Paper 2007/050

Special block cipher family DN and new generation SNMAC-type hash function family HDN

Vlastimil KLIMA

Abstract

Special block cipher is a new cryptographic primitive designed to be a building block of the new generation hash functions SNMAC [Kl06]. Contrary to classical block ciphers it is knowingly designed focusing to those properties which are expected to be just a “side effect” on usual cipher constructions. Its design anticipates that an attacker could exploit or know its key, or even he/she could discretionarily tamper with the key. The design criteria of SNMAC hash functions are publicly known. Limitly, these functions approach a random oracle, they are computationally resistant against pre-image and collision attacks, and different special block cipher instances can be used in their design. In this paper, we present special block cipher family Double Net DN(n,k)-rho with n-bit block, k-bit key and rho rounds, their building blocks construction principles and design criteria. Based on DN, we define hash functions family HDN(n,k)-rho with n-bit hash code working on blocks of k-n bits. We introduce and propose to use DN(512,8192)-10 and HDN(512,8192)-10 as example instances. It turns out these are not just theoretical concepts, but practically employable functions with speeds only 2-3 times lower than SHA-512 and Whirlpool. Basic idea behind the special block cipher DN is simple – contrary to classical block cipher approach, the same attention is paid to key and plaintext processing. One SP network ensures key mixing, while the second one mixes the plaintext with the key. Once the special block cipher concept is examined and accepted in hash functions, it can be used in advance in its original purpose – data encryption. We suggest the transition from the classical block ciphers to more secure special block ciphers in the future. Its advantage is its readiness for various attacks on the secret key; the attacks which have recently started to emerge in classical block cipher cryptanalysis. Among others, these include side-channel attacks, related keys attacks and rectangular attacks (see e.g. [Bi93], [Bi03], [Ki04], [Ho05], [Ki05], [Bi05], and [Bi06]). With the expansion of the cryptographic instruments and cryptanalytic methods, these attacks will appear more and more frequently. Their common traits are the various attempts to exploit the original assumption on the attacker’s limited power over the secret key or its knowledge. The defence against these attacks is illustrated by the evolution of the functions processing the secret key, starting with simple copy-type functions used in DES and TripleDES to weak non-linear functions in AES. We believe that this trend will continue to strong non-linear functions (similar to the ones used in DN). The employment of these stronger functions in the encryption might not seem as a must in the present, but it probably will be in the future. In the hash functions, it is a necessity today already.