code: function CBC(blockcipher, cipherkeybuf, ivbuf) {
    if (!(this instanceof CBC)) {
      return new CBC(blockcipher, cipherkeybuf, ivbuf);
    }
  
    this.blockcipher = blockcipher;
    this.cipherkeybuf = cipherkeybuf;
    this.ivbuf = ivbuf;
  }
docs: Cipher Block Chaining http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher-block_chaining_.28CBC.29