Rasta: A cipher with low ANDdepth and few ANDs per bit

Recent developments in multi party computation (MPC) and fully homomorphic encryption (FHE) promoted the design and analysis of symmetric cryptographic schemes that minimize multiplications in one way or another. In this paper, we propose with Rasta a design strategy for symmetric encryption that has ANDdepth d and at the same time only needs d ANDs per encrypted bit. Even for very low values of d between 2 and 6 we can give strong evidence that attacks may not exist. This contributes to a better understanding of the limits of what concrete symmetric-key constructions can theoretically achieve with respect to AND-related metrics, and is to the best of our knowledge the first attempt that minimizes both metrics simultaneously. Furthermore, we can give evidence that for choices of d between 4 and 6 the resulting implementation properties may well be competitive by testing our construction in the use-case of removing the large ciphertext-expansion when using the BGV scheme.

Publication Details:

Christoph Dobraunig, Maria Eichlseder, Lorenzo Grassi, Virginie Lallemand, Gregor Leander, Eik List, Florian Mendel and Christian Rechberger: Rasta: A cipher with low ANDdepth and few ANDs per bit. Cryptology ePrint Archive 2018


Exploiting Ineffective Fault Inductions on Symmetric Cryptography

Since the seminal work of Boneh et al., the threat of fault attacks has been widely known and new techniques for fault attacks and countermeasures have been studied extensively. The vast majority of the literature on fault attacks focuses on the ability of fault attacks to change an intermediate value to a faulty one, such as differential fault analysis (DFA), collision fault analysis, statistical fault attack (SFA), fault sensitivity analysis, or differential fault intensity analysis. The other aspect of faults—that faults can be induced and do not change a value—has been far less researched. In case of symmetric ciphers, this area is covered by ineffective fault attacks (IFA). However, IFA relies on the ability of an attacker to induce reproducible deterministic faults like stuck-at faults for a smaller intermediate structure (e.g., one bit or byte), which is often considered to be impracticable.

As a consequence, most countermeasures against fault attacks focus on the ability of faults to change intermediate values and usually try to detect such a change (detection-based), or to destroy the exploitable information if a fault happens (infective countermeasures). Such countermeasures implicitly assume that the release of “fault-free” ciphertexts in the presence of a fault-inducing attacker does not reveal any exploitable information. In this work, we challenge this assumption and show attacks that exploit the fact that intermediate values leading to such “fault-free” ciphertexts show a non-uniform distribution, while they should be uniformly distributed. The presented attacks are entirely practical and are demonstrated to work for software implementations of AES and for a hardware co-processor.

Publication Details:

Christoph Dobraunig, Maria Eichlseder, Thomas Korak, Stefan Mangard, Florian Mendel and Robert Primas: Exploiting Ineffective Fault Inductions on Symmetric Cryptography. Cryptology ePrint Archive 2018

Robustness of CAESAR Candidates

Authenticated ciphers rely on the uniqueness of the nonces to meet their security goals. In this work, we investigate the implications of reusing nonces for three third-round candidates of the ongoing CAESAR competition, namely Tiaoxin, AEGIS and MORUS. We show that an attacker that is able to force nonces to be reused can reduce the security of the ciphers with results ranging from full key-recovery to forgeries with practical complexity and a very low number of nonce-misuse queries.

Publication Details:

Daniel Kales, Maria Eichlseder and Florian Mendel: Note on the Robustness of CAESAR Candidates. Cryptology ePrint Archive 2017

