halkyon/docs/adr/0020-sign-in-with-simplelogin.md

# ADR 0020: Signing in with SimpleLogin only

- **Status:** decided 23 Oct 2026, by the developer (D1–D20 below), the *Proposal*
  with it (D15), as D18 to D20 change it.
- **Built (23 Oct 2026), step 1:** the forwarding page (3),
  `/simplelogin-callback` on the register's web server (halkyon_base's
  `addSimpleLoginCallbackRoute`, Caddy forwarding the path), ahead of the sign-in, so
  development signs in through learn.halkyon.eu's page, which its SimpleLogin app
  registers too. The state is `<app>-<way back>.<rest>`: the marker one of the module's
  fixed table (Learn and Todo, the loopback on ports 43823 and 43824, or the app's
  scheme; named `<app>-<platform>` until D9's correction), the rest 1 to 512 characters
  of base64url. A 303 carries the code or
  SimpleLogin's error (`deny`) with the state and nothing else; whatever is malformed,
  repeated or not a GET is refused with a page that repeats nothing. No caching, no
  referrer, nothing logged. Android's `assetlinks.json` for Halkyon Learn signed with
  the release key is answered by `deploy/Caddyfile` on learn.halkyon.eu alone, held to
  `tool/release/android-certificate.sha256` by a test: built for D9's verified links,
  which its correction dropped, and kept, as it costs nothing.
- **Built (23 Oct 2026), step 2:** the sign-in on the server's side (1 to 5), on
  halkyon_oidc (ADR 0023) with its SimpleLogin preset: `simpleLogin.begin` and
  `simpleLogin.complete` in halkyon_base, a member asking the register for both; the
  instance's basics with `simpleLogin`, the ways in with it; the attempts and the
  newcomers' identities forgotten hourly with the sign-up attempts. Otherwise than
  proposed: the identities are halkyon_oidc's (`halkyon_oidc_identity`: account,
  provider, subject; the provider `simplelogin`), not a table of halkyon_base's; the app
  and platform are the state's marker rather than a column; a signed-in caller of a
  member becomes a person on the register as the attempt begins, as linking would make
  them; GitHub links nobody more where SimpleLogin is offered, as where passwords are
  (ADR 0017); `SIGN_IN_WAYS` counts on the register, which members ask, and stops any
  server with a name it doesn't know as it starts; sessions name the method
  `simplelogin` (these deviations accepted by the developer, 23 Oct). Someone new gets
  no account yet: their identity waits (D16), with a
  handle suggested (D17), for the sign-up of step 3. halkyon_oidc's `testing.dart` gives
  hosts a fake SimpleLogin signing real ID tokens, which the module's tests use too.
- **Built (23 Oct 2026), step 3 so far:** the password's own proof key (`halkyon_keys`'
  subkey 3 of the master key, D19), which `account_password` keeps in place of SRP's
  verifier for an account that signs in with SimpleLogin (the verifier becomes nullable:
  a migration that drops nothing); `simpleLogin.signUp` in one call (D18) and
  `simpleLogin.checkHandle`, both for someone new's identity alone; `keys.begin` and
  `keys.unlock`, the proof counted as ADR 0017's; every sign-in into an account kept ten
  minutes under its attempt as D20's confirmation. Then the changes, each through the
  register: the first keys (and handle, where none) of an account SimpleLogin was added
  to (`keys.set`), with the sign-in alone, as there is no password to prove yet; a new
  password, a lost one with the phrase (`keys.beginPhrase`, `keys.resetPassword`), a new
  phrase and the deletion (`account.delete`'s `keysProof`), each with the proof and the
  sign-in, which the change takes: one change for each sign-in. A new or lost password
  ends every other session, as ADR 0017's change does. Then mending a broken link
  (8, D7): `simpleLogin.beginMend(attempt, handle)` and `simpleLogin.mend(attempt,
  proof)`, the phrase's proof binding the attempt, so a relayed proof links nobody
  else's identity. Otherwise than proposed: only an account with a SimpleLogin identity
  is mended (D10: nobody adds SimpleLogin signed out), every other answered with a
  decoy; mending ends every session of the account, as whoever came in through the old
  link may still be in; and SimpleLogin needs `SIGN_IN_DECOY_KEY` alone of ADR 0017's
  three keys, a server with a SimpleLogin app and without it stopping as it starts.
  Then handles (9, D5): `/@handle` and `profiles.byHandle` answer as for a handle nobody
  has until its person goes public (a handle fixed by a publish, or a display name, a
  bio, a link or an avatar), the host's own code still finding whose a handle is.
  Otherwise than proposed: not only asking whether a handle is free is limited, but
  every answer about a new handle, counted together (`countHandleCheck`, 30 an hour per
  person or attempt, 60 per network): saving a profile with one, signing up and the
  first keys with one, and Halkyon Learn's blocks by handle, as each tells the same, so
  a limit on checking alone would limit nothing; a person's own handle and what is no
  handle aren't counted. The handle suggested at sign-up is not counted: it tells at
  most that the name SimpleLogin shared is someone's handle already, once a sign-in.
  Step 3's deviations accepted by the developer (23 Oct, night).
- **Builds on:** ADR 0010 (email sign-in), ADR 0011 (profile pages), ADR 0016 (the
  register), ADR 0017 (a password, TOTP and the keys).
- **Changes:** ADR 0017's way in on an instance that offers SimpleLogin (its keys, their
  wrapping and the recovery phrase stay as they are), and every instance's anonymous
  accounts, which go (ADR 0010's start without an account, ADR 0016's 6, ADR 0017's 9).

