Bart Mennink’s research while affiliated with Radboud University and other places

In order to maintain a similar security level in a post-quantum setting, many symmetric primitives should have to double their keys and increase their state sizes. So far, no generic way for doing this is known that would provide convincing quantum security guarantees. In this paper we propose a new generic construction, QuEME, that allows one to double the key and the state size of a block cipher in such a way that a decent level of quantum security is guaranteed. The QuEME design is inspired by the ECB-Mix-ECB (EME) construction, but is defined for a different choice of mixing function than what we have seen before, in order to withstand a new quantum superposition attack that we introduce as a side result: this quantum superposition attack exhibits a periodic property found in collisions and breaks EME and a large class of its variants. We prove that QuEME achieves n-bit security in the classical setting, where n is the block size of the underlying block cipher, and at least (n/6)-bit security in the quantum setting. We finally propose a concrete instantiation of this construction, called Double-AES, that is built with variants of the standardized AES-128 block cipher.

We employ an algebraic approach to estimate the success rate of a sidechannel adversary attacking secrets of a masked circuit within the Random Probing Model (RPM), where intermediate variables of the implementation leak with a probability p. Our method efficiently handles masked linear circuits, enabling security bound estimation for practically large masking orders. For non-linear circuits, we employ a linearization technique. To reason about the security of complex structures like an S-box, we introduce a composition theorem, reducing the RPM security of a circuit to that of its constituent gadgets. Moreover, we lower the complexity of the multiplication gadget of CHES 2016 from O(n2 log(n)) to O(n2) while demonstrating its conjectured RPM security. Collectively, these novel methods enable the development of a practical masking scheme with O(n2) complexity for AES, maintaining security for a considerably high leakage rate p ≤ 0.02 ≈ 2−5.6.

In this work we present the COLM authenticated encryption (AE) scheme which is the second of the two winners in the defense in depth category of the CAESAR competition. COLM realizes a nonce-based authenticated encryption with associated data and uses the popular AES blockcipher as its underlying primitive. We propose two possible blockcipher instantiations (with key of length 128 or 256 bits). We also define two COLM modes of operation variants: a primary COLM$_0$ mode for general purpose applications, and a COLM$_{\tau }$ variant with intermediate tag generation/verification geared to support low-end devices and applications where frequent verification is required. COLM is designed with security, simplicity, and efficiency in mind. The main design goal of COLM is high security: a primary feature of the defense in depth CAESAR category. COLM provides security beyond the traditional AE security. First, COLM is secure against nonce misuse, namely, it enables security in adversarial settings where the nonce inputs to the AE scheme repeat. In contrast to standardized and popular AE algorithms, such as GCM and OCB1-3 modes, whose AE security trivially breaks down when the nonce is repeated, COLM ensures both confidentiality and authenticity (AE) security with repeated nonces. Second, our COLM$_{\tau }$ variant enables increased security levels in situations where release of unverified ciphertext (RUP) occurs due to its ability to limit a potential leakage by frequent verifications. In this work we prove COLM secure with respect to both confidentiality and authenticity (AE) security under nonce misuse in the well-known provable security framework. Our proofs show that COLM maintains n/2-bit security levels for block sizes of n bits. Furthermore, due to the inherent parallelism on both mode and primitive levels, our software performance results show that the price paid for enhanced security does come at the cost of minimal efficiency losses. More concretely, we implement GCM, COLM, and Deoxys-II on the Kaby Lake and Coffee lake Intel platforms. Compared to the other winner in the defense in depth category Deoxys-II, our AE design COLM$_0$ performs 10–20% faster for the 128-bit key version. Regarding the 256-bit key versions COLM$_0$ is around 5% faster for short and 2% slower than Deoxys-II for the longer messages.

Lightweight cryptographic constructions are often optimized on multiple aspects that put the security bounds to the limit. In this respect, it is important to obtain security bounds that are tight and give an accurate and exact indication of the generic security. However, whereas for black-box security bounds it has become common practice to argue tightness of security bounds, for leakage resilience security bounds this is not the case. This is unfortunate, as for leakage resilience results, tightness is even more important as there is already a lossiness incurred in capturing the actual leakage by a theoretical model in the first place.In this work, we consider the SuKS (Suffix Keyed Sponge) PRF construction and investigate tightness of the leakage resilience bound of Dobraunig and Mennink (ToSC 2019). We observe that, although their black-box security result is tight, their leakage resilience bound is not tight in their bounded leakage term λ. We observe that this is caused by the fact that parts of the security bound contain a term covering multicollisions and a term covering leakage, but an adversary is unable to combine both. We next consider improved security of the SuKS for two types of leakage: fixed position leakage, where the adversary directly learns the value of λ bits of a secret state, and Hamming weight leakage, where the Hamming weight of a fixed part of the state is leaked. For fixed position leakage, a very generous form of bounded leakage, we improve the original bound by making wise use of the multicollision limit function of Daemen et al. (ASIACRYPT 2017). For the more realistic setting of Hamming weight leakage, we structurally revisit the multicollision limit function analysis by including Hamming weight in the computation, a problem that is difficult on its own due to the non-uniform character of this type of leakage. In both cases, we improve and tighten the leakage resilience bound of Dobraunig and Mennink. The improved bound for the SuKS has immediate consequences for the leakage resilience of the NIST lightweight cryptography competition finalist ISAP v2, an authenticated encryption scheme that uses the SuKS internally.

