Residual block termination

In cryptography, residual block termination is a variation of Cipher Block Chaining (CBC) that does not require any padding, at the cost of increased complexity.

Encryption procedure

• If the plaintext length is not a multiple of the block size
• Encrypt the last encrypted block again.
• XOR the incomplete block with leftmost bits of the re-encrypted block.

Decryption procedure

• Decrypt the ciphertext using the Cipher Block Chaining mode.
• Encrypt the last full encrypted block
• XOR the incomplete block with leftmost bits of the re-encrypted block.

Proof that this decryption procedure really is the inverse of the encryption procedure is left as an exercise for the reader.

Short message

For messages shorter than one block residual block termination can use an encrypted IV instead of previously encrypted block.