OCB mode
From Wikipedia, the free encyclopedia
OCB mode (Offset Codebook Mode) is a mode of operation for cryptographic block ciphers.
Contents |
[edit] Encryption and authentication
It was designed to provide both 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 a block cipher encryption for privacy. This results in lower computational cost compared to the application of separate encryption and authentication functions.
OCB mode was designed by Phillip Rogaway, who credits Mihir Bellare, John Black, and Ted Krovetz with assistance and comment on the designs. It is based on the authenticated encryption mode IAPM due to Charanjit S. Jutla (see the OCB FAQ for more details).
There are two versions of OCB: 1.0 and 2.0. OCB 2.0 improves on 1.0 by allowing associated data to be included with the message — that is, data that are not encrypted but should be authenticated — and a new method for generating a sequence of offsets. OCB 2.0 was first published in 2003, originally named AEM (Authenticated-Encryption Mode, or Advanced Encryption Mode).
OCB mode is listed as an optional method in the IEEE 802.11 wireless security standard as an alternative to CCM.
[edit] Patents
A patent application is in place for OCB mode. However, a special exemption has been granted so that OCB mode can be used in software licensed under the GNU General Public License without cost, as well as in software not developed and not sold inside the United States [1].
[edit] Performance
OCB performance overhead is minimal comparing to classical, non-authenticating modes like CBC. OCB requires one block cipher encryption per each block of encrypted and authenticated message and one encryption per each block of additional associated data. There are also two extra encryptions required at the end of process.
For comparison, CCM mode offering similar functionality requires twice as many encryptions per each message block (associated data requires one as in OCB).
[edit] See also
[edit] External links
[edit] References
- Phillip Rogaway, Mihir Bellare, John Black. OCB: A block-cipher mode of operation for efficient authenticated encryption, ACM Transactions on Information and System Security (TISSEC), Volume 6, Issue 3, pp.365-403. August 2003.
- Charanjit S. Jutla, "Encryption Modes with Almost Free Message Integrity", Proc. Eurocrypt 2001, LNCS 2045, May 2001.
