A fixed peer id on a public PeerServer belongs to whoever registers it first, so clients must authenticate the peer before sending

Fact

PeerServer checks only the API key, which every client of a public server shares, and that the id is free and well formed. While the service that normally holds a well-known id is offline, anyone can register that id, and every client that dials it reaches them instead. The encrypted data channel does not help: it is encrypted to whoever answered.

Why it matters

A client that dials a fixed id to upload data (a backup server, a bot, a relay) hands that data to a squatter the moment the real peer goes offline, and nothing on the client side looks wrong.

How to apply

• Pin the peer's public key in the client (shipped with the app, not taken from the URL or from the peer).
• On every connection, send a fresh random nonce first and send nothing else until the peer returns a signature over the nonce and its own id; close otherwise. Signing the id as well keeps a key shared by several services from vouching for one under another's id.
• WebCrypto ECDSA P-256 works for this in browsers and in Node (crypto.subtle in both): sign and verify with SHA-256 use the same raw 64-byte signature on both sides, so no DER conversion is needed.
• Back off for a long time after a failed check; the real peer cannot come online under that id until the squatter leaves.