International Association for Cryptologic Research

International Association
for Cryptologic Research


Ewan Fleischmann


Some Observations on Indifferentiability
Ewan Fleischmann Michael Gorski Stefan Lucks
At Crypto 2005, Coron et al. introduced a formalism to study the presence or absence of structural flaws in iterated hash functions: If one cannot differentiate a hash function using ideal primitives from a random oracle, it is considered structurally sound, while the ability to differentiate it from a random oracle indicates a structural weakness. This model was devised as a tool to see subtle real world weaknesses while in the random oracle world. In this paper we take in a practical point of view. We show, using well known examples like NMAC and the Mix-Compress-Mix (MCM) construction, how we can prove a hash construction secure and insecure at the same time in the indifferentiability setting. These constructions do not differ in their implementation but only on an abstract level. Naturally, this gives rise to the question what to conclude for the implemented hash function. Our results cast doubts about the notion of “indifferentiability from a random oracle” to be a mandatory, practically relevant criterion (as e.g., proposed by Knudsen [16] for the SHA-3 competition) to separate good hash structures from bad ones.
On the Security of Tandem-DM
Ewan Fleischmann Michael Gorski Stefan Lucks
We provide the first proof of security for Tandem-DM one of the oldest and most well-known constructions for turning a blockcipher with n-bit blocklength and 2n-bit keylength into a 2n-bit cryptographic hash function. We prove, that when Tandem-DM is instantiated with AES-256, i.e. blocklength 128 bits and keylength 256 bits, any adversary that asks less than 2^{120.4} queries cannot find a collision with success probability greater than 1/2. We also prove a bound for preimage resistance of Tandem-DM. Interestingly, as there is only one practical construction known (FSE'06, Hirose) turning such an (n,2n)-bit blockcipher into a 2n-bit compression function that has provably birthday-type collision resistance, Tandem-DM is one out of two structures that possess this desirable feature.
Some Observations on SHAMATA
Ewan Fleischmann Michael Gorski
In this note we discuss some observation of the SHA-3 candidate SHAMATA. We observe that its internal block cipher is very weak, which could possibly lead to an attack on the hash function.