A Fast and Compact RISC-V Accelerator for Ascon and Friends

Ascon-p is the core building block of Ascon, the winner in the lightweight category of the CAESAR competition. With ISAP, another Ascon-p-based AEAD scheme is currently competing in the 2nd round of the NIST lightweight cryptography standardization project. In contrast to Ascon, ISAP focuses on providing hardening/protection against a large class of implementation attacks, such as DPA, DFA, SFA, and SIFA, entirely on mode-level. Consequently, Ascon-p can be used to realize a wide range of cryptographic computations such as authenticated encryption, hashing, pseudorandom number generation, with or without the need for implementation security, which makes it the perfect choice for lightweight cryptography on embedded devices. In this paper, we implement Ascon-p as an instruction extension for RISC-V that is tightly coupled to the processors register file and thus does not require any dedicated registers. This single instruction allows us to realize all cryptographic computations that typically occur on embedded devices with high performance. More concretely, with ISAP and Ascon\u27s family of modes for AEAD and hashing, we can perform cryptographic computations with a performance of about 2 cycles/byte, or about 4 cycles/byte if protection against fault attacks and power analysis is desired. As we show, our instruction extension requires only 4.7 kGE, or about half the area of dedicated Ascon co-processor designs, and is easy to integrate into low-end embedded devices like 32-bit ARM Cortex-M or RISC-V microprocessors. Finally, we analyze the provided implementation security of ISAP, when implemented using our instruction extension

Statistical Ineffective Fault Attacks on Masked AES with Fault Countermeasures

Implementation attacks like side-channel and fault attacks are a threat to deployed devices especially if an attacker has physical access. As a consequence, devices like smart cards and IoT devices usually provide countermeasures against implementation attacks, such as masking against side-channel attacks and detection-based countermeasures like temporal or spacial redundancy against fault attacks. In this paper, we show how to attack implementations protected with both masking and detection-based fault countermeasures by using statistical ineffective fault attacks using a single fault induction per execution. Our attacks are largely unaffected by the deployed protection order of masking and the level of redundancy of the detection-based countermeasure. These observations show that the combination of masking plus error detection alone may not provide sufficient protection against implementation attacks

Fault Attacks on Nonce-based Authenticated Encryption: Application to Keyak and Ketje

In the context of fault attacks on nonce-based authenticated encryption, an attacker faces two restrictions. The first is the uniqueness of the nonce for each new encryption that prevents the attacker from collecting pairs of correct and faulty outputs to perform, e.g., differential fault attacks. The second restriction concerns the verification/decryption, which releases only verified plaintext. While many recent works either exploit misuse scenarios (e.g. nonce-reuse, release of unverified plaintext), we turn the fact that the decryption/verification gives us information on the effect of a fault (whether a fault changed a value or not) against it. In particular, we extend the idea of statistical ineffective fault attacks (SIFA) to target the initialization performed in nonce-based authenticated encryption schemes. By targeting the initialization performed during decryption/verification, most nonce-based authenticated encryption schemes provide the attacker with an oracle whether a fault was ineffective or not. This information is all the attacker needs to mount statistical ineffective fault attacks. To demonstrate the practical threat of the attack, we target software implementations of the authenticated encryption schemes Keyak and Ketje. The presented fault attacks can be carried out without the need of sophisticated equipment. In our practical evaluation the inputs corresponding to 24 ineffective fault inductions were required to reveal large parts of the secret key in both scenarios

Out of Oddity – New Cryptanalytic Techniques Against Symmetric Primitives Optimized for Integrity Proof Systems

International audienceThe security and performance of many integrity proof systems like SNARKs, STARKs and Bulletproofs highly depend on the underlying hash function. For this reason several new proposals have recently been developed. These primitives obviously require an in-depth security evaluation, especially since their implementation constraints have led to less standard design approaches. This work compares the security levels offered by two recent families of such primitives, namely GMiMC and HadesMiMC. We exhibit low-complexity distinguishers against the GMiMC and HadesMiMC permutations for most parameters proposed in recently launched public challenges for STARK-friendly hash functions. In the more concrete setting of the sponge construction corresponding to the practical use in the ZK-STARK protocol, we present a practical collision attack on a round-reduced version of GMiMC and a preimage attack on some instances of HadesMiMC. To achieve those results, we adapt and generalize several cryptographic techniques to fields of odd characteristic

