Blockcipher-Based Double-Length Hash Functions for Pseudorandom Oracles

PRO (Pseudorandom Oracle) is an important security of hash functions because it ensures that the hash function inherits all properties of a random oracle up to the PRO bound (e.g., security against length extension attack, collision resistant security, preimage resistant security and so on). In this paper, we propose new blockcipher-based double-length hash functions, which are PROs up to O(2) query complexity in the ideal cipher model. Our hash functions use a single blockcipher, which encrypts an n-bit string using a 2n-bit key, and maps an input of arbitrary length to an n-bit output. Since many blockciphers supports a 2n-bit key (e.g. AES supports a 256-bit key), the assumption to use the 2n-bit key length blockcipher is acceptable. To our knowledge, this is the first time double-length hash functions based on a single (practical size) blockcipher with birthday PRO security.