for Dilithium, the post-quantum signature scheme recently standardized by NIST. We improve the masked generation of the masking vector y, based on a fast Booleanto- arithmetic conversion modulo q. We also describe an optimized gadget for the high-order masked rejection sampling, with a complexity independent from the size of the modulus q. We prove the security of our gadgets in the classical ISW t-probing model. Finally, we detail our open-source C implementation of these gadgets integrated into a fully masked Dilithium implementation, and provide an efficiency comparison with previous works.

The main protection against side-channel attacks consists in computing every function with multiple shares via the masking countermeasure. While the masking countermeasure was originally developed for securing block-ciphers such as AES, the protection of lattice-based cryptosystems is often more challenging, because of the diversity of the underlying algorithms. In this paper, we introduce new gadgets for the high-order masking of the NTRU cryptosystem, with security proofs in the classical ISW probing model. We then describe the first fully masked implementation of the NTRU Key Encapsulation Mechanism submitted to NIST, including the key generation. To assess the practicality of our countermeasures, we provide a concrete implementation on ARM Cortex-M3 architecture, and eventually a t-test leakage evaluation.

The main protection against side-channel attacks consists in computing every function with multiple shares via the masking countermeasure. For IND-CCA secure lattice-based encryption schemes, the masking of the decryption algorithm requires the high-order computation of a polynomial comparison. In this paper, we describe and evaluate a number of different techniques for such high-order comparison, always with a security proof in the ISW probing model. As an application, we describe the full high-order masking of the NIST standard Kyber, with a concrete implementation on ARM Cortex M architecture, and a t-test evaluation.

We present novel and improved high-order masking gadgets for Dilithium, a post-quantum signature scheme that has been standardized by the National Institute of Standards and Technologies (NIST). Our proposed gadgets include the ShiftMod gadget, which is used for efficient arithmetic shifts and serves as a component in other masking gadgets. Additionally, we propose a new algorithm for Boolean-to-arithmetic masking conversion of a μ-bit integer x modulo any integer q, with a complexity that is independent of both μ and q. This algorithm is used in Dilithium to mask the generation of the random variable y modulo q. Moreover, we describe improved techniques for masking the Decompose function in Dilithium. Our new gadgets are proven to be secure in the t-probing model.We demonstrate the effectiveness of our countermeasures by presenting a complete high-order masked implementation of Dilithium that utilizes the improved gadgets described above. We provide practical results obtained from a C implementation and compare the performance improvements provided by our new gadgets with those of previous work.

Masking is the main countermeasure against side-channel attacks on embedded devices. For cryptographic algorithms that combine Boolean and arithmetic masking, one must therefore convert between the two types of masking, without leaking additional information to the attacker. In this paper we describe a new high-order conversion algorithm between Boolean and arithmetic masking, based on table recomputation, and provably secure in the ISW probing model. We show that our technique is particularly efficient for masking structured LWE encryption schemes such as Kyber and Saber. In particular, for Kyber IND-CPA decryption, we obtain an order of magnitude improvement compared to existing techniques.

At Crypto ’99, Nguyen and Stern described a lattice based algorithm for solving the hidden subset sum problem, a variant of the classical subset sum problem where the n weights are also hidden. As an application, they showed how to break the Boyko et al. fast generator of random pairs ( x, g x (mod p )). The Nguyen-Stern algorithm works quite well in practice for moderate values of n , but its complexity is exponential in n . A polynomial-time variant was recently described at Crypto 2020, based on a multivariate technique, but the approach is heuristic only. In this paper, we describe a proven polynomial-time algorithm for solving the hidden subset-sum problem, based on statistical learning. In addition, we show that the statistical approach is also quite efficient in practice: using the FastICA algorithm, we can reach n = 250 in reasonable time.

In this paper we describe the first improvement of the wire shuffling countermeasure against side-channel attacks described by Ishai, Sahai and Wagner at Crypto 2003. More precisely, we show how to get worst case statistical security against t probes with running time \({\mathcal O}(t)\) instead of \({\mathcal O}(t \log t)\); our construction is also much simpler. Recall that the classical masking countermeasure achieves perfect security but with running time \({\mathcal O}(t^2)\). We also describe a practical implementation for AES that outperforms the masking countermeasure for \(t \ge 6\,000\).

