docs(auth): what building the apps' side settled (ADR 0017)

ADR 0017's step-3 section: codes by mail sign nobody in, the ways in
for accounts without a working password, changes while signed in,
sessions ended in every service, the apps' screens (the QR package and
the alternatives considered), the keys in the keyring, anonymous
people's keys waiting for step 4, and libsodium on devices (results
copied before they leave an isolate; a C compiler and make for every
build). The module's and halkyon_app's READMEs and the AGENTS files say
the same; the snap's build installs make for sodium's build hook.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/AGENTS.md

@@ -73,13 +73,13 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 ## Rules the code relies on
 
 - **Authentication**: server-side sessions (`ServerSideSessionsConfigFromPasswords`), not JWT, for immediate revocation. Scopes are copied onto the session row, so changing a person's scopes means updating their session rows too.
-- **Identity**: `AuthUser` is the person; identity providers (anonymous, GitHub, an email address, a password) attach to it. Content references `AuthUser`, never a provider account. A person may sign in with GitHub and with an address, one of each (`account.signInWays`, which also says whether they have a password and a confirmed authenticator); the Profile tab lists their ways in and adds the missing one.
-  - **A password and an authenticator** (ADR 0017; the server's side built 2026-10-20, the app's screens to come; halkyon_base's README): the address, a password that never leaves the device (SRP-6a, `passwordSignIn.begin` and `complete`, the verifier and the account's wrapped keys on the register) and a code from an authenticator app (TOTP, mandatory) or a recovery code. An address nobody has is answered as an account's would be (decoys from `signInDecoyKey`), and every wrong credential is refused alike (`wrongCredentials`). Lost authenticator: the password and a mailed code (or the recovery phrase) turn it off 72 hours later, a mail at the start, a sign-in with a code calling it off; lost password: the recovery phrase's signature and a code set a new one, keys kept, or without the phrase a mailed code resets the whole account (new keys) after the same 72 hours. An account with a password and a confirmed authenticator no longer signs in by a mailed code or GitHub (`usePassword`); where passwords are offered, no new GitHub account is linked (`deprecated`), and from `GITHUB_SIGN_IN_ENDS` on GitHub signs nobody in and pruning forgets every GitHub ID (D2). Password sign-in is on only where the passwords hold `totpKey`, `recoveryCodeKey` and `signInDecoyKey` (`TOTP_KEY`, `RECOVERY_CODE_KEY`, `SIGN_IN_DECOY_KEY` in `deploy/.env`) and email sign-in is set up (`passwordSignIn` in `instance.basics()`).
+- **Identity**: `AuthUser` is the person; identity providers (anonymous, GitHub, an email address with a password) attach to it. Content references `AuthUser`, never a provider account. A person may sign in with GitHub and with an address and password, one of each (`account.signInWays`, which also says whether they have a password and a confirmed authenticator); the Profile tab lists their ways in, adds GitHub where offered, and holds halkyon_app's `AccountSecuritySection` (setting up a password, the authenticators, changing it).
