A Tweakable Enciphering Mode

We describe a block-cipher mode of operation, CMC, that turns an n-bit block cipher intoa tweakable enciphering scheme that acts on strings of mn bits, where m ≥ 2. When theunderlying block cipher is secure in the sense of a strong pseudorandom permutation (PRP),our scheme is secure in the sense of tweakable, strong PRP. Such an object can be used toencipher the sectors of a disk, in-place, offering security as good as can be obtained in thissetting. CMC makes a pass of CBC encryption, xors in a mask, and then makes a pass of CBCdecryption; no universal hashing, nor any other non-trivial operation beyond the block-ciphercalls, is employed. Besides proving the security of CMC we initiate a more general investigationof tweakable enciphering schemes, considering issues like the non-malleability of these objects.