login: guessing from elsewhere no longer slows a browser already in use

The login throttle was one counter for everyone, so a stranger failing
on purpose every thirty seconds kept the owner on 429s too. A browser
that logs in now gets a signed diary_device cookie (__Host- over HTTPS,
SameSite=Strict, 400 days), and a login carrying a valid one is slowed
only by its own throttle. Everything else shares the anonymous one, as
before. At most 64 known browsers are tracked; the least recently used
is forgotten first. The failed-login mail says which kind of browser
the failures came from.

README.md

@@ -604,6 +604,14 @@ runtime dependencies, no build tools on the host. The server will not start
 while `WORKSPACE_SECRET` or `WORKSPACE_PASSWORD` is still the example from
 `.env.example`: with the example secret anyone could sign themselves a session.
 
+Wrong passwords are answered more and more slowly — free for three, then 1 s,
+2 s, 4 s… up to 30 s. A browser that has logged in before keeps a signed
+`diary_device` cookie (`__Host-diary_device` over HTTPS) and is slowed only by
+its own mistakes, so someone guessing at the login from elsewhere cannot keep
+you out of a browser you already use. A browser that has never logged in waits
+in the same line as the guesses. The cookie opens nothing and survives
+logging out.
+
 With `WORKSPACE_SECURE_COOKIE=1` the session cookie is `Secure` and named
 `__Host-diary_session`, so the browser keeps it to this host and HTTPS, and every
 response carries `Strict-Transport-Security` for a year. Turning it on logs

src/auth.rs

@@ -267,9 +267,8 @@ pub fn clear_cookie(config: &Config) -> String {
     format!("{name}=; Path=/; HttpOnly; SameSite=Lax; Max-Age=0{secure}")
 }
 
-fn cookie_value<'a>(parts: &'a Parts, name: &str) -> Option<&'a str> {
-    parts
-        .headers
+pub fn cookie_value<'a>(headers: &'a HeaderMap, name: &str) -> Option<&'a str> {
+    headers
         .get_all(header::COOKIE)
         .iter()
         .filter_map(|v| v.to_str().ok())
@@ -279,6 +278,41 @@ fn cookie_value<'a>(parts: &'a Parts, name: &str) -> Option<&'a str> {
         .map(|(_, v)| v.trim())
 }
 
+/// The cookie a browser keeps once it has logged in, so the login can tell
+/// it from a stranger; see [`crate::throttle::LoginThrottles`]. It opens
+/// nothing, outlives logging out, and only ever picks a throttle.
+const DEVICE_COOKIE: &str = "diary_device";
+const SECURE_DEVICE_COOKIE: &str = "__Host-diary_device";
+const DEVICE_DAYS: i64 = 400;
+
+fn device_cookie_name(config: &Config) -> &'static str {
+    if config.secure_cookie {
+        SECURE_DEVICE_COOKIE
+    } else {
+        DEVICE_COOKIE
+    }
+}
+
+/// The device id a request's cookie carries, if it is one this server signed.
+pub fn device_id(config: &Config, headers: &HeaderMap) -> Option<String> {
+    let (id, signature) = cookie_value(headers, device_cookie_name(config))?.split_once('.')?;
+    let expected = sign(&config.secret, &format!("device:{id}"));
+    bool::from(expected.as_bytes().ct_eq(signature.as_bytes())).then(|| id.to_string())
+}
+
+/// A fresh device id, for a browser that logged in without one.
+pub fn new_device_id() -> String {
+    B64.encode(rand::random::<[u8; 16]>())
+}
+
+pub fn device_cookie(config: &Config, id: &str) -> String {
+    let name = device_cookie_name(config);
+    let signature = sign(&config.secret, &format!("device:{id}"));
+    let max_age = DEVICE_DAYS * 86_400;
+    let secure = if config.secure_cookie { "; Secure" } else { "" };
+    format!("{name}={id}.{signature}; Path=/; HttpOnly; SameSite=Strict; Max-Age={max_age}{secure}")
+}
+
 /// Extractor that rejects the request unless a valid session cookie is present.
 pub struct Session {
     /// The `sessions` row the cookie belongs to.
@@ -292,8 +326,8 @@ impl FromRequestParts<AppState> for Session {
         parts: &mut Parts,
         state: &AppState,
     ) -> Result<Self, Self::Rejection> {
-        let token =
-            cookie_value(parts, cookie_name(&state.config)).ok_or(AppError::Unauthorized)?;
+        let token = cookie_value(&parts.headers, cookie_name(&state.config))
+            .ok_or(AppError::Unauthorized)?;
         let epoch = state.epoch.load(std::sync::atomic::Ordering::SeqCst);
         let id = verify_token(&state.config, token, epoch).ok_or(AppError::Unauthorized)?;
         let due = state.sessions.seen(&id).ok_or(AppError::Unauthorized)?;
@@ -507,6 +541,27 @@ mod tests {
         assert!(verify_token(&config, &token, 4).is_none());
     }
 