+  - **A password and an authenticator** (ADR 0017, built 2026-10-20 for build 22; halkyon_base's and halkyon_app's READMEs): the only way an identified person signs in besides GitHub, which is deprecated. The address, a password that never leaves the device (SRP-6a, `passwordSignIn.begin` and `complete`, the verifier and the account's wrapped keys on the register) and a code from an authenticator app (TOTP, mandatory) or a recovery code. An address nobody has is answered as an account's would be (decoys from `signInDecoyKey`), and every wrong credential is refused alike (`wrongCredentials`). Lost authenticator: the password and a mailed code (or the recovery phrase) turn it off 72 hours later, a mail at the start, a sign-in with a code calling it off; lost password: the recovery phrase's signature and a code set a new one, keys kept, or without the phrase a mailed code resets the whole account (new keys) after the same 72 hours. An account with a password and a confirmed authenticator no longer signs in by a mailed code or GitHub (`usePassword`); where passwords are offered, no new GitHub account is linked (`deprecated`), and from `GITHUB_SIGN_IN_ENDS` on GitHub signs nobody in and pruning forgets every GitHub ID (D2). Password sign-in is on only where the passwords hold `totpKey`, `recoveryCodeKey` and `signInDecoyKey` (`TOTP_KEY`, `RECOVERY_CODE_KEY`, `SIGN_IN_DECOY_KEY` in `deploy/.env`) and email sign-in is set up (`passwordSignIn` in `instance.basics()`). On the device (halkyon_app): signing up shows the recovery phrase once and checks three of its words before anything reaches the instance; signing in checks SRP's M2 and opens the keys before the session is put in place; an authenticator still to set up is set up with the session held back; the unlocked keys stay in the system's keyring per instance and person, gone once another person signs in or nobody. A change of password ends the account's other sessions and a reset all of them, on every service of the instance (`sessions_ended_notice`).
   - **One account across the instance** (ADR 0016, decided 2026-10-19; halkyon_base's README): Learn's server holds the instance's register, so its tables keep every service's ways in, handles and profiles, and an account's ID is the `AuthUser` ID in every service. The instance's other services (Todo) are its members: they ask it over its web server (`/halkyon-base/register/…`, `addInstanceRoutes` in `server.dart`, never forwarded by Caddy) with the secret the services share (`instanceSecret`, `INSTANCE_SECRET` in `deploy/.env`), and a member's anonymous person adding a way in becomes an account here under the same ID. Deleting an account (here, or a member passing it on) keeps a notice per member in the same transaction (`account_deletion_notice`), told at once and hourly until each confirms (`tellMembersOfDeletions`). Learn's endpoints and code go through the module as on any register; nothing changes for Learn's apps.
   - **GitHub keeps only the numeric GitHub ID**: our own slim login (halkyon_base's `github.dart`) on Serverpod's GitHub helpers; Serverpod's stock GitHub login endpoint, which stores name, email and avatar, is never exposed. GitHub sign-in is on only where the passwords hold `githubClientId` and `githubClientSecret` (in production `GITHUB_CLIENT_ID`/`GITHUB_CLIENT_SECRET` in `deploy/.env`); the app asks halkyon_base's `instance.basics()`.
   - **Linking keeps the person** (`GitHubUpgradeEndpoint`, `requireLogin`): the GitHub ID joins the signed-in anonymous person, who becomes identified and loses the anonymous identity, in one transaction. **Signing in** (`GitHubSignInEndpoint`) only reaches people who have a GitHub account already, and only without a session: Serverpod refuses to issue another person's session inside a signed-in call. Refusals are `GitHubSignInRefusedException` with a reason.
-  - **Switching devices' person** (halkyon_app's `switchToGitHub`, `switchToEmail`, on whether the device holds content as Learn counts it, `PersonDependencies.holdsContent`: its studysets, Recently deleted's too): swap the session directly, never through "signed out" (which would leave the device asking how to start), then delete the account left behind if it was anonymous (nobody could sign into it again), never an identified one. Someone identified is never offered a switch: an address or GitHub account that is another account's is said to be so.