## Context

The developer asked on 23 Oct to limit signing in to "only SimpleLogin/Proton single sign
in". Proton offers no OpenID Connect sign-in to other apps; SimpleLogin, which belongs to
Proton, does ("Sign in with SimpleLogin"), and its own login page offers "Log in with
Proton". What SimpleLogin does, from its source (`simple-login/app`, 22 Oct) and from tries
against app.simplelogin.io on 23 Oct:

- **OpenID Connect's code flow:** `https://app.simplelogin.io/oauth2/authorize`,
  `/oauth2/token`, `/jwks`; ID tokens signed with RS256, carrying the `nonce` the
  authorization asked for and what the person shares (address, name, avatar); each code is
  taken once. **No PKCE**; the client secret is always required, so only a server can
  exchange a code.
- **New apps are closed by default since 22 Oct** (`ALLOW_NEW_OAUTH_CLIENTS`); "existing
  websites keep working as usual". app.simplelogin.io still let the developer make apps on
  23 Oct: one for development, one for learn.halkyon.eu ("Halkyon") and a few in reserve.
  Other operators may not get an app any more.
- **Dev Mode:** until SimpleLogin approves an app, its consent page warns "… is in Dev
  Mode and isn't approved (yet) by SimpleLogin". Approval is asked for on the app's page.
- **Redirects:** each must be registered exactly (an unregistered loopback address was
  refused on 23 Oct), and the settings page takes https addresses only.
- **The ID (`sub`) belongs to one person in one app** (`sub` is the person's link to the
  app, `ClientUser`), whatever the discovery document's `public` says. It changes when the
  person removes the app in SimpleLogin's list, deletes the alias they shared with it, or
  deletes their SimpleLogin account. Once someone has allowed an app, later sign-ins skip
  the consent page and give the same ID.
- **The consent page always shares an address, a name and an avatar:** the app can't
  choose (`Client.get_scopes`). The address is the person's own or an alias, by default a
  new one, which counts against their aliases.

