refact(jose-to-der) unroll/manually encode der

10 times faster on my machine, given we can be very specific and avoid
copies and allocations

src/ecdsa-sig-formatter.js

@@ -1,7 +1,6 @@
 'use strict';
 
 var asn1 = require('asn1.js'),
- BN = asn1.bignum,
  base64Url = require('base64-url').escape;
 
 var ECDSASigValue = asn1.define('ECDSASigValue', function () {
@@ -11,6 +10,9 @@ var ECDSASigValue = asn1.define('ECDSASigValue', function () {
  );
 });
 
+var seq = 0x10,
+ int = 0x02;
+
 function getParamSize (keySize) {
  var result = ((keySize / 8) | 0) + (keySize % 8 === 0 ? 0 : 1);
  return result;
@@ -31,11 +33,6 @@ function getParamBytesForAlg (alg) {
  throw new Error('Unknown algorithm "' + alg + '"');
 }
 
-function bufToBignum (buf) {
- var bn = new BN(buf, 10, 'be').iabs();
- return bn;
-}
-
 function bignumToBuf (bn, numBytes) {
  var buf = new Buffer(bn.toString('hex', numBytes), 'hex');
  return buf;
@@ -67,6 +64,34 @@ function derToJose(signature, alg) {
  return signature;
 }
 
+function reduceBuffer (buf) {
+ var padding = 0;
+ for (var n = buf.length; padding < n && buf[padding] === 0;) {
+ ++padding;
+ }
+
+ var needsSign = buf[padding] >= 0x80;
+ if (needsSign) {
+ --padding;
+
+ if (padding < 0) {
+ var old = buf;
+ buf = new Buffer(1 + buf.length);
+ buf[0] = 0;
+ old.copy(buf, 1);
+
+ return buf;
+ }
+ }
+
+ if (padding === 0) {
+ return buf;
+ }
+
+ buf = buf.slice(padding);
+ return buf;
+}
+
 function joseToDer(signature, alg) {
  signature = signatureAsBuffer(signature);
  var paramBytes = getParamBytesForAlg(alg);
@@ -76,15 +101,30 @@ function joseToDer(signature, alg) {
  throw new TypeError('"' + alg + '" signatures must be "' + paramBytes * 2 + '" bytes, saw "' + signatureBytes + '"');
  }
 
- var r = signature.slice(0, paramBytes);
- r = bufToBignum(r);
- var s = signature.slice(paramBytes);
- s = bufToBignum(s);
+ var r = reduceBuffer(signature.slice(0, paramBytes));
+ var s = reduceBuffer(signature.slice(paramBytes));
 
- signature = ECDSASigValue.encode({
- r: r,
- s: s
- }, 'der');
+ var rsBytes = 1 + 1 + r.length + 1 + 1 + s.length;
+
+ var oneByteLength = rsBytes < 0x80;
+
+ signature = new Buffer((oneByteLength ? 2 : 3) + rsBytes);
+
+ var offset = 0;
+ signature[offset++] = (seq | 0x20) | 0 << 6;
+ if (oneByteLength) {
+ signature[offset++] = rsBytes;
+ } else {
+ signature[offset++] = 0x80 | 1;
+ signature[offset++] = rsBytes & 0xff;
+ }
+ signature[offset++] = int | (0 << 6);
+ signature[offset++] = r.length;
+ r.copy(signature, offset);
+ offset += r.length;
+ signature[offset++] = int | (0 << 6);
+ signature[offset++] = s.length;
+ s.copy(signature, offset);
 
  return signature;
 }