The masking countermeasure is among the most powerful countermeasures to counteract side-channel attacks. Leakage models have been exhibited to theoretically reason on the security of such masked implementations. So far, the most widely used leakage model is the probing model defined by Ishai, Sahai, and Wagner at (CRYPTO 2003). While it is advantageously convenient for security proofs, it does not capture an adversary exploiting full leakage traces as, e.g., in horizontal attacks. Those attacks target the multiple manipulations of the same share to reduce noise and recover the corresponding value. To capture a wider class of attacks another model was introduced and is referred to as the random probing model. From a leakage parameter p, each wire of the circuit leaks its value with probability p. While this model much better reflects the physical reality of side channels, it requires more complex security proofs and does not yet come with practical constructions. In this paper, we define the first framework dedicated to the random probing model. We provide an automatic tool, called VRAPS, to quantify the random probing security of a circuit from its leakage probability. We also formalize a composition property for secure random probing gadgets and exhibit its relation to the strong non-interference (SNI) notion used in the context of probing security. We then revisit the expansion idea proposed by Ananth, Ishai, and Sahai (CRYPTO 2018) and introduce a compiler that builds a random probing secure circuit from small base gadgets achieving a random probing expandability property. We instantiate this compiler with small gadgets for which we verify the expected properties directly from our automatic tool. Our construction can tolerate a leakage probability up to $$2^{-8}$$ , against $$2^{-25}$$ for the previous construction, with a better asymptotic complexity.

Abstract At Crypto 2018, Aggarwal, Joux, Prakash and Santha (AJPS) described a new public-key encryption scheme based on Mersenne numbers. Shortly after the publication of the cryptosystem, Beunardeau et al. described an attack with complexity 𝓞(22h). In this paper, we describe an improved attack with complexity 𝓞(21.75h).

At Crypto ’99, Nguyen and Stern described a lattice based algorithm for solving the hidden subset sum problem, a variant of the classical subset sum problem where the n weights are also hidden. While the Nguyen-Stern algorithm works quite well in practice for moderate values of n, we argue that its complexity is actually exponential in n; namely in the final step one must recover a very short basis of a n-dimensional lattice, which takes exponential-time in n, as one must apply BKZ reduction with increasingly large block-sizes.

We consider the problem of recovering the entries of diagonal matrices $\{U_a\}_a$ for $a = 1,\ldots,t$ from multiple "incomplete" samples $\{W_a\}_a$ of the form $W_a=PU_aQ$, where $P$ and $Q$ are unknown matrices of low rank. We devise practical algorithms for this problem depending on the ranks of $P$ and $Q$. This problem finds its motivation in cryptanalysis: we show how to significantly improve previous algorithms for solving the approximate common divisor problem and breaking CLT13 cryptographic multilinear maps.

Many constructions based on multilinear maps require independent slots in the plaintext, so that multiple computations can be performed in parallel over the slots. Such constructions are usually based on CLT13 multilinear maps, since CLT13 inherently provides a composite encoding space, with a plaintext ring \(\bigoplus _{i=1}^n \mathbb {Z}/g_i\mathbb {Z}\) for small primes \(g_i\)’s. However, a vulnerability was identified at Crypto 2014 by Gentry, Lewko and Waters, with a lattice-based attack in dimension 2, and the authors have suggested a simple countermeasure. In this paper, we identify an attack based on higher dimension lattice reduction that breaks the author’s countermeasure for a wide range of parameters. Combined with the Cheon et al. attack from Eurocrypt 2015, this leads to the recovery of all the secret parameters of CLT13, assuming that low-level encodings of almost zero plaintexts are available. We show how to apply our attack against various constructions based on composite-order CLT13. For the [FRS17] construction, our attack enables to recover the secret CLT13 plaintext ring for a certain range of parameters; however, breaking the indistinguishability of the branching program remains an open problem.