## Decisions (23 Oct 2026, the developer's)

- **D1 SimpleLogin only** on learn.halkyon.eu (over SimpleLogin as an extra way in beside
  ADR 0017's, which was recommended): no address, no TOTP and no password signs in there.
- **D2 A password still unlocks the keys** (recommended, over dropping the keys): it never
  leaves the device; ADR 0017's keys, wrapping and recovery phrase stay.
- **D3 Per instance:** each operator chooses the ways; operators without a SimpleLogin app
  keep ADR 0017's.
- **D4 No anonymous accounts, anywhere** (recommended): every account has a way in and a
  handle; the first start needs a connection and an account.
- **D5 A handle for everyone**, suggested at sign-up from the name SimpleLogin shares
  (recommended), confirmed or changed by the person; the name is then dropped. Its public
  page exists only once the person goes public (recommended): something published or the
  profile filled in.
- **D6 An ID nobody has, signed out: ask first** (recommended): "New here?" makes an
  account; "I already have one" takes the handle and the recovery phrase (8).
- **D7 A broken link is mended with the handle and the recovery phrase** (the developer's,
  over the phrase alone, which was recommended): every account has a handle now (D5).
- **D8 A hard cut-off on learn.halkyon.eu, 30 days after the release** (over the day
  GitHub ends, 22 Jan 2027, which was recommended): from then on every session of an
  account without SimpleLogin ends, GitHub and email codes sign nobody in there, and the
  instance offers SimpleLogin alone.
- **D9 Apps come back through their scheme or the loopback** (corrected 23 Oct, night,
  by the developer, after research; decided first as verified links for phones on
  learn.halkyon.eu, Android's App Links and iOS's https callbacks): Android, iOS and
  macOS come back through the app's scheme, which the system's browser sheet catches
  (Chrome's Auth Tab, since Chrome 132, even after the forwarding page's redirect;
  `ASWebAuthenticationSession`), Linux and Windows through the loopback.
  `simpleLogin.begin` takes the way back (`SimpleLoginReturn`: `loopback`, `scheme`),
  not the platform. Verified links protected nothing here: a code caught on its way is
  worthless without the attempt's ID (4), and a hostile app beginning an attempt of its
  own can ask for the scheme or the loopback, which other platforms need open (an
  Android app can listen on `127.0.0.1` too). They cost: debug builds and other
  instances need the scheme anyway, Firefox has no Auth Tab, Samsung Internet stays on
  the page, and flutter_web_auth_2 (v5.1.0) has open bugs with https callbacks (#183,
  #215). The scheme is what AppAuth calls the most widely supported.
- **D10 Accounts without SimpleLogin are deleted 30 days after the cut-off**
  (recommended, over keeping them unreachable): nobody can add SimpleLogin signed out;
  the reminder names both days.
- **D11 An account left unfinished (no keys) is deleted after a day** (recommended, over
  finishing it at the next sign-in); its handle is free again.
- **D12 The ID token checked with `dart_jsonwebtoken` itself** (23 Oct, evening,
  recommended), already in the lock through Serverpod's sign-ins. The developer first
  chose Serverpod's own verifier (`IdTokenVerifier`, which Google's and Firebase's
  sign-ins use; not exported, so imported from its `src/`), which can't: SimpleLogin's
  ID tokens carry no `typ` in their header (`jose_utils.make_id_token` writes `alg` and
  `kid` alone, and jwcrypto 0.8 adds nothing), and the verifier calls
  `JWT.verify` with its default `checkHeaderType: true`, so it refuses every one, and
  takes no options to change that. It also tries every key rather than the one the
