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

Abstract

We describe a parallelizable block-cipher mode of operation that simultaneously provides privacy and authenticity. OCB encrypts-and-authenticates a nonempty string Mϵ{0,1}* using [|M|/n]+2 block-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. © 2003, ACM. All rights reserved.