function addPadding(input) {
  var paddingLength = (16 - (input.length % 16)) % 16;
  for (var i = 0; i < paddingLength; i++) {
    input.push(0);
  }
  return input;
}
Helper function to pad block for easier array byte array comparison.
This will pad the input to reach the closest multiple of a block.
@param {!Array<number>} input The input byte array.
@return {!Array<number>} The padded input.