+    #[test]
+    fn only_a_device_cookie_this_server_signed_counts() {
+        use axum::http::{header, HeaderMap};
+        let config = config(30);
+        let with = |cookie: &str| {
+            let mut headers = HeaderMap::new();
+            headers.insert(header::COOKIE, cookie.parse().unwrap());
+            super::device_id(&config, &headers)
+        };
+        let set = super::device_cookie(&config, "laptop");
+        let pair = set.split(';').next().unwrap();
+        assert_eq!(with(pair).as_deref(), Some("laptop"));
+        assert_eq!(with("diary_device=laptop"), None);
+        assert_eq!(with(&pair.replace("laptop.", "phone.")), None);
+
+        let mut other = Config::for_tests(30);
+        other.secret = b"a-different-secret-entirely".to_vec();
+        let foreign = super::device_cookie(&other, "laptop");
+        assert_eq!(with(foreign.split(';').next().unwrap()), None);
+    }
+
     #[test]
     fn a_cookie_without_a_session_id_is_refused() {
         let config = config(30);

src/notify/mod.rs

@@ -315,7 +315,7 @@ impl Notifier {
 
     /// A run of wrong passwords. Enqueued at most once an hour, because the
     /// point is to notice an attempt, not to be mailed by the attacker.
-    pub async fn note_failed_logins(&self, failures: u32) {
+    pub async fn note_failed_logins(&self, failures: u32, known_device: bool) {
         if !self.config.mail.sends("login") {
             return;
         }
@@ -327,8 +327,14 @@ impl Notifier {
                 &format!("login_failed:{hour}"),
                 "workspace — repeated failed logins",
                 &format!(
-                    "{failures} failed login attempts. The login is being answered more \
-                     slowly with every one of them.\n"
+                    "{failures} failed login attempts {}. The login is being answered more \
+                     slowly with every one of them.\n",
+                    if known_device {
+                        "from a browser that has logged in before"
+                    } else {
+                        "from browsers that have not logged in before, which do not slow \
+                         down the browsers that have"
+                    }
                 ),
             )
             .await;

src/routes/session.rs

@@ -1,7 +1,7 @@
 use axum::{
     extract::{Path, Query, State},
     http::{header, HeaderMap, StatusCode},
-    response::IntoResponse,
+    response::{AppendHeaders, IntoResponse},
     Json,
 };
 use serde::{Deserialize, Serialize};
@@ -25,8 +25,13 @@ pub async fn login(
     headers: HeaderMap,
     Json(body): Json<LoginRequest>,
 ) -> AppResult<impl IntoResponse> {
-    let attempt = state.login_throttle.begin().map_err(|wait| {
-        tracing::warn!("login refused, {wait}s of backoff remaining");
+    let device = auth::device_id(&state.config, &headers);
+    let throttle = state.login_throttle.for_device(device.as_deref());
+    let attempt = throttle.begin().map_err(|wait| {
+        tracing::warn!(
+            known_device = device.is_some(),
+            "login refused, {wait}s of backoff remaining"
+        );
         AppError::TooManyRequests(wait)
     })?;
 
@@ -34,7 +39,10 @@ pub async fn login(
         let failures = attempt.failed();
         // Worth a mail once someone is past guessing and into trying.
         if failures > FREE_ATTEMPTS {
-            state.notify.note_failed_logins(failures).await;
+            state
+                .notify
+                .note_failed_logins(failures, device.is_some())
+                .await;
         }
         return Err(AppError::Unauthorized);
     }
@@ -54,12 +62,20 @@ pub async fn login(
         &id,
         &state.config,
     );
+    // Signed again on every login, so a browser in use never runs out of it.
+    let device = device.unwrap_or_else(auth::new_device_id);
     Ok((
         StatusCode::OK,
-        [(
+        AppendHeaders([
+            (
                 header::SET_COOKIE,
                 auth::session_cookie(&state.config, &token),
-        )],
+            ),
+            (
+                header::SET_COOKIE,
+                auth::device_cookie(&state.config, &device),
+            ),
+        ]),
         Json(json!({ "user": state.config.username })),
     ))
 }

src/state.rs

@@ -3,7 +3,11 @@ use std::sync::{atomic::AtomicI64, Arc};
 use sqlx::SqlitePool;
 
 use crate::{
-    auth::Sessions, backup::Backup, config::Config, notify::Notifier, throttle::LoginThrottle,
+    auth::Sessions,
+    backup::Backup,
+    config::Config,
+    notify::Notifier,
+    throttle::{LoginThrottle, LoginThrottles},
     vault::Vault,
 };
 
