fix(ecdh): allow base64 to be left-0-padded, needed for JWK format

Fixes #97

src/crypto/ecdh-browser.js

@@ -117,8 +117,8 @@ function unmarshalPublicKey (curve, key) {
   return {
     kty: 'EC',
     crv: curve,
-    x: toBase64(x),
-    y: toBase64(y),
+    x: toBase64(x, byteLen),
+    y: toBase64(y, byteLen),
     ext: true
   }
 }

src/crypto/util.js

@@ -5,8 +5,9 @@ const Buffer = require('safe-buffer').Buffer
 
 // Convert a BN.js instance to a base64 encoded string without padding
 // Adapted from https://tools.ietf.org/html/draft-ietf-jose-json-web-signature-41#appendix-C
-exports.toBase64 = function toBase64 (bn) {
-  let s = bn.toArrayLike(Buffer, 'be').toString('base64')
+exports.toBase64 = function toBase64 (bn, len) {
+  // if len is defined then the bytes are leading-0 padded to the length
+  let s = bn.toArrayLike(Buffer, 'be', len).toString('base64')
 
   return s
     .replace(/(=*)$/, '') // Remove any trailing '='s