CLOC: Authenticated Encryption for Short Input

We define and analyze the security of a blockcipher mode of operation, \(\mathrm {CLOC}\), for provably secure authenticated encryption with associated data. The design of \(\mathrm {CLOC}\) aims at optimizing previous schemes, CCM, EAX, and EAX-prime, in terms of the implementation overhead beyond the blockcipher, the precomputation complexity, and the memory requirement. With these features, \(\mathrm {CLOC}\) is suitable for handling short input data, say 16 bytes, without needing precomputation nor large memory. This property is especially beneficial to small microprocessors, where the word size is typically 8 bits or 16 bits, and there are significant restrictions in the size and the number of registers. \(\mathrm {CLOC}\) uses a variant of CFB mode in its encryption part and a variant of CBC MAC in the authentication part. We introduce various design techniques in order to achieve the above mentioned design goals. We prove \(\mathrm {CLOC}\) secure, in a reduction-based provable security paradigm, under the assumption that the blockcipher is a pseudorandom permutation. We also present our preliminary implementation results.