Function constant_time_eq 

pub fn constant_time_eq(a: &str, b: &str) -> bool

Constant-time comparison of two x5t#S256 thumbprints (or any two strings) — a short, dependency-free byte-XOR loop rather than pulling in subtle for a single fixed-length string comparison. Mirrors authkestra_op::attestation::constant_time_eq, used for the analogous cnf.jkt device-attestation check.