Design, Analysis, and Optimization of Isogeny-Based Key Establishment Protocols

Abstract

We analyze the Commutative Supersingular Isogeny Diffie-Hellman protocol (CSIDH), a novel supersingular isogeny-based key establishment protocol. Our analysis is from three perspectives:

Quantum Cryptanalysis. Building upon quantum attacks on ordinary isogeny-based cryptography, we propose a subexponential-time quantum algorithm for inverting the complex multiplication group action for supersingular elliptic curves, which uses only polynomial quantum space. This improves upon previously-known algorithms which required subexponential quantum space.

Optimization. We develop more efficient algorithms for evaluating the class group action in the context of CSIDH. We consider "strategies"—formerly only considered for Supersingular Isogeny Diffie-Hellman (SIDH)—in the context of CSIDH, and develop systematic methods for optimizing "permutations" of the small primes used in CSIDH, which previously had been treated only in an ad hoc fashion. We also develop a systematic technique to optimize the CSIDH keyspace. These optimizations are complementary to prior work on optimizing CSIDH, including improved field arithmetic, Splitting Isogenies into Multiple Batches (SIMBA), and the two-point method. We apply our optimizations to the CSIDH-512 parameter set and give experimental results.

Fault Attacks. We consider physical attacks on static/ephemeral CSIDH in which limited information about which isogenies are "real" and which are "dummy" is revealed. We determine bounds on the number of fault injections required to recover the static secret key, and show that simply reordering the real and dummy isogenies from the ubiquitous "real-then-dummy" ordering to a dynamic random ordering dramatically increases the number of faults required, with negligible impact on the running time of the key exchange protocol (in contrast with prior fault attack countermeasures, which prevent fault attacks entirely at the cost of doubling the running time for key exchange).