[Security] Add challenge-response auth with scrypt verifiers and stop storing plaintext passwords (#7193)

* [Security] Add challenge-response auth with scrypt verifiers and stop storing plaintext passwords

Challenge-response authentication: the client derives a scrypt verifier
(RFC 7914, EVP_PBE_scrypt, N=32768, r=8, p=1) and authenticates with
HMAC-SHA256(key, nonce), so neither the password nor its hash is
transmitted. The stored format becomes
"$scrypt$<n>$<r>$<p>$<salt>$<verifier>" and Response_PasswordSalt
now carries the cost parameters. Strict servers only accept scrypt
verifiers; legacy accounts are migrated after a successful login.

Fix #344 for challenge-response servers: a saved profile stores the
derived verifier under the password key instead of the plaintext password.
The connect dialog loads it without revealing it, autoconnect passes it
through, the change-password dialog no longer prefills the old password
field with it, and the client only persists the verifier when
"Save password" is checked.

Took 3 minutes

Took 1 minute

Took 10 seconds

Took 7 minutes

* [Security] Harden credential format acceptance and challenge nonce validation

* [Security] Refuse KDF downgrade; trim profile name and warn on silent credential drop

* [Security] Add SupportsAll ServerOptions enumerator to fix enum validation assert

The server advertised both SupportsPasswordHash (1) and
SupportsChallengeResponseAuth (2) via a bitwise-OR that produced the
value 3, which no enumerator declared. Protobuf's proto2 enum
validation rejects the value in _internal_set_server_options, crashing
servatrice on the debug assertion. Add a SupportsAll = 3 enumerator with
an identical wire value so the bitmask reads unchanged while the value
is now a documented, valid member of the enum.

* [Security] Normalize username when issuing challenge nonce

cmdRequestPasswordSalt bound the challenge nonce to the raw, un-simplified
username while cmdLogin verifies it against the simplified name. A user
who typed leading or trailing whitespace in the username box would be
silently rejected during challenge-response login even with the correct
password. Simplify at the nonce-issuing site so both commands agree on
the normalized name.

Also drive-by formatting: collapse the now single-expression
serverOptions initializer onto one line.

* [Security] Keep an encrypted legacy password backup for old-client logins

Migrating an account to scrypt stranded pre-challenge clients, which can
only present the legacy salt+hash: after migration the server served them
a scrypt salt they cannot use, so the only recovery was a password reset.

When security/legacy_backup_key is configured (64 hex chars,
openssl rand -hex 32), submitPasswordVerifier now seals the legacy
credential with AES-256-GCM under that key before overwriting the row,
storing it in password_legacy_backup with the time it was last used. Old
clients keep working: cmdRequestPasswordSalt serves the backup's 16-char
salt in password_salt so they can derive the legacy hash they were
always able to compute, while new clients derive the verifier from the
new scrypt_salt field, which always carries the verifier salt. The
non-challenge login path accepts the plaintext or legacy hash against
the decrypted backup and refreshes its window; a migrated row with no
usable backup fails closed.

Backups age out so a database dump taken after the window closes never
carries a crackable legacy hash: getUsableLegacyBackup purges on first
use past security/legacy_backup_ttl_days (default 90, clamped 1..3650),
and a six-hourly sweep drops backups idle accounts never touch.
changeUserPassword clears the backup since a new credential invalidates
it. Without a configured key nothing is sealed or purged, so a
temporarily missing key cannot destroy existing backups.

The AAD is the simplified user name on both sides, sealing the blob to
its row so it cannot be moved between accounts. Sealing failures
degrade to the previous lockout behaviour rather than blocking
migration. The columns extend the unmerged 0036->0037 migration, so the
schema version stays 37.

