Cryptology ePrint Archive: Report 2001/026

OCB Mode

Phillip Rogaway and Mihir Bellare and John Black and Ted Krovetz

Abstract: This paper was prepared for NIST, which is considering new block-cipher modes of operation. It describes a parallelizable mode of operation that simultaneously provides both privacy and authenticity. "OCB mode" encrypts-and-authenticates an arbitrary message $M\in\bits^*$ using only $\lceil |M|/n\rceil + 2$ 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 Jutla [IACR-2000/39], who was the first to devise an authenticated-encryption mode with minimal overhead compared to standard modes. Desirable new properties of OCB include: very cheap offset calculations; operating on an arbitrary message $M\in\bits^*$; producing ciphertexts of minimal length; using a single underlying cryptographic key; making a nearly optimal number of block-cipher calls; avoiding the need for a random IV; and rendering it infeasible for an adversary to find "pretag collisions". The paper provides a full proof of security for OCB.

Category / Keywords: secret-key cryptography / AES, secret-key cryptography, modes of operation

Publication Info: unpublished NIST submission

Date: received 1 Apr 2001, last revised 18 Apr 2001

Contact author: rogaway at cs ucdavis edu

Available format(s): Postscript (PS) | Compressed Postscript (PS.GZ) | PDF | BibTeX Citation

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