Recommendation for Block Cipher Modes of Operation: Three Variants of Ciphertext Stealing for CBC Mode

A limitation to Cipher Block Chaining (CBC) mode, as specified in NIST Special Publication 800-38A, is that the plaintext input must consist of a sequence of blocks. Ciphertext stealing is a padding method in which the required padding bits are "stolen" from the penultimate ciphertext block. This addendum to SP 800-38A specifies three variants of CBC mode with ciphertext stealing. These variants, which differ only in the ordering of the ciphertext bits, can encrypt any input whose bit length is greater than or equal to the block size. Unlike conventional padding methods, these variants do not expand the length of the data.