>     0xff
+ *    128 =>   0x0080
+ *   -128 =>   0x8080
+ *    255 =>   0x00ff
+ *   -255 =>   0x80ff
+ *  16300 =>   0x3fac
+ * -16300 =>   0xbfac
+ *  62300 => 0x00f35c
+ * -62300 => 0x80f35c
+ */
+BigInteger.prototype.toByteArraySigned = function () {
+  var val = this.abs().toByteArrayUnsigned();
+  var neg = this.compareTo(BigInteger.ZERO) < 0;
+
+  if (neg) {
+    if (val[0] & 0x80) {
+      val.unshift(0x80);
+    } else {
+      val[0] |= 0x80;
+    }
+  } else {
+    if (val[0] & 0x80) {
+      val.unshift(0x00);
+    }
+  }
+
+  return val;
+};
+
+/**
+ * Parse a signed big integer byte representation.
+ *
+ * For details on the format please see BigInteger.toByteArraySigned.
+ */
+BigInteger.fromByteArraySigned = function (ba) {
+  // Check for negative value
+  if (ba[0] & 0x80) {
+    // Remove sign bit
+    ba[0] &= 0x7f;
+
+    return BigInteger.fromByteArrayUnsigned(ba).negate();
+  } else {
+    return BigInteger.fromByteArrayUnsigned(ba);
+  }
+};
+
 // Console ignore
 var names = ["log", "debug", "info", "warn", "error", "assert", "dir",
              "dirxml", "group", "groupEnd", "time", "timeEnd", "count",