decrypt block

lib/expmt/cbc.js

@@ -1,5 +1,6 @@
 var Random = require('../random');
 
+// Cipher Block Chaining
 // http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher-block_chaining_.28CBC.29
 var CBC = function CBC(blockcipher, cipherkeybuf, ivbuf) {
   if (!(this instanceof CBC))
@@ -40,6 +41,12 @@ CBC.encryptblock = function(blockbuf, ivbuf, blockcipher, cipherkeybuf) {
   return encbuf;
 };
 
+CBC.decryptblock = function(encbuf, ivbuf, blockcipher, cipherkeybuf) {
+  var xorbuf = blockcipher.decrypt(encbuf, cipherkeybuf);
+  var blockbuf = CBC.xorbufs(xorbuf, ivbuf);
+  return blockbuf;
+};
+
 CBC.encryptblocks = function(blockbufs, ivbuf, blockcipher, cipherkeybuf) {
   var encbufs = [];
 
@@ -55,6 +62,21 @@ CBC.encryptblocks = function(blockbufs, ivbuf, blockcipher, cipherkeybuf) {
   return encbufs;
 };
 
+CBC.decryptblocks = function(encbufs, ivbuf, blockcipher, cipherkeybuf) {
+  var blockbufs = [];
+
+  for (var i = 0; i < encbufs.length; i++) {
+    var encbuf = encbufs[i];
+    var blockbuf = CBC.decryptblock(encbuf, ivbuf, blockcipher, cipherkeybuf);
+
+    blockbufs.push(blockbuf);
+
+    ivbuf = encbuf;
+  }
+
+  return encbufs;
+};
+
 CBC.pkcs7pad = function(buf, blocksize) {
   var bytesize = blocksize / 8;
   var padbytesize = bytesize - buf.length;

test/test.cbc.js

@@ -90,6 +90,29 @@ describe('CBC', function() {
 
   });
   
+  describe('@decryptblock', function() {
+
+    it('should decrypt an encrypted block', function() {
+      var messagebuf = new Buffer(128 / 8);
+      messagebuf.fill(0);
+      var ivbuf = new Buffer(128 / 8);
+      ivbuf.fill(0x10);
+      var cipherkeybuf = new Buffer(128 / 8);
+      cipherkeybuf.fill(0);
+      var blockcipher = {};
+      blockcipher.encrypt = function(messagebuf, cipherkeybuf) {
+        return messagebuf;
+      };
+      blockcipher.decrypt = function(messagebuf, cipherkeybuf) {
+        return messagebuf;
+      };
+      var encbuf = CBC.encryptblock(messagebuf, ivbuf, blockcipher, cipherkeybuf);
+      var buf = CBC.decryptblock(encbuf, ivbuf, blockcipher, cipherkeybuf);
+      buf.toString('hex').should.equal(messagebuf.toString('hex'));
+    });
+
+  });
+  
   describe('@encryptblocks', function() {
 
     it('should return this known value', function() {