International Association
for Cryptologic Research

<p>We analyze the multi-user (mu) security of a family of nonce-based authentication encryption (nAE) schemes based on a tweakable block cipher (TBC). The starting point of our work is an analysis of the mu security of the SCT-II mode which underlies the nAE scheme Deoxys-II, winner of the CAESAR competition for the defense-in-depth category. We extend this analysis in two directions, as we detail now.</p><p>First, we investigate the mu security of several TBC-based variants of the counter encryption mode (including CTRT, the encryption mode used within SCT-II) that differ by the way a nonce, a random value, and a counter are combined as tweak and plaintext inputs to the TBC to produce the keystream blocks that will mask the plaintext blocks. Then, we consider the authentication part of SCT-II and study the mu security of the nonce-based MAC Nonce-as-Tweak (NaT) built from a TBC and an almost universal (AU) hash function. We also observe that the standard construction of an AU hash function from a (T)BC can be proven secure under the assumption that the underlying TBC is unpredictable rather than pseudorandom, allowing much better conjectures on the concrete AU advantage. This allows us to derive the mu security of the family of nAE modes obtained by combining these encryption/MAC building blocks through the NSIV composition method.</p><p>Some of these modes require an underlying TBC with a larger tweak length than what is usually available for existing ones. We then show the practicality of our modes by instantiating them with two new TBC constructions, Deoxys-TBC-512 and Deoxys-TBC-640, which can be seen as natural extensions of the Deoxys-TBC family to larger tweak input sizes. Designing such TBCs with unusually large tweaks is prone to pitfalls: Indeed, we show that a large-tweak proposal for SKINNY published at EUROCRYPT 2020 presents an inherent construction flaw. We therefore provide a sound design strategy to construct large-tweak TBCs within the Superposition Tweakey (STK) framework, leading to new Deoxys-TBC and SKINNY variants. We provide software benchmarks indicating that while ensuring a very high security level, the performances of our proposals remain very competitive. </p>

Elisabeth-4 is a stream cipher tailored for usage in hybrid homomorphic encryption applications that has been introduced by Cosseron et al. at ASIACRYPT 2022. In this paper, we present several variants of a key-recovery attack on the full Elisabeth-4 that break the 128-bit security claim of that cipher. Our most optimized attack is a chosen-IV attack with a time complexity of 2^88 elementary operations, a memory complexity of 2^54 bits and a data complexity of 2^41 bits. Our attack applies the linearization technique to a nonlinear system of equations relating some keystream bits to the key bits and exploits specificities of the cipher to solve the resulting linear system efficiently. First, due to the structure of the cipher, the system to solve happens to be very sparse, which enables to rely on sparse linear algebra and most notably on the Block Wiedemann algorithm. Secondly, the algebraic properties of the two nonlinear ingredients of the filtering function cause rank defects which can be leveraged to solve the linearized system more efficiently with a decreased data and time complexity. We have implemented our attack on a toy version of Elisabeth-4 to verify its correctness. It uses the efficient implementation of the Block Wiedemann algorithm of CADO-NFS for the sparse linear algebra.

We present the Deoxys family of authenticated encryption schemes, which consists of Deoxys-I and Deoxys-II . Both are nonce-based authenticated encryption schemes with associated data and have either 128- or 256-bit keys. Deoxys-I is similar to OCB : It is single-pass but insecure when nonces are repeated; in contrast, Deoxys-II is nonce-misuse resistant. Deoxys-II was selected as first choice in the final portfolio of the CAESAR competition for the defense-in-depth category. Deoxys uses a new family of tweakable block ciphers as internal primitive, Deoxys-TBC , which follows the TWEAKEY framework (Jean, Nikolić, and Peyrin, ASIACRYPT 2014) and relies on the AES round function. Our benchmarks indicate that Deoxys does not sacrifice efficiency for security and performs very well both in software (e.g., Deoxys-I efficiency is similar to AES-GCM ) and hardware.

