One-key MAC

One-key MAC, usually shortened to OMAC, is a family of message authentication codes built from a block cipher in the manner of CBC-MAC. A message authentication code lets a receiver check that data came from a holder of the shared secret key and was not altered on the way. Two versions are defined. The original OMAC of February 2003 is rarely used and is now called OMAC2, while the refinement OMAC1 became a NIST recommendation in May 2005 under the name CMAC. The design grew out of XCBC, which Black and Rogaway proposed to repair the security weaknesses of CBC-MAC at the cost of needing three keys. Iwata and Kurosawa then reduced the key material to a single key and named the result One-Key CBC-MAC. To tag a message, the algorithm first derives two subkeys from the encryption of a zero block, shifting the value and conditionally combining it with a constant that depends only on the block size. It then chains the cipher over the message blocks and mixes one of the two subkeys into the final block, choosing which according to whether that block is complete. OMAC is not covered by any patents and is free for all uses. The AES form is specified for Internet use in RFC 4493.