It is known that the sponge construction is tightly indifferentiable from a random oracle up to around 2c/2 queries, where c is the capacity. In particular, it cannot provide generic security better than half of the underlying permutation size. In this paper, we aim to achieve hash function security beating this barrier. We present a hashing mode based on two b-bit permutations named the double sponge. The double sponge can be seen as the sponge embedded within the double block length hashing paradigm, making two permutation calls in parallel interleaved with an efficient mixing function. Similarly to the sponge, the permutation size is split as b = r+c, and the underlying compression function absorbs r bits at a time. We prove that the double sponge is indifferentiable from a random oracle up to around 22c/3 queries. This means that the double sponge achieves security beyond the birthday bound in the capacity. In addition, if c > 3b/4, the double sponge beats the birthday bound in the primitive size, to our knowledge being the first hashing mode based on a permutation that accomplices this feature.

The duplex construction is already well analyzed with many papers proving its security in the random permutation model. However, so far, the first phase of the duplex, where the state is initialized with a secret key and an initialization vector ( IV ), is typically analyzed in a worst case manner. More detailed, it is always assumed that the adversary is allowed to choose the IV at will. However, in practice, the adversary can be stripped of its power to control the IV in several ways. One prominent way of doing this is the use of a nonce ( IV ) masked with a secret, as done in AES-GCM in TLS 1.3. In this paper, we analyze how the security of the duplex construction changes if restrictions on the choice of the IV are imposed. In particular, we evaluate several strategies that can achieve this, varying from the IV on key case over the global nonce case to the random IV case. We apply our findings to duplex-based encryption and authenticated encryption, compare the different strategies, and discuss the practical applications of our results.

We provide security foundations for SAFE, a recently introduced API framework for sponge-based hash functions tailored to prime-field-based protocols. SAFE aims to provide a robust and foolproof interface, has been implemented in the Neptune hash framework and some zero-knowledge proof projects, but despite its usability and applicability it currently lacks any security proof. Such a proof would not be straightforward as SAFE abuses the inner part of the sponge and fills it with protocol-specific data. In this work we identify the SAFECore as versatile variant sponge construction underlying SAFE, we prove indifferentiability of SAFECore for all (binary and prime) fields up to around $|\mathbb {F}_p |^{c/2}$ queries, where $\mathbb {F}_p$ is the underlying field and c the capacity, and we apply this security result to various use cases. We show that the SAFE-based protocols of plain hashing, authenticated encryption, verifiable computation, non-interactive proofs, and commitment schemes are secure against a wide class of adversaries, including those dealing with multiple invocations of a sponge in a single application. Our results pave the way of using SAFE with the full taxonomy of hash functions, including SNARK-, lattice-, and x86-friendly hashes.

The sum of two n-bit pseudorandom permutations is known to behave like a pseudorandom function with n bits of security. A recent line of research has investigated the security of two public n-bit permutations and its degree of indifferentiability. Mandal et al. (INDOCRYPT 2010) proved 2n/3-bit security, Mennink and Preneel (ACNS 2015) pointed out a non-trivial flaw in their analysis and re-proved $(2n/3-\log _2(n))$-bit security. Bhattacharya and Nandi (EUROCRYPT 2018) eventually improved the result to n-bit security. Recently, Gunsing at CRYPTO 2022 already observed that a proof technique used in this line of research only holds for sequential indifferentiability. We revisit the line of research in detail, and observe that the strongest bound of n-bit security has two other serious issues in the reasoning, the first one is actually the same non-trivial flaw that was present in the work of Mandal et al., while the second one discards biases in the randomness influenced by the distinguisher. More concretely, we introduce two attacks that show limited potential of different approaches. We (i) show that the latter issue that discards biases only holds up to $2^{3n/4}$ queries, and (ii) perform a differentiability attack against their simulator in $2^{5n/6}$ queries. On the upside, we revive the result of Mennink and Preneel and show $(2n/3-\log _2(n))$-bit regular indifferentiability security of the sum of public permutations.Keywordsindifferentiabilitysum of permutationsattacksresolutions

At SAC 2011, Bertoni et al. introduced the keyed duplex construction as a tool to build permutation based authenticated encryption schemes. The construction was generalized to full-state absorption by Mennink et al. (ASIACRYPT 2015). Daemen et al. (ASIACRYPT 2017) generalized it further to cover much more use cases, and proved security of this general construction, and Dobraunig and Mennink (ASIACRYPT 2019) derived a leakage resilience security bound for this construction. Due to its generality, the full-state keyed duplex construction that we know today has plethora applications, but the flip side of the coin is that the general construction is hard to grasp and the corresponding security bounds are very complex. Consequently, the state-of-the-art results on the full-state keyed duplex construction are not used to the fullest. In this work, we revisit the history of the duplex construction, give a comprehensive discussion of its possibilities and limitations, and demonstrate how the two security bounds (of Daemen et al. and Dobraunig and Mennink) can be interpreted in particular applications of the duplex.