Function wan_instance_id 

pub fn wan_instance_id(instance_public_key: &[u8; 32]) -> String

An instance’s id: the first 128 bits of SHA-256(instance public key), lowercase unpadded base32 — 26 chars of [a-z2-7] (§2.2).

Not secret. It names the key, so anything signed by the instance key can be checked against the id without trusting whoever supplied the key.