# ADR 0017: A password, TOTP and the account's keys

- **Status:** decided 20 Oct 2026, by the developer (D1–D5 below).
- **Decided before:** 19 Oct 2026, the developer: people sign in with a password that never
  leaves the device (SRP) and a mandatory TOTP code; the password unlocks the account's keys;
  recovery codes and a recovery phrase; Sign in with SimpleLogin later, as an extra way in.
- **Builds on:** ADR 0002 (the content envelope: `keyId`, `nonce`, `payload`), ADR 0010
  (email sign-in), ADR 0016 (the register).
- **Followed by:** ADR 0018 (the agenda), the first data encrypted with these keys.

## Context

Todo is to be the agenda of every Halkyon service, both ways (ADR 0016), and what people
plan there may need to be encrypted end to end later: readable on their devices, never by an
instance. Proton does this with one account key pair per person, unlocked on the device by a
key derived from the password, and random keys per store (a calendar, a vault) wrapped with
it; times a server must index stay readable but signed.

Today people sign in with a code by mail (ADR 0010), GitHub, or no account. No secret exists
that the instance doesn't also know, so nothing could be encrypted for the person alone.

## Proposal

1. **Signing in** (identified accounts): the email address (kept as its keyed hash, as ADR
   0010 keeps it), a password and a TOTP code. The password never leaves the device. It
   replaces email codes for signing in outright (nobody signs in with them yet); a code by
   mail serves only resets (5, 6).
2. **The protocol (D1).** SRP-6a (RFC 5054, the 3072-bit group, SHA-256), as Proton runs it:
   the device proves it knows the password without sending it or anything that would sign in
   in its place; the register keeps a verifier. Its private value is derived from the
   password with Argon2id, so guessing a leaked verifier costs Argon2id per guess.
   Implemented in Dart (BigInt), held to RFC 5054's test vectors, and reviewed before release.
3. **The keys.** At sign-up the device makes the account's key pairs (X25519 to encrypt,
   Ed25519 to sign) and encrypts their private halves (XChaCha20-Poly1305) with a key derived
   from the password (Argon2id, a salt per account, parameters a phone manages in about a
   second; HKDF splits it from SRP's value). The register keeps the encrypted private keys,
   the public ones and the salt; it can never use them.
4. **Data keys** are random per store (Todo's agenda, later an app's space), sealed to the
   account's public key, so every device that unlocks the account opens them; sharing with
   someone later seals them to that person's key too, as Proton shares a calendar. Content
   goes into ADR 0002's envelope (`keyId`, `nonce`, the encrypted `payload`), signed by its
   author; what a server must read (a reminder's time) stays readable and signed.
5. **TOTP** (RFC 6238: SHA-1, 6 digits, 30 seconds, one step either way), mandatory for every
   identified account; its secret kept by the register, encrypted with a key of the instance's
   (a new password, `totpKey`). Ten recovery codes, shown once, kept as HMAC-SHA256 under a key
   of the instance (`recoveryCodeKey`, outside the database: a code's 40 bits leak nothing
   from a database without it), each used once.
6. **Recovery.** A recovery phrase (24 words, BIP-39's English list: 256 bits) also wraps the
   private keys: a lost password with the phrase keeps everything. Without it, the address
   (a code by mail) resets the account: new keys, and what was encrypted with the old ones
   is lost, as Proton warns. A lost TOTP device: a recovery code (D3).
7. **Libraries.** On devices libsodium through `sodium` and `sodium_libs` (Android, iOS,
   Linux, Windows, macOS: Argon2id, X25519, XChaCha20-Poly1305, Ed25519); on servers only SRP's
   arithmetic, SHA-256 and HMAC (`crypto`): no server ever decrypts a key.
8. **Where it lives:** with the register (ADR 0016); a member forwards SRP's two steps to it.
9. **Anonymous accounts** stay without a password. Their device makes the key pairs itself
   and keeps the private halves in its keyring, so their private studysets are encrypted as
   anyone's; setting an address and password makes them identified, wrapping those keys with
   the password and handing them to the register.
10. **Private or not, per studyset** (D5): a studyset, with everything inside it, is private
   (end to end encrypted with a data key of its own) or not, chosen when it is made and
   changeable later (the device re-encrypts or decrypts it and syncs it anew). New ones are
   private. A private studyset can't be published, read by the Claude connector, counted for
   "learn this here" or checked by the instance's LanguageTool; asking an AI assistant from
   the device (Lumo's hand-off) works as before. Other apps choose per store alike; Todo's
   agenda is always private.
11. **GitHub sign-in is deprecated** (D2): people who sign in with it add an address, a
   password and TOTP at their next sign-in; three months after the release GitHub signs no
   one in any more and the instance forgets the GitHub IDs.

## Decisions (20 Oct 2026, the developer's)

- **D1** SRP-6a, as decided on 19 Oct (over OPAQUE, which has no Dart implementation, and
  Bitwarden's hash, which signs in in the password's place should TLS be broken).
- **D2** GitHub sign-in is deprecated: its people set an address, a password and TOTP at
  their next sign-in; it ends three months after the release (11).
- **D3** A lost authenticator without recovery codes, as Proton: the password plus a code by
  mail (or the recovery phrase) turns TOTP off, but only 72 hours later, with a mail at the
  start, and called off if the person signs in meanwhile; several authenticators may be
  linked.
- **D4** Nobody signs in with email codes yet, so the password replaces them outright (1).
- **D5** Privacy is the person's choice per studyset, private by default (10).
- **Also (20 Oct, at the crypto core's review):** recovery codes kept under the instance's
  key (5); passwords taken as RFC 8265's OpaqueString (other spaces to a plain space, NFC,
  control characters refused) before Argon2id, so a password unlocks alike on every system.

## Consequences

- People keep a password, an authenticator app and a recovery phrase: more to keep than a
  code by mail, and the reason the agenda can be encrypted.
- Every app gets the sign-up and sign-in screens (halkyon_app), the key store on the device
  (the keyring, as sessions are), and the recovery screens.
- The instance can't read what is encrypted, can't recover it, and can't reset a password
  without the data encrypted under it being lost.
- Learn's server-side features (publishing, the connector, the learner count, LanguageTool)
  ask whether a studyset is private and refuse it; each says why, and how to open it up.
- SRP and the key handling are security-critical code of our own: test vectors, a second
  reading of every step, and no release before both.