Cryptography

Encrypting plaintext using Advanced Encryption Standard (AES) in Cipher Block Chaining (CBC) mode requires the plaintext to be a multiple of 16 bytes long. All plaintext is padded before encryption. The padding method is described in Section 6.3 of PKCS #7.

The Padding Technique

If the last block of the plaintext message has 1 byte (e.g., if the plaintext is 17, 33, or 49 bytes), then 15 bytes of the value 0x0f (15) are added to the end of the plaintext. Finding the number of bytes in the last block is trivial using modulo math in python.