* [Security] Address review: proto compat, plaintext verify, AAD case, strict scope

Fix the two blockers and follow-ups from the 2026-10-16 review:

- Advertise challenge-response in a new supports_challenge_response
  field instead of the ServerOptions enum and drop SupportsAll:
  proto2 enums are closed, so already-released clients parsed
  server_options = 3 as NoOptions and fell back to plaintext
  passwords. server_options stays exactly SupportsPasswordHash
  on the wire; old clients ignore the new field.
- Verify a plaintext password directly against a migrated scrypt
  row with verifyPassword() so email changes (cmdAccountEdit)
  and old clients sending password keep working without a
  legacy backup key. Pre-hashed values are still checked
  against the backup, and its TTL window is refreshed only
  when the backup is what actually matched.
- Use the stored row name as the AES-GCM AAD on both seal
  and open so a login whose case differs from the stored
  name (utf8mb4_unicode_ci) still opens its backup.
- Require challenge-response in strict mode only for
  registered SQL accounts (userExists gate), leaving
  guest/unregistered logins and none/password auth
  methods unaffected.
- Document in servatrice.ini.example that a captured
  legacy hash can take over a not-yet-migrated account
  and that strict only constrains registered SQL
  accounts. Schema version is 38.

* [Security] Raise legacy backup TTL default to 180 days

Gives operators and slow-to-upgrade users a longer downgrade
window for pre-challenge clients before unused encrypted
legacy password backups age out.

* [Security] Fix servatrice.cpp formatting per clang-format-18

* [Security] Add missing forward declaration for Command_SubmitPasswordVerifier

The cmdSubmitPasswordVerifier declaration references the proto-generated
type, which otherwise does not name a type in this header and breaks
every C++ build.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>
Co-authored-by: BruebachL <bruebachl@cockatrice.example>

servatrice/servatrice.ini.example

@@ -420,6 +420,48 @@ max_forgot_password_requests_per_ip=3
 
 ; Length in seconds of the forgot-password window. Default is 3600 (1 hour).
 forgot_password_window_seconds=3600
+; How strictly new authentication features are enforced.  Possible values:
+; * legacy: accounts that still use the legacy 1000-round SHA-512 hash keep logging in with it
+;           (they are served the legacy salt, never a challenge-response nonce) and are never
+;           auto-migrated to scrypt. Already-migrated scrypt rows keep logging in via
+;           challenge-response. New credentials may use either format;
+; * mixed: accept both legacy hashes and challenge-response authentication (default);
+; * strict: only accept challenge-response authentication from clients that support it,
+;           and reject plain password submissions.  Legacy accounts are migrated to scrypt
+;           automatically on their next successful login. Strict only constrains
+;           registered SQL accounts: guest/unregistered logins and servers with
+;           authentication/method set to none or password are unaffected, since
+;           they have no stored verifier to challenge against.
+;
+; Note: anyone holding only the legacy salt+hash of a not-yet-migrated account
+; (e.g. from a wire capture of an old client, or an old database dump) can log in
+; with it and then set a brand new password of their choosing. Before
+; challenge-response a captured hash yielded a session, but changing the password
+; still required the plaintext old password. There is no way around this without
+; a plaintext proof at migration time, so not-yet-migrated rows should be treated
+; accordingly.
+;
+; Challenge-response verifiers are scrypt (RFC 7914, N=32768, r=8, p=1) stored as
+; "$scrypt$<n>$<r>$<p>$<salt>$<verifier>". The stored verifier is password-equivalent:
+; plaintext passwords never reach the client configuration and never go over the wire, but
+; a database dump yields credentials that can answer a login challenge directly.
+authentication_strictness=mixed
+
+; Optional key that seals the legacy password backup (64 hex characters).
+; Migrating an account to scrypt normally strands old clients, which can only present
+; the legacy hash. With a key configured, the server keeps that hash encrypted next to
+; the scrypt verifier, so old clients can still log in during the window below without
+; the database holding a fast-crackable hash in the clear.
+; Generate with: openssl rand -hex 32
+; If unset, migrated accounts have no downgrade window: old clients must use a password
+; reset to get back in. Rotating the key invalidates existing backups (they become
+; unusable immediately; expired ones are still purged).
+legacy_backup_key=
+
+; Days a legacy password backup may go unused before it is purged from the database.
+; Logging in with an old client refreshes the window; backups that are never touched
+; age out. Default 180.
+legacy_backup_ttl_days=180
 
 ; You may want to allow an unlimited number of users from a trusted source. This setting can contain a
 ; comma-separed list of IP addresses which will allow an unlimited number of connections from each of the