Cryptography Engineering on Cortex-M4

This project is the final project for Cryptography Engineering (2025 Fall) at National Taiwan University, lectured by Matthias J. Kannwischer. It focuses on aggressive performance optimization of two modern cryptographic primitives on resource-constrained embedded systems.

The project implements highly optimized versions of:

• ML-KEM (FIPS 203): NIST’s lattice-based post-quantum key encapsulation mechanism
• X25519 (ECDH25519): Elliptic Curve Diffie-Hellman key exchange

Target Platform: ARM Cortex-M4 microcontroller
Constraints: Constant-time execution, strict code-size budgets for NTT routines

Key Optimizations

X25519 (ECDH25519)

• Finite Field Arithmetic (mod p = 2^255 - 19): Hand-optimized assembly for fe25519_mul (field multiplication) and fe25519_sqr (field squaring), achieving 81-85% cycle reduction.
• Constant-Time Scalar Multiplication: No secret-dependent branches or table indexing to prevent timing side-channel attacks.
• Fixed-Base Acceleration: Window method with precomputed tables using constant-time selection, reducing fixed-base scalar multiplication by up to 94% against the course baseline.

ML-KEM (FIPS 203)

• NTT/iNTT Kernels: Optimized Number-Theoretic Transform operations with reduced memory traffic, layer fusion, and loop unrolling, achieving 2.85-3.79x speedup.
• Polynomial Arithmetic (mod q = 3329): Leveraged ARMv7E-M DSP instructions (SIMD 16-bit operations) for packed polynomial addition and subtraction, achieving 2-2.2x speedup.
• Code-Size Budgeting: NTT routines implemented under strict per-function size constraints while maintaining performance.
• Course-Baseline Result: Reduced the cycle count of ML-KEM poly_ntt by up to 62% under the same benchmarking setup.

This project demonstrates deep understanding of low-level optimization, cryptographic algorithm engineering, and secure implementation practices on embedded ARM platforms.

• View Code on GitHub