bugfix: equalise login response time across user-existence branches (0.34.1)

A login attempt against a non-existent username returned 401 in <1ms,
while the wrong-password branch ran argon2 verify (~50-100ms). Timing
the difference let an attacker enumerate valid usernames without ever
seeing a successful response. Run verify_password against a fixed
dummy argon2id hash on the no-user branch so both paths spend the
same compute.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/api/auth.rs

@@ -4,6 +4,8 @@
 //! expire naturally rather than being explicitly invalidated, so other
 //! devices keep their existing logins).
 
+use std::sync::OnceLock;
+
 use axum::extract::{Path, State};
 use axum::http::StatusCode;
 use axum::response::IntoResponse;
@@ -102,9 +104,15 @@ async fn login(
         ));
     }
 
-    let user = repo::user::find_by_username(&state.db, username)
-        .await?
-        .ok_or(AppError::Unauthenticated)?;
+    let user = repo::user::find_by_username(&state.db, username).await?;
+    let Some(user) = user else {
+        // No such user. Run argon2 against a stable dummy hash so the
+        // response time matches the wrong-password branch — otherwise
+        // an attacker can enumerate usernames by timing the no-user
+        // 401 against the wrong-password 401.
+        let _ = verify_password(&input.password, dummy_password_hash());
+        return Err(AppError::Unauthenticated);
+    };
     if !verify_password(&input.password, &user.password_hash) {
         return Err(AppError::Unauthenticated);
     }
@@ -113,6 +121,21 @@ async fn login(
     Ok((StatusCode::OK, jar, Json(AuthResponse { user })))
 }
 
+/// Lazily-computed argon2 hash used to equalise login response time
+/// across the "no such user" and "wrong password" branches. Computing
+/// it once (on the first login of the process) is enough — the hash is
+/// never compared against a real password, only used to force argon2
+/// to do the same amount of work it would for a real verify.
+fn dummy_password_hash() -> &'static str {
+    static DUMMY: OnceLock<String> = OnceLock::new();
+    DUMMY
+        .get_or_init(|| {
+            crate::auth::password::hash_password("login-timing-equaliser")
+                .expect("hash_password on a fixed input cannot fail")
+        })
+        .as_str()
+}
+
 async fn logout(
     State(state): State<AppState>,
     jar: CookieJar,