Passkey

Security-Critical Rules

• Challenge must be unique per ceremony — reused challenges enable replay attacks
• Origin validation is mandatory — missing check allows phishing from similar domains
• Store and verify sign count — detects cloned authenticators when count doesn't increment
• Never implement CBOR parsing manually — use established libraries, crypto is hard

Server Implementation

• Use battle-tested libraries — SimpleWebAuthn (JS), py_webauthn (Python), webauthn-rs (Rust)
• Challenges expire quickly — 60-120 seconds max, store server-side
• Store credential ID as base64 — binary data, needs encoding for database
• Store public key in COSE format — libraries handle this, don't transform it

Credential Storage Requirements

• Credential ID — unique identifier, used in allowCredentials
• Public key — COSE format, for signature verification
• Sign count — integer, increment check detects cloning
• Transports hint — helps browser suggest correct authenticator