users: a user manages its own keys

Only a user who may manage users could change a user's keys: a user
couldn't add a new laptop's key, or remove a lost one.

A user now manages its own keys, whatever else it may do:

* GET /auth/keys: its keys, its entry, and the envelope holding it
  under a strict policy (`signed`), to sign against.
* PUT /auth/keys/{id} with an authorized_keys line, DELETE
  /auth/keys/{id}: one change, as the user. The rules take a write of
  _continuum/users/<name>/keys/… from the user it names.
* Under a strict policy, the user signs its entry with only its keys
  changed, with a key it holds already (envelope::check takes that from
  the user itself), PUT to its own entry even without write on the
  registry; continuum-signed key-add and key-remove do so.

2 files changed, 93 insertions, 1 deletions

diff --git a/src/access/envelope.rs b/src/access/envelope.rs
index e0c8909..7cd0083 100644
--- a/src/access/envelope.rs
+++ b/src/access/envelope.rs
@@ -207,7 +207,7 @@ pub(crate) fn check(
     if !user.keys.values().any(|known| *known == key) {
         return Err(format!("{} has no key {}", payload.user, key.fingerprint()));
     }
-    if !user.caps().allows(Right::Write, path) {
+    if !user.caps().allows(Right::Write, path) && !own_keys_only(doc, &payload, &user, path) {
         return Err(format!(
             "{} may not write /{}",
             payload.user,
@@ -217,6 +217,33 @@ pub(crate) fn check(
     Ok(Signed { payload, key })
 }
 
+/// Whether `payload`, signed by its user (`user`, enabled), is that user's own entry
+/// with only its keys changed from the one at `path` in `doc`: a user manages its own
+/// keys, whatever else it may do.
+fn own_keys_only(
+    doc: &impl ReadDoc,
+    payload: &Payload,
+    user: &registry::User,
+    path: &[String],
+) -> bool {
+    if user.disabled || path != registry::user_path(&payload.user) {
+        return false;
+    }
+    let Ok(Some(mut before)) = store::read_plain(doc, path) else {
+        return false;
+    };
+    let mut after = payload.value.clone();
+    for entry in [&mut before, &mut after] {
+        match entry.as_object_mut() {
+            Some(entry) => {
+                entry.remove("keys");
+            }
+            None => return false,
+        }
+    }
+    before == after
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -307,6 +334,29 @@ mod tests {
         assert!(check(&doc, &elsewhere, &path("config"), None).is_err());
     }
 
+    /// A user signs its own entry with its keys changed, though it may not write the
+    /// registry; not with anything else changed, nor another's.
+    #[test]
+    fn users_sign_their_own_keys() {
+        let key = SigningKey::from_bytes(&[5; 32]);
+        let doc = registry_with(&key);
+        let entry = store::read_plain(&doc, &registry::user_path("alice"))
+            .unwrap()
+            .unwrap();
+        let line =
+            sshsig::PublicKey(SigningKey::from_bytes(&[8; 32]).verifying_key().to_bytes()).line();
+        let mut rekeyed = entry.clone();
+        rekeyed["keys"]["phone"] = json!(line);
+        let at = "_continuum/users/alice";
+        let signed_as = |user: &str, value: JsonValue| signed(&key, &payload(user, at, value, ""));
+        assert!(check(&doc, &signed_as("alice", rekeyed.clone()), &path(at), None).is_ok());
+        let mut more = rekeyed.clone();
+        more["caps"] = json!({"x": {"path": [], "rights": ["write"]}});
+        assert!(check(&doc, &signed_as("alice", more), &path(at), None).is_err());
+        // bob, holding the same key, for alice: not his entry.
+        assert!(check(&doc, &signed_as("bob", rekeyed), &path(at), None).is_err());
+    }
+
     #[test]
     fn listed_paths() {
         let mut doc = Document::new();
diff --git a/src/access/rules.rs b/src/access/rules.rs
index 3eca1c9..450addb 100644
--- a/src/access/rules.rs
+++ b/src/access/rules.rs
@@ -193,6 +193,13 @@ pub(crate) fn judge(doc: &Document, epoch: Epoch, change: &Change<'_>) -> Result
                     || user_may(path)
                     || user_may(&meta::path(&[meta::USERS]))
             }
+            // A user's own keys (`/auth/keys`), whatever else it may do.
+            (Some(meta::USERS), Some(owner))
+                if path.get(3).map(String::as_str) == Some("keys")
+                    && change.user == Some(owner) =>
+            {
+                true
+            }
             // A node forgotten while alive cancels its own record (`swarm::peer::forget`).
             (Some(meta::FORGOTTEN), Some(owner)) => owner == node || user_may(path),
             (Some(meta::REMAKES), Some(id)) => match path.get(3).map(String::as_str) {
@@ -788,6 +795,41 @@ mod tests {
         assert!(decide(Some("carol"), "committed", 0).is_ok());
     }
 
+    /// A user's own keys: its own to change, whatever else it may do; another's, or the
+    /// rest of its own entry, not.
+    #[test]
+    fn users_change_their_own_keys() {
+        let doc = swarm(true);
+        assert!(
+            judged(
+                &doc,
+                "a",
+                Some("bob"),
+                &["_continuum/users/bob/keys/laptop"]
+            )
+            .is_ok()
+        );
+        assert!(judged(&doc, "a", Some("bob"), &["_continuum/users/bob/keys"]).is_ok());
+        assert!(refused(judged(
+            &doc,
+            "a",
+            Some("bob"),
+            &["_continuum/users/alice/keys/laptop"]
+        )));
+        assert!(refused(judged(
+            &doc,
+            "a",
+            Some("bob"),
+            &["_continuum/users/bob/caps"]
+        )));
+        assert!(refused(judged(
+            &doc,
+            "a",
+            None,
+            &["_continuum/users/bob/keys/laptop"]
+        )));
+    }
+
     /// A node that isn't a member may only register itself.
     #[test]
     fn strangers_register_themselves_only() {