Improved Conditional Cube Attacks on Keccak Keyed Modes with MILP Method

Conditional cube attack is an efficient key-recovery attack on Keccak keyed modes proposed by Huang et al. at EUROCRYPT 2017. By assigning bit conditions, the diffusion of a conditional cube variable is reduced. Then, using a greedy algorithm (Algorithm 4 in Huang et al.\u27s paper), Huang et al. find some ordinary cube variables, that do not multiply together in the 1st round and do not multiply with the conditional cube variable in the 2nd round. Then the key-recovery attack is launched. The key part of conditional cube attack is to find enough ordinary cube variables. Note that, the greedy algorithm given by Huang et al. adds ordinary cube variable without considering its bad effect, i.e. the new ordinary cube variable may result in that many other variables could not be selected as ordinary cube variable (they multiply with the new ordinary cube variable in the first round). In this paper, we bring out a new MILP model to solve the above problem. We show how to model the CP-like-kernel and model the way that the ordinary cube variables do not multiply together in the 1st round as well as do not multiply with the conditional cube variable in the 2nd round. Based on these modeling strategies, a series of linear inequalities are given to restrict the way to add an ordinary cube variable. Then, by choosing the objective function of the maximal number of ordinary cube variables, we convert Huang et al.\u27s greedy algorithm into an MILP problem and the maximal ordinary cube variables are found. Using this new MILP tool, we improve Huang et al.\u27s key-recovery attacks on reduced-round Keccak-MAC-384 and Keccak-MAC-512 by 1 round, get the first 7-round and 6-round key-recovery attacks, respectively. For Ketje Major, we conclude that when the nonce is no less than 11 lanes, a 7-round key-recovery attack could be achieved. In addition, for Ketje Minor, we use conditional cube variable with 6-6-6 pattern to launch 7-round key-recovery attack

An Algebraic Attack on Ciphers with Low-Degree Round Functions: Application to Full MiMC

Item does not contain fulltextAdvances in Cryptology – ASIACRYPT 202

Lightweight Authenticated Encryption Mode Suitable for Threshold Implementation

This paper proposes tweakable block cipher (TBC) based modes $\mathsf{PFB\_Plus}$ and $\mathsf{PFB}\omega$ that are efficient in threshold implementations (TI). Let $t$ be an algebraic degree of a target function, e.g.~$t=1$ (resp.~$t>1$) for linear (resp.~non-linear) function. The $d$-th order TI encodes the internal state into $d t + 1$ shares. Hence, the area size increases proportionally to the number of shares. This implies that TBC based modes can be smaller than block cipher (BC) based modes in TI because TBC requires $s$-bit block to ensure $s$-bit security, e.g. \textsf{PFB} and \textsf{Romulus}, while BC requires $2s$-bit block. However, even with those TBC based modes, the minimum we can reach is 3 shares of $s$-bit state with $t=2$ and the first-order TI ($d=1$). Our first design $\mathsf{PFB\_Plus}$ aims to break the barrier of the $3s$-bit state in TI. The block size of an underlying TBC is $s/2$ bits and the output of TBC is linearly expanded to $s$ bits. This expanded state requires only 2 shares in the first-order TI, which makes the total state size $2.5s$ bits. We also provide rigorous security proof of $\mathsf{PFB\_Plus}$. Our second design $\mathsf{PFB}\omega$ further increases a parameter $\omega$: a ratio of the security level $s$ to the block size of an underlying TBC. We prove security of $\mathsf{PFB}\omega$ for any $\omega$ under some assumptions for an underlying TBC and for parameters used to update a state. Next, we show a concrete instantiation of $\mathsf{PFB\_Plus}$ for 128-bit security. It requires a TBC with 64-bit block, 128-bit key and 128-bit tweak, while no existing TBC can support it. We design a new TBC by extending \textsf{SKINNY} and provide basic security evaluation. Finally, we give hardware benchmarks of $\mathsf{PFB\_Plus}$ in the first-order TI to show that TI of $\mathsf{PFB\_Plus}$ is smaller than that of \textsf{PFB} by more than one thousand gates and is the smallest within the schemes having 128-bit security

