Abstract In this paper, we re-investigate the Lai–Massey scheme, originally proposed in the cipher IDEA. Due to the similarity with the Feistel networks, and due to the existence of invariant subspace attacks as originally pointed out by Vaudenay at FSE 1999, the Lai–Massey scheme has received only little attention by the community. As first contribution, we propose two new generalizations of such scheme that are not (extended) affine equivalent to any generalized Feistel network proposed in the literature so far. Then, inspired by the recent construction, we propose the structure as a generalization of the Lai–Massey scheme, in which the linear combination in the Lai–Massey scheme can be replaced by a non-linear one. Besides proposing concrete examples of the construction, we analyze its cryptographic properties in the context of MPC-/HE-/ZK-friendly symmetric primitives.

We propose a new hash function Reinforced Concrete, which is the first generic purpose hash that is fast both for a zero-knowledge prover and in native x86 computations. It is suitable for a various range of zero-knowledge proofs and protocols, from set membership to generic purpose verifiable computation. Being up to 15x faster than its predecessor Poseidon hash, Reinforced Concrete inherits security from traditional time-tested schemes such as AES, whereas taking the zero-knowledge performance from a novel and efficient decomposition of a prime field into compact buckets.

Abstract Known-key distinguishers have been introduced by Knudsen and Rijmen in 2007 to better understand the security of block ciphers in situations where the key can not be considered to be secret, i.e. the “thing between secret-key model and hash function use-cases”. Trying to find a rigorous model to fit this intuition is still ongoing. The most recent advance by Gilbert (Asiacrypt 2014) describes a new model that—even if it is well justified—seemingly does not match this intuition. AES is often considered as a target of such analyses, simply because AES or its building blocks are used in many settings that go beyond classical encryption. Consider AES-128. Results in the secret-key model cover up to 6 rounds, while results in the chosen-key model reach up to 9 rounds. Gilbert however showed a result in the known-key model that goes even further, covering 10 rounds. Does it mean that the use cases corresponding to the cryptanalysis of hash-function use-cases are inherently less efficient, or is it rather an artifact of the new model? In this paper we give strong evidence for the latter. In Gilbert’s work, two types of arguments or rather conjectures are put forward suggesting that the new model is meaningful. Firstly that the number of “extension rounds” due to the new model is limited to two. And secondly that only a distinguisher that exploits the uniform distribution property can be extended in such way. We disprove both conjectures and arrive at the following results: First, we are also able to show that more than two extension rounds are possible. As a result of this, we describe the first known-key distinguishers on 12 rounds of AES that fit into Gilbert’s model. The second conjecture is disproven by showing that the technique proposed by Gilbert can also be used to extend a known-key distinguisher based on another property: truncated differentials. A potential conclusion of this work would be that the counter-intuitive gap between Gilbert’s known-key model and the chosen-key model is wider than initially thought. We however conclude that results in Gilbert’s model are due to an artifact in the model. To remedy this situation, we propose a refinement of the known-key model which restores its original intent to fit the original intuition.

We discuss the design of symmetric primitives, in particular Pseudo-Random Functions (PRFs) which are suitable for use in a secret-sharing based MPC system. We consider three different PRFs: the Naor-Reingold PRF, a PRF based on the Legendre symbol, and a specialized block cipher design called MiMC. We present protocols for implementing these PRFs within a secret-sharing based MPC system, and discuss possible applications. We then compare the performance of our protocols. Depending on the application, different PRFs may offer different optimizations and advantages over the classic AES benchmark. Thus, we cannot conclude that there is one optimal PRF to be used in all situations.