-  - **Email keeps only a keyed hash of the address** (ADR 0010, decided 2026-10-15): our own slim login (halkyon_base's `email.dart`) on Serverpod's code challenges and rate limiter, as Serverpod's email sign-in keeps the address in plain text and passwords. `email_identity` holds HMAC-SHA256 of the address in lower case, keyed with `emailSignInPepper`; the instance mails only an address someone has just typed, and the device remembers the address it signed in with (`SignInAddresses`, `sign_in.json` in the person's folder). A code by mail each time, no password: eight Crockford characters (halkyon_base's `short_code.dart`, shared with connect codes), 15 minutes, 3 tries, a newer one replacing the older; at most 3 codes an hour per address and 10 per network. `emailCode.sendCode` answers alike for every address; `emailCode.verifyCode` (`requireLogin`) makes a new address the caller's (as linking GitHub does) or, for another person's, returns a switch token that `emailSignIn.signIn` (without a session) turns into a session for them. Refusals are `EmailSignInRefusedException` with a reason. Codes go by mail over SMTP (halkyon_base's `code_mail.dart`, naming the app the host configures, pub.dev's `mailer`, TLS only), through the service the passwords name (`emailSmtpHost`, `emailSmtpPort`, `emailSmtpUser`, `emailSmtpPassword`, `emailFrom`; in production `EMAIL_SMTP_…` and `EMAIL_FROM` in `deploy/.env`; learn.halkyon.eu's is Proton's), as plain text with the code in the subject; without one, into the server's log in development, and elsewhere there is no email sign-in (`emailSignIn` in halkyon_base's `instance.basics()`, which the app asks). Nothing logs an address or a code (a failed mail logs its error's type), and never attach a handler to `package:logging`'s root logger, through which `mailer` would.
+  - **Switching devices' person** (halkyon_app's `switchToGitHub` and `useSession`, on whether the device holds content as Learn counts it, `PersonDependencies.holdsContent`: its studysets, Recently deleted's too): swap the session directly, never through "signed out" (which would leave the device asking how to start), then delete the account left behind if it was anonymous (nobody could sign into it again), never an identified one. Someone identified is never offered a switch: an address or GitHub account that is another account's is said to be so.
+  - **Email keeps only a keyed hash of the address** (ADR 0010, decided 2026-10-15): our own slim login (halkyon_base's `email.dart`) on Serverpod's code challenges and rate limiter, as Serverpod's email sign-in keeps the address in plain text and passwords. `email_identity` holds HMAC-SHA256 of the address in lower case, keyed with `emailSignInPepper`; the instance mails only an address someone has just typed, and the device remembers the address it signed in with (`SignInAddresses`, `sign_in.json` in the person's folder). A code by mail signs nobody in since ADR 0017 (build 22): it proves an address for a password's setup, a sign-up, a setup never finished and the resets. Eight Crockford characters (halkyon_base's `short_code.dart`, shared with connect codes), 15 minutes, 3 tries, a newer one replacing the older; at most 3 codes an hour per address and 10 per network. `emailCode.sendCode` (without a session) answers alike for every address. Refusals are `EmailSignInRefusedException` with a reason. Codes go by mail over SMTP (halkyon_base's `code_mail.dart`, naming the app the host configures, pub.dev's `mailer`, TLS only), through the service the passwords name (`emailSmtpHost`, `emailSmtpPort`, `emailSmtpUser`, `emailSmtpPassword`, `emailFrom`; in production `EMAIL_SMTP_…` and `EMAIL_FROM` in `deploy/.env`; learn.halkyon.eu's is Proton's), as plain text with the code in the subject; without one, into the server's log in development, and elsewhere there is no email sign-in (`emailSignIn` in halkyon_base's `instance.basics()`, which the app asks). Nothing logs an address or a code (a failed mail logs its error's type), and never attach a handler to `package:logging`'s root logger, through which `mailer` would.
   - **On Linux and Windows the browser half is the app's own** (`LoopbackWebAuth`, halkyon_app): the system browser and a loopback address (RFC 8252), put in place of flutter_web_auth_2's own implementation (checked with v5.1.0) before each sign-in. The package's front ends every pending sign-in when the app is resumed, and on the desktop the window regaining focus resumes it, so its loopback server closed before GitHub answered. Ours waits for GitHub, the profile's Cancel (shown only there), choosing another tab, a change of person, or 5 minutes; the regression test drives the package's front through a focus change. Remove it once the package stops ending sign-ins on focus there. Serverpod (v4.0.3) reports every cancelled browser round trip as an unknown error, so the gateway knows a cancel only by having asked for it; a cancel on GitHub's own page, or leaving the browser on Android, still shows "Couldn't sign in with GitHub."
