Pad implements PKCS#7 padding as defined in RFC2315. It pads the data to
the given blockSize in the range [1, 255]. This is normally used in AES-CBC
encryption.
Unpad implements PKCS#7 unpadding as defined in RFC2315. It unpads the
data by reading the padding amount from the last byte of the data. This is
normally used in AES-CBC decryption.