OCB mode

OCB mode (Offset Codebook Mode) is an authenticated encryption mode of operation for cryptographic block ciphers. OCB mode was designed by Phillip Rogaway, who credits Mihir Bellare, John Black, and Ted Krovetz with assistance and comments on the designs. It is based on the authenticated encryption mode IAPM due to Charanjit S. Jutla.

OCB mode was designed to provide both message authentication and privacy. It is essentially a scheme for integrating a Message Authentication Code (MAC) into the operation of a block cipher. In this way, OCB mode avoids the need to use two systems: a MAC for authentication and encryption for privacy. This results in lower computational cost compared to using separate encryption and authentication functions.

There are three versions of OCB: OCB1, OCB2 and OCB3. OCB1 was published in 2001. OCB2 improves on OCB1 by allowing associated data to be included with the message (providing AEAD) — that is, data that are not encrypted but should be authenticated — and a new method for generating a sequence of offsets. OCB2 was first published in 2003, originally named AEM (Authenticated-Encryption Mode, or Advanced Encryption Mode). OCB3, published in 2011, changes again the way offsets are computed and introduces minor performance improvements.

OCB mode is listed as an optional method in the IEEE 802.11 wireless security standard as an alternative to CCM. OCB2 is standardized in ISO/IEC 19772:2009 and a modified OCB3 in RFC 7253. The RFC encodes the tag length into the internally formatted nonce.

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