OCB mode
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OCB mode (Offset Codebook Mode) is a mode of operation for cryptographic block ciphers.
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[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 simplifies the secure communication process for end users.
OCB mode was designed by Phillip Rogaway, who credits Mihir Bellare, John Black, and Ted Krovetz with assistance and comment on the designs.
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 two also extra encryptions added once at the end of process.
For comparison, CCM mode offering similar functionality requires twice as more encryptions per each message block (associated data requires one as in OCB).
