KyberSlash: Exploiting secret-dependent division timings in Kyber implementations​ ​

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• Collaboration with Daniel J. Bernstein (University of Illinois at Chicago), Karthikeyan Bhargavan (Inria, Paris), Shivam Bhasin (Nanyang Technological University, Singapore), Anupam Chattopadhyay (Nanyang Technological University, Singapore), Tee Kiah Chia (Nanyang Technological University, Singapore), Franziskus Kiefer (Cryspen, Berlin), Thales Paiva (University of Sao Paulo, Brazil), Prasanna Ravi (Nanyang Technological University, Singapore), and Goutam Tamvada (Cryspen, Berlin)
   

• Abstract: This paper presents KyberSlash1 and KyberSlash2 – two timing vulnerabilities in several implementations (including the official reference code) of the Kyber Post-Quantum Key Encapsulation Mechanism, currently undergoing standardization as ML-KEM. We demonstrate the exploitability of both KyberSlash1 and KyberSlash2 on two popular platforms: the Raspberry Pi 2 (Arm Cortex-A7) and the Arm Cortex-M4 microprocessor. Kyber secret keys are reliably recovered within minutes for KyberSlash2 and a few hours for KyberSlash1.
  We responsibly disclosed these vulnerabilities to maintainers of various libraries and they have swiftly been patched. We present two approaches for detecting and avoiding similar vulnerabilities. First, we patch the dynamic analysis tool Valgrind to allow detection of variable-time instructions operating on secret data, and apply it to more than 1000 implementations of cryptographic primitives in SUPERCOP. We report multiple findings. Second, we propose a more rigid approach to guarantee the absence of variable-time instructions in cryptographic software using formal methods.
   

Read More: https://eprint.iacr.org/2024/1049

Read More: https://kyberslash.cr.yp.to/ 

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Nibbling MAYO: Optimized Implementations for AVX2 and Cortex-M4​​

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• Collaboration with Ward Beullens (IBM Research), Fabio Campos (RheinMain University of Applied Science), Sofía Celi (Brave Software), Basil Hess (IBM Research)

• Abstract: MAYO is a popular high-calorie condiment as well as an auspicious candidate in the ongoing NIST competition for additional post-quantum signature schemes achieving competitive signature and public key sizes.
  In this work, we present high-speed implementations of MAYO using the AVX2 and Armv7E-M instruction sets targeting recent x86 platforms and the Arm Cortex-M4.                                                                                                                                                                                                                                                                                Moreover, the main contribution of our work is showing that MAYO can be even faster when switching from a bitsliced representation of keys to a nibble-sliced representation.                                                                                                                                                                                                                                                                While the bitsliced representation was primarily motivated by faster arithmetic on microcontrollers, we show that it is not necessary for achieving high performance on Cortex-M4.On Cortex-M4, we instead propose to implement the large matrix multiplications of MAYO using the Method of the Four Russians (M4R), which allows us to achieve better performance than when using the bitsliced approach. This results in up to 21% faster signing.                                                                                                                                                                                                                                                                                                                                              For AVX2, the change in representation allows us to implement the arithmetic much faster using shuffle instructions. Signing takes up to  3.2x fewer cycles and key generation and verification enjoy similar speedups. This shows that MAYO is competitive with lattice-based signature schemes on x86 CPUs, and a factor of 2-6 slower than lattice-based signature schemes on Cortex-M4 (which can still be considered competitive).

Read More: https://eprint.iacr.org/2023/1683

​Fast and Clean: Auditable high-performance assembly via constraint solving

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• Collaboration with Amin Abdulrahman  (Max Planck Institute for Security and Privacy), Hanno Becker (AWS), Fabien Klein (Arm) 

• Abstract: Handwritten assembly is a widely used tool in the development of high-performance cryptography: By providing full control over instruction selection, instruction scheduling, and register allocation, highest performance can be unlocked. On the flip side, developing handwritten assembly is not only time-consuming, but the artifacts produced also tend to be difficult to review and maintain – threatening their suitability for use in practice.                                                                                                                                                                                                                                                                                                                      In this work, we present SLOTHY (Super (Lazy) Optimization of Tricky Handwritten assemblY), a framework for the automated superoptimization of assembly with respect to instruction scheduling, register allocation, and loop optimization (software pipelining): With SLOTHY, the developer controls and focuses on algorithm and instruction selection, providing a readable “base” implementation in assembly, while SLOTHY automatically finds optimal and traceable instruction scheduling and register allocation strategies with respect to a model of the target (micro)architecture.                                                                                                                                                                                                                                                                      We demonstrate the flexibility of SLOTHY by instantiating it with models of the Cortex-M55, Cortex-M85, Cortex-A55 and Cortex-A72 microarchitectures, implementing the Armv8.1-M+Helium and AArch64+Neon architectures. We use the resulting tools to optimize three workloads: First, for Cortex-M55 and Cortex-M85, a radix-4 complex Fast Fourier Transform (FFT) in fixed-point and floating-point arithmetic, fundamental in Digital Signal Processing. Second, on Cortex-M55, Cortex-M85, Cortex-A55 and Cortex-A72, the instances of the Number Theoretic Transform (NTT) underlying CRYSTALS-Kyber and CRYSTALS-Dilithium, two recently announced winners of the NIST Post-Quantum Cryptography standardization project. Third, for Cortex-A55, the scalar multiplication for the elliptic curve key exchange X25519. The SLOTHY-optimized code matches or beats the performance of prior art in all cases, while maintaining compactness and readability.

Read More: https://eprint.iacr.org/2022/1303