An inverse-free single-keyed tweakable enciphering scheme

Abstract

In CRYPTO 2003, Halevi and Rogaway proposed CMC, a tweakable enciphering scheme (TES) based on a blockcipher. It requires two blockcipher keys and it is not inverse-free (i.e., the decryption algorithm uses the inverse (decryption) of the underlying blockcipher). We present here a new inverse-free, single-keyed TES. Our construction is a tweakable strong pseudorandom permutation (TSPRP), i.e., it is secure against chosen-plaintext-ciphertext adversaries assuming that the underlying blockcipher is a pseudorandom permutation (PRP), i.e., secure against chosen-plaintext adversaries. In comparison, SPRP assumption of the blockcipher is required for the TSPRP security of CMC. Our scheme can be viewed as a mixture of type-1 and type-3 Feistel cipher and so we call it FMix or mixed-type Feistel cipher.