A code holds 40 bits, which plain SHA-256 hashes in a leaked database
would give up to guessing; HMAC-SHA256 under a key of the instance kept
outside the database (recoveryCodeKey, at least 32 characters) gives
nothing to guess against without it. Decided 20 Oct 2026.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>
ADR 0017's second factor, in halkyon_base_server (base.dart):
- RFC 6238 with HMAC-SHA1, 6 digits, 30-second steps, one step either
way. checkTotp takes the account's last accepted step as a required
argument and refuses a code of that step or before, so no code is
accepted twice; every step of the window is computed and compared.
- 160-bit secrets in RFC 4648 Base32 (strict decoding); the otpauth
URI with issuer and account escaped.
- Ten recovery codes of eight Crockford characters (short_code.dart),
kept as SHA-256 hashes, each used once, every hash compared in
constant time.
- Tests: RFC 6238 Appendix B (8 digits, SHA-1, SHA-256, SHA-512), the
window, replay, RFC 4648's Base32 vectors, the recovery codes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>
