Design Space for a Post-Quantum Public Key Registry for Ethereum Validators

This Ethereum Research post proposes a Public Key Registry as a warmup phase before validators switch from BLS signatures to a post-quantum scheme, with hash-based XMSS named as the strongest candidate. The authors say the registry fork will come first, letting validators register post-quantum keys, with the signature switch following several forks later. Per the post, an XMSS public key is 52 bytes, compared with 48 bytes for a BLS12-381 key, and each per-slot signature is 3,112 bytes, versus 96 bytes for BLS. The post says a top-bottom tree strategy reduces key generation and signing to O(√lifetime), requiring only a few megabytes of storage.