Analysis of RIPEMD-160

In this paper, we propose an improved cryptanalysis of the double-branch hash function RIPEMD-160 standardized by ISO/IEC. We show how to theoretically calculate the step differential probability of RIPEMD-160, which was stated as an open problem by Mendel et al. at ASIACRYPT 2013. Secondly, based on the method proposed by Mendel et al. to automatically find a differential path of RIPEMD-160, we construct a 30-step differential path where the left branch is sparse and the right branch is controlled as sparse as possible. To ensure the message modification techniques can be applied to RIPEMD-160, some extra bit conditions should be pre-deduced and well controlled. These extra bit conditions are used to ensure that the modular difference can be correctly propagated. This way, we can find a collision of 30-step RIPEMD-160 with complexity 267. This is the first collision attack on round-reduced RIPEMD-160. Moreover, by a different choice of the message words to merge two branches and adding some conditions to the starting point, the semi-free-start collision attack on the first 36-step RIPEMD-160 from ASIACRYPT 2013 can be improved by a factor of 215.3 to 255.1.

Publication Details:

Fukang Liu, Florian Mendel, Gaoli Wang: Collisions and Semi-Free-Start Collisions for Round-Reduced RIPEMD-160. ASIACRYPT 2017

Gimli: A cross-platform permutation

This paper presents Gimli, a 384-bit permutation designed to achieve high security with high performance across a broad range of platforms, including 64-bit Intel/AMD server CPUs, 64-bit and 32-bit ARM smartphone CPUs, 32-bit ARM microcontrollers, 8-bit AVR microcontrollers, FPGAs, ASICs without side-channel protection, and ASICs with side-channel protection.
Like other permutations with sufficiently large state sizes, Gimli can easily be used to build high-security block ciphers, tweakable block ciphers, stream ciphers, message-authentication codes, authenticated ciphers, hash functions, etc.

Publication Details:

Daniel J. Bernstein, Stefan Kölbl, Stefan Lucks, Pedro Maat Costa Massolino, Florian Mendel, Kashif Nawaz, Tobias Schneider, Peter Schwabe, François-Xavier Standaert, Yosuke Todo and Benoît Viguier: Gimli: a cross-platform permutation. CHES 2017

Analysis of Round-Reduced SKINNY

At CRYPTO’16, Beierle et al. presented SKINNY, a family of lightweight tweakable block ciphers intended to compete with SIMON. SKINNY can be implemented efficiently in both soft- and hardware, possesses a Substitution-Permutation-Network structure, and supports block sizes of 64 and 128 bits as well as key and tweak sizes of 64, 128, 192, and 256 bits. This paper outlines a related-tweakey impossible-differential attack on 21 rounds of SKINNY-64/128 and two attacks on 22 and 23 rounds of SKINNY-64/128 under the assumption that 48 bits of the tweakey are public.

Publication Details:

Ralph Ankele, Subhadeep Banik, Avik Chakraborti, Eik List, Florian Mendel, Siang Meng Sim and Gaoli Wang: Related-Key Impossible-Differential Attack on Reduced-Round SKINNY. ACNS 2017

Side-Channel Analysis of Keymill

One prominent countermeasure against side-channel attacks, especially differential power analysis (DPA), is fresh re-keying. In such schemes, the so-called re-keying function takes the burden of protecting a cryptographic primitive against DPA. To ensure the security of the scheme against side-channel analysis, the used re-keying function has to withstand both simple power analysis (SPA) and differential power analysis (DPA). Recently, at SAC 2016, Keymill – a side-channel resilient key generator (or re-keying function) – has been proposed, which is claimed to be inherently secure against side-channel attacks. In this work, however, we present a DPA attack on Keymill, which is based on the dynamic power consumption of a digital circuit that is tied to the 01 and 10 switches of its logical gates. Hence, the power consumption of the shift-registers used in Keymill depends on the 01 and 1switches of its internal state. This information is sufficient to obtain the internal differential pattern (up to a small number of bits, which have to be brute-forced) of the 4 shift-registers of Keymill after the nonce (or IV) has been absorbed. This leads to a practical key-recovery attack on Keymill.

Publication Details:

Christoph Dobraunig, Maria Eichlseder, Thomas Korak, Florian Mendel: Side-Channel Analysis of Keymill. COSADE 2017