MARS (cipher)

MARS is a symmetric block cipher that IBM submitted to the Advanced Encryption Standard selection process run by the United States standards agency NIST. It was voted the fifth and last finalist at the second AES conference in March 1999 and was named a finalist in August 1999, alongside four other candidate designs. The IBM design team included Don Coppersmith, who had also taken part in creating the earlier Data Encryption Standard about twenty years before. MARS works on a 128-bit block and accepts a variable key length from 128 to 448 bits in steps of 32 bits. Its structure is unusual for a block cipher because it is heterogeneous: several rounds of a keyed cryptographic core are wrapped, or jacketed, by unkeyed mixing rounds together with key whitening. The designers said they chose this layered, compartmentalized approach to resist future advances in cryptanalysis, and IBM reported that MARS and Serpent were the only two finalists to build in an explicit safety net of that kind. Published analysis found that subkeys with long runs of ones or zeroes may allow efficient attacks, and that a meet-in-the-middle attack by John Kelsey and Bruce Schneier in 2004 could break 21 of its 32 rounds.