code: function calculate(digest) {
    var offset = digest[19] & 0xf;
    var code = (digest[offset] & 0x7f) << 24 | (digest[offset + 1] & 0xff) << 16 | (digest[offset + 2] & 0xff) << 8 | (digest[offset + 3] & 0xff);
    return code.toString();
  }
docs: Computes a htop code based on a hmac result
  See: http://tools.ietf.org/html/rfc4226#section-5.4
  
  @param     {String}    digest
  @return    {String}