 - **Authorization lives in endpoint code**; Serverpod has no row-level security. Load content filtered by the session's `authUserId`, never by an owner ID the client sends. A row another person owns must be indistinguishable from a missing one. Features involving other people require the `identified` scope, declared via `requiredScopes`, never checked ad hoc.
   - **The reference is `DevicesEndpoint`** (`apps/learn/halkyon_learn_server/lib/src/auth/`) with its tests: the owner check sits in the same query (`WHERE "id" = $1 AND "authUserId" = $2`), zero rows means `NotFoundException` (shared, `lib/src/common/`), and every rule has a test that asserts the refusal. Each query carries its SQL as a comment.
@@ -121,6 +121,6 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 - **Session logs are not persisted** (`persistentEnabled: false` in every run mode) for privacy. Don't re-enable them.
 - **Secrets** live in `apps/learn/halkyon_learn_server/config/passwords.yaml` (git-ignored). Never change `serverSideSessionKeyHashPepper`: it would sign everyone out, and anonymous accounts can't sign back in. Never change `emailSignInPepper` either: every address's hash would be lost, and with it those people's way in. Never change `totpKey` (every authenticator stops working) or `recoveryCodeKey` (every recovery code), nor `signInDecoyKey` without need (it shows which addresses were unused to whoever watched).
 - **Checks on a device** live in `apps/learn/halkyon_learn_flutter/integration_test/` and run by hand, never in `tool/check.dart` or CI: `flutter test integration_test/<file> -d <device>`. `pdf_engine_test.dart` checks the real PDF engine (on the Linux desktop too), `youtube_player_test.dart` the embedded YouTube player (Android), `jellyfin_player_test.dart` the video player streaming from a Jellyfin server (`--dart-define`s for the server and the person, in the file), `japanese_analyser_test.dart` MeCab as its build hook compiles it, with the curator's IPADIC pack and an instance's JMdict export (`--dart-define`s for both). Never on the emulator from an agent session: it takes the editor down with it.
-- **Every build needs a C++ compiler**, `flutter test` too: `mecab_for_dart`'s build hook compiles MeCab for the platform (ADR 0012; on Linux g++ or clang, on Android the NDK's, on Windows Visual Studio's, on macOS and iOS Xcode's). The snap's build installs clang; CI's runner needs one too.
+- **Every build needs a C++ compiler and make**, `flutter test` too: `mecab_for_dart`'s build hook compiles MeCab for the platform (ADR 0012; on Linux g++ or clang, on Android the NDK's, on Windows Visual Studio's, on macOS and iOS Xcode's), and `sodium`'s compiles libsodium from the source it ships with its configure script and make (ADR 0017, halkyon_keys and halkyon_app). The snap's build installs clang and make; CI's runner needs both too.
 - **Windows builds need Developer Mode**: building makes symbolic links (Flutter links each plugin's native code into the build, and pdfrx's build does too), which Windows lets ordinary users make only in Developer Mode. PDFium itself comes with the build as a Dart native asset.
 - **Linux builds need** `libwebkit2gtk-4.1-dev`, `libsecret-1-dev`, for sounds (`audioplayers`) `libgstreamer1.0-dev` and `libgstreamer-plugins-base1.0-dev`, and for videos (`media_kit_video`) `libmpv-dev`: the video player links libmpv, so the app doesn't start where `libmpv.so.2` is missing (the snap brings Ubuntu's `libmpv2` with ffmpeg's libraries, and its `cleanup` part removes every staged file the GNOME runtime or core24 holds at the same path, since older copies of cairo, pango or librsvg would hide the runtime's from its GTK; staging never runs packages' install scripts, so the snap makes the `update-alternatives` links for BLAS and LAPACK itself); that build also downloads mimalloc's source from GitHub (checked against its MD5) and links it in. If a Linux build tries to install to `/usr/local`, the CMake cache is stale: delete `build/linux`.