Efficient Constructions of Variable-Input-Length Block Ciphers

Abstract

Existing block ciphers operate on a fixed-input-length (FIL) block size (e.g., 64-bits for DES). Often, one needs a variable-input-length (VIL) primitive that can operate on a different size input; it is, however, undesirable to construct this primitive from “scratch.” This paper contains two constructions that start with a fixed-input-length block cipher and show how to securely convert it to a variable-input-length block cipher without making any additional cryptographic assumptions. Both constructions model the FIL block cipher as a pseudorandom permutation (PRP) – that is, indistinguishable from a random permutation against adaptive chosen plaintext attack. The first construction converts it to a VIL PRP and is an efficiency improvement over the scheme of Bellare and Rogaway [4]. The second construction converts it to a VIL super pseudorandom permutation (SPRP) – that is, the resulting VIL block cipher is indistinguishable from a random permutation against adaptive chosen plaintext and ciphertext attack.