The Rosenpass static keypair was regenerated on every engine start
(rp.GenerateKeyPair in NewManager), so the local ~512KB public key —
and its fingerprint — changed on each client restart.
Persist the keypair to <StateDir>/rosenpass_key.json with 0600
permissions (same protection tier as the WireGuard private key), and
reload it on start so the public key stays stable across restarts. A
missing, corrupt, or version-incompatible file degrades gracefully to
generating a fresh ephemeral keypair (previous behaviour); an empty
StateDir keeps the ephemeral path for callers without a state dir.
This is the foundation for fingerprint-based RP pubkey caching over
signalling (NET-1407): a stable local key lets remote peers keep their
cached copy valid across our restart.
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