halkyon/docs/adr/0017-password-totp-and-keys.md

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.

Settled in building the register's side (20 Oct 2026)

In halkyon_base_server (its README has the details); members forward every step through AccountRegister, the session being theirs.

• Sign-up (password.set) needs an address: the person's own, or one a mailed code proves in the same call, which makes an anonymous person identified under their ID. The password signs in only once an authenticator is confirmed; until then a new setup replaces it. The register checks what it can: salt 16 bytes, Argon2id argon2id13 at 3–16 passes and 64 MiB–1 GiB, the verifier 1 < v < N padded to N, public keys 32 bytes, each wrapping a 24-byte nonce and 112 bytes.
• SRP's identity I is the sign-in's challenge ID, not the account's: x comes from Argon2id over the password and salt, so I only binds the proof to this sign-in, and an account ID handed to whoever types an address would tell who has one.
• Addresses nobody has (or whose account has no password) get a salt and verifier from a third instance key, signInDecoyKey, and the address's hash, the same queries and modular arithmetic (both are computed for every sign-in), a challenge row of their own, and in the second step the same refusal. One refusal, wrongCredentials, for a wrong password, an unknown address, and a wrong, used or missing code; setupIncomplete (no confirmed authenticator) and notDue (a reset waiting) only after a right password. Begun sign-ins are limited to 10 an hour per address and 30 per network; challenges last two minutes and are taken once.
• TOTP secrets are sealed with XChaCha20-Poly1305 (pub.dev's cryptography, pure Dart, pinned) under a key derived from totpKey, bound to the row's account and ID; each step's code passes once (a conditional update of the last step). At most 10 authenticators; one not confirmed within a day is pruned; the last confirmed stays; removing one, or new recovery codes, takes a code (10 tries an hour per account).
• Resets (D3): an authenticator reset takes effect 72 hours after the password and a mailed code or the recovery phrase ask, with a mail to the address at once (none sent, none started); a sign-in with a code calls off every reset waiting. Once due, the first sign-in with the password alone carries it out (authenticators and recovery codes go), and the password alone signs in until a new authenticator is confirmed. The recovery phrase is proved by the account's Ed25519 signature over halkyon key proof v1|, the challenge's ID and a nonce. A password set with the phrase, or an account reset, ends the account's sessions on the register's server.
• The account reset without the phrase waits 72 hours too, with a mail at the start (a mailed code asks, a new one carries it out once due; a sign-in with the password calls it off): a proposal, for the developer to confirm. Carried out: a new password and new keys, no authenticators.
• Keys: without all three keys (or without email sign-in) the instance offers no password sign-in (passwordSignIn in its basics), so an instance can take these releases before its operator makes keys; a key set but shorter than 32 characters stops the server.
• GitHub (D2): an account with a password and a confirmed authenticator is refused (usePassword), one without signs in as before and the app leads to setting up an address, a password and an authenticator (its ways in say what is missing): a refusal there would lock out GitHub's people on a new device, as nothing else signs them in. Where passwords are offered, no new GitHub account is linked (deprecated). From GITHUB_SIGN_IN_ENDS (a day the operator sets) GitHub signs nobody in (ended), the basics stop offering it, and the daily pruning forgets every GitHub ID.
• Email codes still sign in (until step 3), but not into an account with a password and a confirmed authenticator (usePassword), which a code alone would bypass.

Step 3 (email codes go): sign-up keeps password.set with its code; emailCode.verifyCode and emailSignIn.signIn go, and with them the switch token; accounts with an address but no password, and accounts whose setup never finished, need a way in (a mailed code and a new password, as the account reset, or a password plus a mailed code until an authenticator is confirmed); a member's sessions should end too when the register resets an account; and a signed-in change of password (with the keys at hand) is still missing.

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.