This paper introduces Pyjamask, a new block cipher family and authenticated encryption proposal submitted to the NIST lightweight cryptography standardization process. Pyjamask targets side-channel resistance as one of its main goal. More precisely, it strongly minimizes the number of nonlinear gates used in its internal primitive in order to allow efficient masked implementations, especially for high-order masking in software. Compared to other block ciphers, our proposal has thus among the smallest number of binary AND computations per input bit at the time of writing. Even though Pyjamask minimizes such an important criterion, it remains rather lightweight and efficient, thanks to a general bitslice construction that enables to computation of all nonlinear gates in parallel. For authenticated encryption, we adopt the provably secure AEAD mode OCB which has been extensively studied and has the benefit to offer full parallelization. Of course, other block cipher-based modes can be considered as well if other performance profiles are to be targeted.The paper first gives the specification of the Pyjamask block cipher and the associated AEAD proposal. We also provide a detailed design rationale for the block cipher which is guided by our aim of software efficiency in the presence of high-order masking. The security of the design is analyzed against most commonly known cryptanalysis techniques. We finally describe efficient (masked) implementations in software and provide implementation results with aggressive performances for masking of very high orders (up to 128). We also provide a rough estimation of the hardware performances which remain much better than those of an AES round-based implementation.

We present the family of authenticated encryption schemes SKINNY-AEAD and the family of hashing schemes SKINNY-Hash. All of the schemes employ a member of the SKINNY family of tweakable block ciphers, which was presented at CRYPTO 2016, as the underlying primitive. In particular, for authenticated encryption, we show how to instantiate members of SKINNY in the Deoxys-I-like ΘCB3 framework to fulfill the submission requirements of the NIST lightweight cryptography standardization process. For hashing, we use SKINNY to build a function with larger internal state and employ it in a sponge construction. To highlight the extensive amount of third-party analysis that SKINNY obtained since its publication, we briefly survey the existing cryptanalysis results for SKINNY-128-256 and SKINNY-128-384 as of February 2020. In the last part of the paper, we provide a variety of ASIC implementations of our schemes and propose new simple SKINNY-AEAD and SKINNY-Hash variants with a reduced number of rounds while maintaining a very comfortable security margin. https://csrc.nist.gov/Projects/Lightweight-Cryptography

This paper presents a cryptanalysis of full Kravatte, an instantiation of the Farfalle construction of a pseudorandom function (PRF) with variable input and output length. This new construction, proposed by Bertoni et al., introduces an efficiently parallelizable and extremely versatile building block for the design of symmetric mechanisms, e.g. message authentication codes or stream ciphers. It relies on a set of permutations and on so-called rolling functions: it can be split into a compression layer followed by a two-step expansion layer. The key is expanded and used to mask the inputs and outputs of the construction. Kravatte instantiates Farfalle using linear rolling functions and permutations obtained by iterating the Keccak round function.We develop in this paper several attacks against this PRF, based on three different attack strategies that bypass part of the construction and target a reduced number of permutation rounds. A higher order differential distinguisher exploits the possibility to build an affine space of values in the cipher state after the compression layer. An algebraic meet-in-the-middle attack can be mounted on the second step of the expansion layer. Finally, due to the linearity of the rolling function and the low algebraic degree of the Keccak round function, a linear recurrence distinguisher can be found on intermediate states of the second step of the expansion layer. All the attacks rely on the ability to invert a small number of the final rounds of the construction. In particular, the last two rounds of the construction together with the final masking by the key can be algebraically inverted, which allows to recover the key.The complexities of the devised attacks, applied to the Kravatte specifications published on the IACR ePrint in July 2017, or the strengthened version of Kravatte recently presented at ECC 2017, are far below the security claimed.

NORX is an authenticated encryption scheme with associated data being publicly scrutinized as part of the ongoing CAESAR competition, where 14 other primitives are also competing. It is based on the sponge construction and relies on a simple permutation that allows efficient and versatile implementations. Thanks to research on the security of the sponge construction, the design of NORX, whose permutation is inspired from the permutations used in BLAKE and ChaCha, has evolved throughout three main versions (v1.0, v2.0 and v3.0). In this paper, we investigate the security of the full NORX v2.0 primitive that has been accepted as third-round candidate in the CAESAR competition. We show that some non-conservative design decisions probably motivated by implementation efficiency considerations result in at least one strong structural distinguisher of the underlying sponge permutation that can be turned into an attack on the full primitive. This attack yields a ciphertext-only forgery with time and data complexity 266 (resp. 2130) for the variant of NORX v2.0 using 128-bit (resp. 256-bit) keys and breaks the designers’ claim of a 128-bit, resp. 256-bit security. Furthermore, we show that this forgery attack can be extended to a key-recovery attack on the full NORX v2.0 with the same time and data complexities. We have implemented and experimentally verified the correctness of the attacks on a toy version of NORX. We emphasize that the scheme has recently been tweaked to NORX v3.0 at the beginning of the third round of the CAESAR competition: the main change introduces some key-dependent internal operations, which make NORX v3.0 immune to our attacks. However, the structural distinguisher of the permutation persists.

