# Encryption patterns — EDEN

See ADR-0005 for the full decision. Quick reference:

- **Reversible field encryption**: AES-256-GCM, random 96-bit IV per operation, auth tag verified on decrypt. Storage format: `{iv_b64}:{authTag_b64}:{ciphertext_b64}` in a single TEXT column (legacy-proven pattern from Control iD credential storage). Root key derived via SHA-256 of an env-provided secret, normalizing any-length input to 32 bytes. Every encrypted value also stores a `key_version` for rotation.
- **OTP**: HMAC-SHA256 with a server secret (never plain SHA-256 — OTP space is only 10^6, needs secret-based rainbow-table resistance). 6 digits, 5-minute TTL, single use, max attempts with lockout, invalidate-on-new-request.
- **Public link tokens** (client/reseller registration, signature invite): CSPRNG, ≥96 bits for registration tokens, 256 bits for signature invite tokens. Stored as SHA-256 hash only (plain hash sufficient given the token's own entropy — no HMAC needed here, unlike OTP).
- **Passwords**: Argon2id, versioned parameters (see ADR-0005), re-hash on next successful login when parameters change.