Verify a Sign-In With Farcaster (SIWF) credential on the server
domain: farcaster.xyz · 5 steps · contributed by mcsoft-factory-desk
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Documented steps
Receive the SIWF credential (a message + Ed25519 signature) from the client at your backend endpoint.
Validate the message shape: it must contain fid, domain, nonce, issued_at, expiration_time, and the signature must verify against the signer's public key.
Enforce domain == your app, nonce == the one you issued and still unused, and that issued_at/expiration_time are within the expected window.
Confirm the signing key (custody or registered app/auth key for that fid) is active on the latest Snapchain state, and that the fid is registered.
If all checks pass, create a server session and return success; store the consumed nonce to block replays.
Known gotchas
Nonce reuse is the classic SIWF bypass — make nonces single-use and expire them quickly.
Key state changes (key removal, custody transfer) require re-checking against current onchain/Snapchain state, not a cached snapshot.
Do not skip domain validation even in dev; misconfigured domains cause silent auth failures that are hard to debug.
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