  `kid` names, never fetches the keys again for an unknown `kid` (an hour of failed
  sign-ins after SimpleLogin changes its key) and fetches them without a timeout. So:
  `checkHeaderType` off, `RS256` alone, the key the `kid` names, fetched again for one
  unknown, with a timeout (over adapting a copy of Serverpod's verifier, or `jose`); in
  the OpenID Connect module the sign-in is built on (ADR 0023).
- **D13 Attempts in a table** (recommended, over the server's memory): a restart or a
  deploy in the middle of a sign-in loses none, and the database takes each once.
- **D14 Steps 2 to 4 in one release** (recommended, over the server's side alone first):
  the module's side, the keys without SRP with handles and without anonymous accounts,
  and the apps' screens come as one release, so people get a sign-in that works; the
  cut-off (step 5) follows 30 days after it.
- **D15 The proposal (1–12) as written** (23 Oct, evening); what building it shows
  otherwise comes back to the developer.
- **D16 A newcomer's identity waits 30 minutes** (23 Oct, night, recommended, over the
  attempt's ten minutes or a day): long enough to choose a handle and make the keys, or
  to find the recovery phrase and mend the link (8); after that they sign in with
  SimpleLogin again, whose consent page they then skip. Nothing is reserved meanwhile,
  the suggested handle neither.
- **D17 Handles suggested as `anna_mueller`** (23 Oct, night, recommended): the name in
  lower case, its words joined by underscores; German's letters as German writes them
  without (ü as ue, ß as ss), æ, œ and þ as ae, oe and th, every other Latin letter
  without its mark (é as e, ø as o); apostrophes dropped, anything else between words;
  cut to 30 characters, and where the handle is taken (whatever its case) or kept by the
  instance, a number from 2 to 99 after it. None where nothing of the name is left (a
  name in another script): the person types one.
- **D18 Signing up in one call** (23 Oct, night, recommended, over the proposal's two
  steps): the device takes the handle, makes the keys, asks the password twice and shows
  the recovery phrase (three of its words typed back), then `simpleLogin.signUp` makes
  the account and its session together, the newcomer's identity taken once. No account is
  ever unfinished, so D11's deletion after a day has nothing left to delete.
- **D19 The password proven with a proof key of its own, the keys only after it** (23
  Oct, night, recommended, over the keys coming with the session, as Bitwarden's SSO
  has it, and over keeping SRP): a third subkey of the password's Argon2id seeds an
  Ed25519 key, whose public half the register keeps; the device signs a one-time nonce
  with it as it signs with the recovery phrase's (ADR 0017, the same challenges and
  message). A new device gets the wrapped keys once it has proven the password, so
  someone who took over a SimpleLogin account can only guess the password online,
  against the limits, never offline. SimpleLogin accounts keep no SRP verifier.
- **D20 Changes take the password and a fresh SimpleLogin sign-in** (23 Oct, night,
  recommended, over the password's proof alone): a new password, a new recovery phrase
  and deleting the account take the password's proof (the phrase's, for a lost
  password) and a SimpleLogin sign-in of the last ten minutes, the two factors of ADR
  0017's third reading with SimpleLogin's and Proton's own second factor in TOTP's
  place.
- **Also (23 Oct):** the developer's own account moves by adding SimpleLogin to it while
  signed in with GitHub; the instance's SimpleLogin app goes up for approval once
  halkyon.app/learn exists, before learn.halkyon.eu switches.

