docs(auth): what the third reading found, and Proton's rule (ADR 0017)

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

@@ -74,12 +74,12 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 
 - **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 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`).
+  - **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 session alone never gets a code checked (ADR 0017's third reading, as Proton has it): every signed-in change of how the account is secured (adding an authenticator once one is confirmed, removing a confirmed one, new recovery codes, a new phrase, a new password, deleting) takes the password and a code (`OwnerProof`), each sheet asking for both; a session's wrong proofs count for it alone and the tenth in a row ends it, never making the account's codes wait. 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` 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, the resets and deleting an account whose second factor doesn't work, each code asked for one of those purposes (`EmailCodePurpose`), taken by it alone, its mail saying what it would do and to type it only in the app on this instance. Eight Crockford characters (halkyon_base's `short_code.dart`, shared with connect codes), 15 minutes, 3 tries, up to 3 valid at once for an address (a fourth voids the oldest); at most 3 codes an hour per address and network, 10 per address from every network 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.
+  - **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, the resets and deleting an account whose second factor doesn't work, each code asked for one of those purposes (`EmailCodePurpose`), taken by it alone, its mail saying what it would do and to type it only in the app on this instance. Eight Crockford characters (halkyon_base's `short_code.dart`, shared with connect codes), 15 minutes, 3 tries, up to 3 valid at once for an address (a fourth voids the oldest); at most 3 codes an hour per address and network, 10 per address from every network 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 and what it is for 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.