Gilles Van Assche

On the indifferentiability of the sponge construction

In this paper we prove that the sponge construction introduced in [4] is indifferentiable from a random oracle when being used with a random transformation or a random permutation and discuss its implications. To our knowledge, this is the first time indifferentiability has been shown for a construction calling a random permutation (instead of an ideal compression function or ideal block cipher) and for a construction generating outputs of any length (instead of a fixed length).

Duplexing the sponge: single-pass authenticated encryption and other applications

This paper proposes a novel construction, called duplex, closely related to the sponge construction, that accepts message blocks to be hashed and---at no extra cost---provides digests on the input blocks received so far. It can be proven equivalent to a cascade of sponge functions and hence inherits its security against single-stage generic attacks. The main application proposed here is an authenticated encryption mode based on the duplex construction. This mode is efficient, namely, enciphering and authenticating together require only a single call to the underlying permutation per block, and is readily usable in, e.g., key wrapping. Furthermore, it is the first mode of this kind to be directly based on a permutation instead of a block cipher and to natively support intermediate tags. The duplex construction can be used to efficiently realize other modes, such as a reseedable pseudo-random bit sequence generators and a sponge variant that overwrites part of the state with the input block rather than to XOR it in.

Sponge-based pseudo-random number generators

This paper proposes a new construction for the generation of pseudo-random numbers. The construction is based on sponge functions and is suitable for embedded security devices as it requires few resources. We propose a model for such generators and explain how to define one on top of a sponge function. The construction is a novel way to use a sponge function, and inputs and outputs blocks in a continuous fashion, allowing to interleave the feed of seeding material with the fetch of pseudo-random numbers without latency. We describe the consequences of the sponge indifferentiability results to this construction and study the resistance of the construction against generic state recovery attacks. Finally, we propose a concrete example based on a member of the KECCAK family with small width.

Efficient and First-Order DPA Resistant Implementations of Keccak

In October 2012 NIST announced that the SHA-3 hash standard will be based on Keccak. Besides hashing, Keccak can be used in many other modes, including ones operating on a secret value. Many applications of such modes require protection against side-channel attacks, preferably at low cost. In this paper, we present threshold implementations (TI) of Keccak with three and four shares that build further on unprotected parallel and serial architectures. We improve upon earlier TI implementations of Keccak in the sense that the latter did not achieve uniformity of shares. In our proposals we do achieve uniformity at the cost of an extra share in a four-share version or at the cost of injecting a small number of fresh random bits for each computed round. The proposed implementations are efficient and provably secure against first-order side-channel attacks.

Security of Keyed Sponge Constructions Using a Modular Proof Approach

Sponge functions were originally proposed for hashing, but find increasingly more applications in keyed constructions, such as encryption and authentication. Depending on how the key is used we see two main types of keyed sponges in practice: inner- and outer-keyed. Earlier security bounds, mostly due to the well-known sponge indifferentiability result, guarantee a security level of c / 2 bits with c the capacity. We reconsider these two keyed sponge versions and derive improved bounds in the classical indistinguishability setting as well as in an extended setting where the adversary targets multiple instances at the same time. For cryptographically significant parameter values, the expected workload for an attacker to be successful in an n-target attack against the outer-keyed sponge is the minimum over \(2^k/n\) and \(2^c/\mu \) with k the key length and \(\mu \) the total maximum multiplicity. For the inner-keyed sponge this simplifies to \(2^k/\mu \) with maximum security if \(k=c\). The multiplicity is a characteristic of the data available to the attacker. It is at most twice the data complexity, but will be much smaller in practically relevant attack scenarios. We take a modular proof approach, and our indistinguishability bounds are the sum of a bound in the PRP model and a bound on the PRP-security of Even-Mansour type block ciphers in the ideal permutation model, where we obtain the latter result by using Patarin’s H-coefficient technique.

Bitslice Ciphers and Power Analysis Attacks

In this paper, we present techniques to protect bitslice block ciphers against power analysis attacks. We analyze and extend a technique proposed in [12]. We apply the technique to BaseKing, a variant of 3-Way [9] that was published in [7]. We introduce an alternative method to protect against power analysis specific for BaseKing. Finally, we discuss the applicability of the methods to the other known bitslice ciphers 3-Way and Serpent [1].

Differential propagation analysis of keccak

In this paper we introduce new concepts that help read and understand low-weight differential trails in Keccak. We then propose efficient techniques to exhaustively generate all 3-round trails in its largest permutation below a given weight. This allows us to prove that any 6-round differential trail in Keccak-f[1600] has weight at least 74. In the worst-case diffusion scenario where the mixing layer acts as the identity, we refine the lower bound to 82 by systematically constructing trails using a specific representation of states.

Sakura: A Flexible Coding for Tree Hashing

We propose a flexible, fairly general, coding for tree hash modes. The coding does not define a tree hash mode, but instead specifies a way to format the message blocks and chaining values into inputs to the underlying function for any topology, including sequential hashing. The main benefit is to avoid input clashes between different tree growing strategies, even before the hashing modes are defined, and to make the SHA-3 standard tree-hashing ready.

Power analysis of hardware implementations protected with secret sharing

We analyze the security of three-share hardware implementations against differential power analysis and advanced variants such as mutual information analysis. We present dedicated distinguishers that allow to recover secret key bits from any cryptographic primitive that is implemented as a sequence of quadratic functions. Starting from the analytical treatment of such distinguishers and information-theoretic arguments, we derive the success probability and required number of traces in the presence of algorithmic noise. We show that attacks on three-share hardware implementation require a number of traces that scales in the third power of the algorithmic noise variance. Finally, we apply and test our model on Keccak in a keyed mode.

Producing collisions for PANAMA, instantaneously

We present a practical attack on the Panama hash function that generates a collision in 26 evaluations of the state updating function. Our attack improves that of Rijmen and coworkers that had a complexity 282, too high to produce a collision in practice. This improvement comes mainly from the use of techniques to transfer conditions on the state to message words instead of trying many message pairs and using the ones for which the conditions are satisfied. Our attack works for any arbitrary prefix message, followed by a pair of suffix messages with a given difference. We give an example of a collision and make the collision-generating program available. Our attack does not affect the Panama stream cipher, that is still unbroken to the best of our knowledge.