unpad

lib/expmt/cbc.js

@@ -82,7 +82,20 @@ CBC.pkcs7pad = function(buf, blocksize) {
   var padbytesize = bytesize - buf.length;
   var pad = new Buffer(padbytesize);
   pad.fill(padbytesize);
-  return Buffer.concat([buf, pad]);
+  var paddedbuf = Buffer.concat([buf, pad]);
+  return paddedbuf;
+};
+
+CBC.pkcs7unpad = function(paddedbuf, blocksize) {
+  var bytesize = blocksize / 8;
+  var padbytesize = bytesize - paddedbuf.length;
+  var padlength = paddedbuf[paddedbuf.length - 1];
+  var padbuf = paddedbuf.slice(paddedbuf.length - padlength, paddedbuf.length);
+  var padbuf2 = new Buffer(padlength);
+  padbuf2.fill(padlength);
+  if (padbuf.toString('hex') !== padbuf2.toString('hex'))
+    throw new Error('invalid padding');
+  return paddedbuf.slice(0, paddedbuf.length - padlength);
 };
 
 CBC.xorbufs = function(buf1, buf2) {

test/test.cbc.js

@@ -176,6 +176,18 @@ describe('CBC', function() {
 
   });
 
+  describe('@pkcs7unpad', function() {
+    
+    it('should unpad this padded 32 bit buffer', function() {
+      var buf = new Buffer(32 / 8);
+      buf.fill(0);
+      var paddedbuf = CBC.pkcs7pad(buf, 128);
+      var unpaddedbuf = CBC.pkcs7unpad(paddedbuf, 128);
+      unpaddedbuf.toString('hex').should.equal(buf.toString('hex'));
+    });
+
+  });
+
   describe('@xorbufs', function() {
     
     it('should xor 1 and 0', function() {