Paper 2026/1660

Transient Quantum Resistance, with Application to Ethereum Consensus

Abstract

Candidates for post-quantum migration carry additional costs compared to their pre-quantum counterparts, especially for signatures, and they lose attractive properties of schemes such as BLS: homomorphism, and hence direct signature aggregation. We propose a methodology through which a pre-quantum primitive may still be securely used past Q-day (the advent of quantum computers) in settings where forgery of signatures or cryptographic proofs need only be prevented for a bounded lifespan (transient quantum security). The idea is to bind a fresh, ephemeral, ordinary pre-quantum key to a long-term post-quantum identity once per lifespan window, so a forgery under the fresh key is useful only for that window, and to derive the fresh key so that its exposure never leaks the long-term secret. Our main case study is the post-quantum migration of Ethereum consensus, where we give a solution that keeps relying on BLS and (i) retains signature aggregation, central at Ethereum's scale, without a SNARK prover, so the consensus-critical aggregate stays a single ~96-byte BLS signature, about three orders of magnitude smaller than the hundreds of kilobytes a SNARK-aggregated hash-based alternative needs per aggregate, as in the current proposal for post-quantum Ethereum consensus; and (ii) needs only an additional ~590-690 bytes per epoch of per-validator reveal traffic. Security holds as long as a standard pairing-based variant of the computational Diffie-Hellman problem (co-CDH') cannot be broken by a quantum computer within the 6.4-minute duration of an Ethereum epoch; we discuss the hardware and time budgets such a break would require today, and how they may shrink as quantum hardware improves. We also provide a formal model and security analysis for the construction, and, of independent interest, a new analysis of BLS where the secret key is sampled similarly to the Dodis-Yampolskiy VRF (IACR PKC 2005) and the adversary is given a related group element as leakage.