## Proposal

1. **The instance's ways.** The operator's SimpleLogin app: `simpleLoginClientId` and
   `simpleLoginClientSecret` in the passwords, on the register (ADR 0016), which alone
   exchanges codes. `SIGN_IN_WAYS`: `simplelogin`, `password` or both; unset, every way
   whose keys are there. On learn.halkyon.eu `simplelogin`. The instance's basics say
   which ways it offers.
2. **One SimpleLogin app per instance, the register's.** Every service of the instance
   signs in through it (Todo's too), as ADR 0016 has them sign in through the register;
   one app means one ID per person on the instance.
3. **The flow.** The app asks its server to begin (`simpleLogin.begin`, with or without a
   session; a member forwards to the register): the register keeps an attempt (a random
   ID the app holds, a `nonce`, which app and platform, the caller if signed in; ten
   minutes, taken once) and answers the authorization address. The app opens it in the
   browser (`flutter_web_auth_2`, as GitHub's). SimpleLogin sends the browser to
   `https://<register's host>/simplelogin-callback`, the one address registered. The
   instance's page there forwards code and `state` to the app (D9): the loopback on
   Linux and Windows (`http://127.0.0.1:43823/simplelogin-callback`, each app its own
   port) or the app's scheme on Android, iOS and macOS
   (`app.halkyon.learn://simplelogin-callback`). Its targets
   are fixed per app and way back, chosen by a marker in `state`, so it sends nobody
   elsewhere; SimpleLogin's errors (a denial) are forwarded alike. The app hands the code
   with its attempt's ID to `simpleLogin.complete`.
4. **Checking.** The register exchanges the code and checks the ID token: RS256 against
   SimpleLogin's keys (`/jwks`, kept, fetched again for an unknown key ID), the issuer
   `https://app.simplelogin.io`, the audience its client ID, not expired, and the `nonce`
   the attempt's. So a code caught on its way to the app is worthless: it can only
   complete the attempt it was asked for, whose ID never left the app. (A hostile app
   on the device beginning an attempt of its own isn't stopped, verified links or not
   (D9): SimpleLogin answers without a consent page for an app already allowed.) It keeps the `sub` alone
   (`simplelogin_identity`: account ID, `sub`, each unique); the name serves only as the
   handle's suggestion at sign-up (D5); address, name and avatar are never stored, and
   SimpleLogin's userinfo is never asked.
5. **A known ID** gives a session for its account (on a member, the member's own, ADR
   0016). A signed-in caller adds SimpleLogin to their account; a `sub` another account
   has is refused, and nobody is switched to another account.
6. **Signing up** (D6, "New here?"): the handle (suggested, D5), then the keys on the
   device: made (ADR 0017, 3), a password to unlock them (asked twice, never sent), the
   recovery phrase shown once and three of its words typed back; the register keeps what
   ADR 0017 keeps of them (`keys.set`: salt, costs, public keys, both wrappings), without
   SRP's verifier, an address or TOTP where SimpleLogin is the way in. Until the keys are
   set the account is unfinished: its session does nothing else, and one unfinished for a
   day is deleted (D11).
7. **A new device** gets the wrapped keys and the salt with the session, and the password
   unwraps them there. Changing it takes the old one on the device (the keys rewrapped and
   sent); a lost password with the phrase rewraps them; without it, new keys, and what was
   encrypted with the old ones is lost (ADR 0017, 6). No TOTP of ours: SimpleLogin and
   Proton have their own second factor.
8. **Mending a broken link** (D7, "I already have one"): the handle and the recovery
   phrase. The register answers the keys wrapped under the phrase and a nonce; the device
   unwraps them and signs (`halkyon key proof v1|`, as ADR 0017's resets); the new `sub`
   takes the old one's place. A handle nobody has gets a decoy wrapping and the same
   refusal, as ADR 0017 does for addresses; at most 10 tries an hour per handle and 30
   per network.
9. **Handles** (D5): required at sign-up, on every instance; `/@handle` answers as for an
   unknown handle until the person publishes something or fills in their profile; asking
   whether a handle is free stays limited per person and network, since its answer tells
   that the handle exists.
10. **Anonymous accounts go** (D4): `anonymousIdp` is removed; the first start offers the
    instance's ways and other instances. On learn.halkyon.eu, anonymous people can add
    SimpleLogin (and a handle and keys) until the cut-off (D8); another instance's
    anonymous people add a password until a cut-off their operator sets
    (`ANONYMOUS_SESSIONS_END`, a day, as `GITHUB_SIGN_IN_ENDS`).
11. **The cut-off on learn.halkyon.eu** (D8), 30 days after the release: the app reminds
    everyone without SimpleLogin from the release on, saying the day and what follows;
    then their sessions end, and 30 days later their accounts are deleted (D10).
12. **The privacy page** says that SimpleLogin learns when someone signs in to Halkyon,
    that the instance keeps only SimpleLogin's ID for its app, and that the name helps
    suggest a handle once and the rest of what the consent page shares is dropped; the
    sign-in screen says so before the browser opens. Store listings, the README and the
    website stop saying that the app starts with an anonymous account.

## Consequences

- People need a SimpleLogin account (a Proton account gives one) and a connection to sign
  up or sign in on a new device; nothing works signed out, on any instance.
- The instance depends on SimpleLogin's sign-in for apps, which SimpleLogin closed to new
  apps on 22 Oct: if it ends altogether, the operator turns `password` on, and people set
  a password and an address (ADR 0017); their keys stay.
- Operators who can't get a SimpleLogin app keep ADR 0017's way in; the code keeps both.
- Everyone has a handle, which stays private until they go public.
- The keys' password is one more thing to keep, and the recovery phrase also mends a lost
  SimpleLogin link.
- A hostile app on a person's device can begin a sign-in of its own and, once the person
  has allowed Halkyon at SimpleLogin, come back with a session without a consent page
  (4, D9); what it gets is the account's server-side data, while private studysets,
  tags and settings stay sealed under the keys (ADR 0019). Whether SimpleLogin honours
  OpenID Connect's `prompt=consent`, which would show the consent page every time, is
  still to be checked.