@@ -11,7 +15,7 @@ use crate::{
 pub struct AppState {
     pub db: SqlitePool,
     pub config: Arc<Config>,
-    pub login_throttle: Arc<LoginThrottle>,
+    pub login_throttle: Arc<LoginThrottles>,
     /// The vault password gets the same slowing-down as the login, kept apart
     /// so guessing at one does not slow the other.
     pub vault_throttle: Arc<LoginThrottle>,

src/throttle.rs

@@ -1,12 +1,15 @@
-use std::sync::Mutex;
+use std::{
+    collections::HashMap,
+    sync::{Arc, Mutex},
+};
 
 /// Failed logins get progressively slower to answer.
 ///
 /// The workspace has exactly one account, so this is one counter rather than a map
 /// keyed by address: an attacker cannot step around it by changing source IP,
-/// and there is no per-client table to grow. The cost of that choice is that
-/// someone hammering the login can slow the owner's own attempt down — which is
-/// why the wait is capped at `MAX_DELAY` seconds instead of locking the account.
+/// and there is no per-client table to grow. Someone hammering it slows down
+/// every browser it covers, so the login keeps one for browsers it does not
+/// know and one apiece for those it does; see [`LoginThrottles`].
 #[derive(Debug, Default)]
 pub struct LoginThrottle {
     state: Mutex<State>,
@@ -62,6 +65,50 @@ impl LoginThrottle {
     }
 }
 
+/// How many browsers that have logged in before get a throttle of their own.
+/// Past that, the one used longest ago is forgotten, and falls back to
+/// sharing the anonymous one until it next logs in.
+const KNOWN_DEVICES: usize = 64;
+
+/// The login's throttles. A browser that has logged in before carries a
+/// signed device cookie and is slowed only by its own mistakes; every other
+/// request shares the anonymous throttle. A stranger guessing at the
+/// password therefore slows strangers, never a browser the owner already
+/// uses — a new one still waits in line with the guesses.
+#[derive(Debug, Default)]
+pub struct LoginThrottles {
+    anonymous: Arc<LoginThrottle>,
+    known: Mutex<HashMap<String, (Arc<LoginThrottle>, i64)>>,
+}
+
+impl LoginThrottles {
+    /// The throttle for a login from `device`, a device id whose signature
+    /// has already been checked, or from a browser without one.
+    pub fn for_device(&self, device: Option<&str>) -> Arc<LoginThrottle> {
+        let Some(device) = device else {
+            return Arc::clone(&self.anonymous);
+        };
+        let mut known = self.known.lock().unwrap_or_else(|e| e.into_inner());
+        let now = crate::now();
+        if let Some((throttle, used)) = known.get_mut(device) {
+            *used = now;
+            return Arc::clone(throttle);
+        }
+        if known.len() >= KNOWN_DEVICES {
+            let oldest = known
+                .iter()
+                .min_by_key(|(_, (_, used))| *used)
+                .map(|(id, _)| id.clone());
+            if let Some(oldest) = oldest {
+                known.remove(&oldest);
+            }
+        }
+        let throttle = Arc::<LoginThrottle>::default();
+        known.insert(device.to_string(), (Arc::clone(&throttle), now));
+        throttle
+    }
+}
+
 /// One attempt that [`LoginThrottle::begin`] let through. Dropping it without
 /// a verdict — a request that failed for some other reason — counts for
 /// nothing either way.
@@ -94,7 +141,7 @@ impl Drop for Attempt<'_> {
 
 #[cfg(test)]
 mod tests {
-    use super::{delay_after, Attempt, LoginThrottle, MAX_DELAY};
+    use super::{delay_after, Attempt, LoginThrottle, LoginThrottles, KNOWN_DEVICES, MAX_DELAY};
 
     #[test]
     fn the_first_few_mistakes_are_free() {
@@ -138,4 +185,35 @@ mod tests {
         drop(first);
         assert!(throttle.begin().is_ok());
     }
+
+    #[test]
+    fn guesses_from_strangers_do_not_slow_a_known_browser() {
+        let throttles = LoginThrottles::default();
+        let anonymous = throttles.for_device(None);
+        for _ in 0..10 {
+            Attempt {
+                throttle: &anonymous,
+            }
+            .failed();
+        }
+        assert!(throttles.for_device(None).begin().is_err());
+        assert!(throttles.for_device(Some("laptop")).begin().is_ok());
+
+        // And a known browser's own mistakes stay its own.
+        let laptop = throttles.for_device(Some("laptop"));
+        for _ in 0..10 {
+            Attempt { throttle: &laptop }.failed();
+        }
+        assert!(throttles.for_device(Some("laptop")).begin().is_err());
+        assert!(throttles.for_device(Some("phone")).begin().is_ok());
+    }
+
+    #[test]
+    fn known_browsers_are_capped() {
+        let throttles = LoginThrottles::default();
+        for n in 0..KNOWN_DEVICES * 3 {
+            throttles.for_device(Some(&format!("device-{n}")));
+        }
+        assert_eq!(throttles.known.lock().unwrap().len(), KNOWN_DEVICES);
+    }
 }