Pad implements PKCS#7 padding as defined in RFC2315. It pads the plaintext
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
plaintext by reading the padding amount from the last byte of the plaintext.
This is normally used in AES-CBC decryption.