code: function rightXorMut(data, padding) {
    const offset = data.length - blockLength;
    for (let i = 0; i < blockLength; i++) {
      data[i + offset] ^= padding[i];
    }
    return data;
  }
docs: xor `padding` into the end of `data`. This function implements "the
  operation xor→ [which] xors the shorter string into the end of longer
  one". Since data is always as least as long as padding, we can
  simplify the implementation.
  @param {Uint8Array} data
  @param {Uint8Array} padding