ABSTRACT OCB: A Block-Cipher Mode of Operation for Efficient Authenticated Encryption ∗

We describe a parallelizable block-cipher mode of operation that simultaneously provides privacy and authenticity. OCB encrypts-and-authenticates a nonempty string M ∈{0, 1} ∗ using ⌈|M|/n ⌉ + 2block-cipher invocations, where n is the block length of the underlying block cipher. Additional overhead is small. OCB refines a scheme, IAPM, suggested by Charanjit Jutla. Desirable properties of OCB include: the ability to encrypt a bit string of arbitrary length into a ciphertext of minimal length; cheap offset calculations; cheap key setup; a single underlying cryptographic key; no extended-precision addition; a nearly optimal number of block-cipher calls; and no requirement for a random IV. We prove OCB secure, quantifying the adversary’s ability to violate the mode’s privacy or authenticity in terms of the quality of its block cipher as a pseudorandom permutation (PRP) or as a strong PRP, respectively. ∗ A full version of this paper is available as [33]