We study the synthesis of small functions used as building blocks in lightweight cryptographic designs in terms of hardware implementations. This phase most notably appears during the ASIC implementation of cryptographic primitives. The quality of this step directly affects the output circuit, and while general tools exist to carry out this task, most of them belong to proprietary software suites and apply heuristics to any size of functions. In this work, we focus on small functions (4- and 8-bit mappings) and look for their optimal implementations on a specific weighted instructions set which allows fine tuning of the technology. We propose a tool named LIGHTER, based on two related algorithms, that produces optimized implementations of small functions. To demonstrate the validity and usefulness of our tool, we applied it to two practical cases: first, linear permutations that define diffusion in most of SPN ciphers; second, non-linear 4-bit permutations that are used in many lightweight block ciphers. For linear permutations, we exhibit several new MDS diffusion matrices lighter than the state-of-the-art, and we also decrease the implementation cost of several already known MDS matrices. As for non-linear permutations, LIGHTER outperforms the area-optimized synthesis of the state-of-the-art academic tool ABC. Smaller circuits can also be reached when ABC and LIGHTER are used jointly.

Area minimization is one of the main efficiency criterion for lightweight encryption primitives. While reducing the implementation data path is a natural strategy for achieving this goal, Substitution-Permutation Network (SPN) ciphers are usually hard to implement in a bit-serial way (1-bit data path). More generally, this is hard for any data path smaller than its Sbox size, since many scan flip-flops would be required for storage, which are more area-expensive than regular flip-flops.In this article, we propose the first strategy to obtain extremely small bit-serial ASIC implementations of SPN primitives. Our technique, which we call bit-sliding, is generic and offers many new interesting implementation trade-offs. It manages to minimize the area by reducing the data path to a single bit, while avoiding the use of many scan flip-flops.Following this general architecture, we could obtain the first bit-serial and the smallest implementation of AES-128 to date (1560 GE for encryption only, and 1738 GE for encryption and decryption with IBM 130 nm standard-cell library), greatly improving over the smallest known implementations (about 30% decrease), making AES-128 competitive to many ciphers specifically designed for lightweight cryptography. To exhibit the generality of our strategy, we also applied it to the PRESENT and SKINNY block ciphers, again offering the smallest implementations of these ciphers thus far, reaching an area as low as 1065 GE for a 64-bit block 128-bit key cipher. It is also to be noted that our bit-sliding seems to obtain very good power consumption figures, which makes this implementation strategy a good candidate for passive RFID tags.

In this paper, we describe attacks on the recently proposed Haraka hash functions. First, for the two hash functions Haraka-256/256 and Haraka-512/256 in the family, we show how two colliding messages can be constructed in about 216 function evaluations. Second, we invalidate the preimage security claim for Haraka-512/256 with an attack finding one preimage in about 2192 function evaluations. These attacks are possible thanks to symmetries in the internal state that are preserved over several rounds.

We present an invariant subspace attack on the block cipher Midori64, proposed at Asiacrypt 2015. Our analysis shows that Midori64 has a class of 232 weak keys. Under any such key, the cipher can be distinguished with only a single chosen query, and the key can be recovered in 216 time with two chosen queries. As both the distinguisher and the key recovery have very low complexities, we confirm our analysis by implementing the attacks. Some tweaks of round constants make Midori64 more resistant to the attacks, but some lead to even larger weak-key classes. To eliminate the dependency on the round constants, we investigate alternative S-boxes for Midori64 that provide certain level of security against the found invariant subspace attacks, regardless of the choice of the round constants. Our search for S-boxes is enhanced with a dedicated tool which evaluates the depth of any given 4-bit S-box that satisfies certain design criteria. The tool may be of independent interest to future S-box designs.

We show generic attacks on unbalanced Feistel ciphers based on the meet-in-the-middle technique. We analyze two general classes of unbalanced Feistel structures, namely contracting Feistels and expanding Feistels. In both of the cases, we consider the practical scenario where the round functions are keyless and known to the adversary. In the case of contracting Feistels with 4 branches, we show attacks on 16 rounds when the key length k (in bits) is as large as the block length n (in bits), and up to 24 rounds when k = 2n. In the case of expanding Feistels, we consider two scenarios: one, where different nonlinear functions without particular structures are used in the round function, and a more practical one, where a single nonlinear is used but different linear functions are introduced in the state update. In the former case, we propose generic attacks on 13 rounds when k = n, and up to 21 rounds when k = 2n. In the latter case, 16 rounds can be attacked for k = n, and 24 rounds for k = 2n.