Indistinguishability obfuscation constructions based on matrix branching programs generally proceed in two steps: first apply Kilian’s randomization of the matrix product computation, and then encode the matrices using a multilinear map scheme. In this paper we observe that by applying Kilian’s randomization after encoding, the complexity of the best attacks is significantly increased for CLT13 multilinear maps. This implies that much smaller parameters can be used, which improves the efficiency of the constructions by several orders of magnitude.

Boneh et al. showed at Crypto 99 that moduli of the form \(N=p^rq\) can be factored in polynomial time when \(r \ge \log p\). Their algorithm is based on Coppersmith’s technique for finding small roots of polynomial equations. Recently, Coron et al. showed that \(N=p^rq^s\) can also be factored in polynomial time, but under the stronger condition \(r \ge \log ^3 p\). In this paper, we show that \(N=p^rq^s\) can actually be factored in polynomial time when \(r \ge \log p\), the same condition as for \(N=p^rq\).

We describe a technique to formally verify the security of masked implementations against side-channel attacks, based on elementary circuit transforms. We describe two complementary approaches: a generic approach for the formal verification of any circuit, but for small attack orders only, and a specialized approach for the verification of specific circuits, but at any order. We also show how to generate security proofs automatically, for simple circuits. We describe the implementation of CheckMasks, a formal verification tool for side-channel countermeasures. Using this tool, we formally verify the security of the Rivain-Prouff countermeasure for AES, and also the recent Boolean to arithmetic conversion algorithms from CHES 2017.

In this paper we describe a new error-correcting code (ECC) inspired by the Naccache-Stern cryptosystem. While by far less efficient than Turbo codes, the proposed ECC happens to be more efficient than some established ECCs for certain sets of parameters. The new ECC adds an appendix to the message. The appendix is the modular product of small primes representing the message bits. The receiver recomputes the product and detects transmission errors using modular division and lattice reduction.

Masking is an effective countermeasure against side-channel attacks. In this paper, we improve the efficiency of the high-order masking of look-up tables countermeasure introduced at Eurocrypt 2014, based on a combination of three techniques, and still with a proof of security in the Ishai-Sahai-Wagner (ISW) probing model. The first technique consists in proving security under the stronger t-SNI definition, which enables to use n = t+1 shares instead of n = 2t+1 against t-th order attacks. The second technique consists in progressively incrementing the number of shares within the countermeasure, from a single share to n, thereby reducing the complexity of the countermeasure. The third technique consists in adapting the common shares approach introduced by Coron et al. at CHES 2016, so that half of a randomized look-up table can be pre-computed for multiple SBoxes. We show that our techniques perform well in practice. In theory, the combination of the three techniques should lead to a factor 10.7 improvement in efficiency, for a large number of shares. For a practical implementation with a reasonable number of shares, we get a 4.8 speed-up factor for AES.

In this work, we describe a new polynomial-time attack on the multilinear maps of Coron, Lepoint, and Tibouchi (CLT13), when used in candidate indistinguishability obfuscation (iO) schemes. More specifically, we show that given the obfuscation of the simple branching program that computes the always zero functionality previously considered by Miles, Sahai and Zhandry (Crypto 2016), one can recover the secret parameters of CLT13 in polynomial time via an extension of the zeroizing attack of Coron et al. (Crypto 2015). Our attack is generalizable to arbitrary oblivious branching programs for arbitrary functionality, and allows (1) to recover the secret parameters of CLT13, and then (2) to recover the randomized branching program entirely. Our analysis thus shows that almost all single-input variants of iO over CLT13 are insecure.

Masking is a very common countermeasure against side channel attacks. When combining Boolean and arithmetic masking, one must be able to convert between the two types of masking, and the conversion algorithm itself must be secure against side-channel attacks. An efficient high-order Boolean to arithmetic conversion scheme was recently described at CHES 2017, with complexity independent of the register size. In this paper we describe a simplified variant with fewer mask refreshing, and still with a proof of security in the ISW probing model. In practical implementations, our variant is roughly 25% faster.