Strong Authenticity with Leakage under Weak and Falsifiable Physical Assumptions

Authenticity can be compromised by information leaked via side-channels (e.g., power consumption). Examples of attacks include direct key recoveries and attacks against the tag verification which may lead to forgeries. At FSE 2018, Berti et al. described two authenticated encryption schemes which provide authenticity assuming a “leak-free implementation” of a Tweakable Block Cipher (TBC). Precisely, security is guaranteed even if all the intermediate computations of the target implementation are leaked in full but the TBC long-term key. Yet, while a leak-free implementation reasonably models strongly protected implementations of a TBC, it remains an idealized physical assumption that may be too demanding in many cases, in particular, if hardware engineers mitigate the leakage to a good extent but (due to performance constraints) do not reach leak-freeness. In this paper, we get rid of this important limitation by introducing the notion of “Strong Unpredictability with Leakage” for BC\u27s and TBC\u27s. It captures the hardness for an adversary to provide a fresh and valid input/output pair for a (T)BC, even having oracle access to the (T)BC, its inverse and their leakages. This definition is game-based and may be verified/falsified by laboratories. Based on it, we then provide two Message Authentication Codes (MAC) which are secure if the (T)BC on which they rely are implemented in a way that maintains a sufficient unpredictability. Thus, we improve the theoretical foundations of leakage-resilient MAC and extend them towards engineering constraints that are easier to achieve in practice

Boolean functions for homomorphic-friendly stream ciphers

The proliferation of small embedded devices having growing but still limited computing and data storage facilities, and the related development of cloud services with extensive storage and computing means, raise nowadays new privacy issues because of the outsourcing of data processing. This has led to a need for symmetric cryptosystems suited for hybrid symmetric-FHE encryption protocols, ensuring the practicability of the FHE solution. Recent ciphers meant for such use have been introduced, such as LowMC, Kreyvium, FLIP, and Rasta. The introduction of stream ciphers devoted to symmetric-FHE frameworks such as FLIP and its recent modification has in its turn posed new problems on the Boolean functions to be used in them as filter functions. We recall the state of the art in this matter and present further studies (without proof)

Mind the Gap - A Closer Look at the Security of Block Ciphers against Differential Cryptanalysis

Resistance against differential cryptanalysis is an important design criteria for any modern block cipher and most designs rely on finding some upper bound on probability of single differential characteristics. However, already at EUROCRYPT'91, Lai et al. comprehended that differential cryptanalysis rather uses differentials instead of single characteristics. In this paper, we consider exactly the gap between these two approaches and investigate this gap in the context of recent lightweight cryptographic primitives. This shows that for many recent designs like Midori, Skinny or Sparx one has to be careful as bounds from counting the number of active S-boxes only give an inaccurate evaluation of the best differential distinguishers. For several designs we found new differential distinguishers and show how this gap evolves. We found an 8-round differential distinguisher for Skinny-64 with a probability of 2−56.932−56.93, while the best single characteristic only suggests a probability of 2−722−72. Our approach is integrated into publicly available tools and can easily be used when developing new cryptographic primitives. Moreover, as differential cryptanalysis is critically dependent on the distribution over the keys for the probability of differentials, we provide experiments for some of these new differentials found, in order to confirm that our estimates for the probability are correct. While for Skinny-64 the distribution over the keys follows a Poisson distribution, as one would expect, we noticed that Speck-64 follows a bimodal distribution, and the distribution of Midori-64 suggests a large class of weak keys