Compressed verification for post-quantum signatures with long-term public keys

Many signature applications-such as root certificates, secure software updates, and authentication protocols-involve long-lived public keys that are transferred or installed once and then used for many verifications. This key longevity makes post-quantum signature schemes with conservative assumptions (e.g., structure-free lattices) attractive for long-term security. But many such schemes, especially those with short signatures, suffer from extremely large public keys. Even in scenarios where bandwidth is not a major concern, large keys increase storage costs and slow down verification. We address this with a method to replace large public keys in GPV-style signatures with smaller, private verification keys. This significantly reduces verifier storage and runtime while preserving security. Applied to the conservative, short-signature schemes Wave and Squirrels, our method compresses Squirrels-I keys from 665 kB to 20.7 kB and Wave822 keys from 3.5 MB to 207.97 kB.