Paper 2025/1458 Original: eprint.iacr.org/2025/1458 ↗

INKE: Isogeny-Based PKE Using Intermediate Curves

Abstract

POKE (POint-based Key Exchange), proposed by Basso and Maino at Eurocrypt 2025, is currently the fastest known isogeny-based public-key encryption scheme. Although POKE is secure against currently known key-recovery attacks, there is no known reduction from key-recovery security to IND-CPA security. In this work, we propose INKE, a variant of POKE that replaces torsion points in the encryption process with intermediate elliptic curves. This modification enables a quantum reduction from key-recovery security to IND-CPA security in the algebraic isogeny model (AIM), while maintaining the practical performance. Although INKE is overall slower than POKE and has larger public-key and ciphertext sizes, it remains more efficient than other group-action-based key exchange protocols such as CSIDH and CORAL that admit reductions from key-recovery security to shared-secret security in algebraic group action model (AGAM). To illustrate the practical overhead of INKE compared to POKE, we provide an optimized C implementation together with detailed benchmark comparisons at each security level.