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.

9 files changed, 286 insertions, 7 deletions

diff --git a/docs/access.md b/docs/access.md
index 3541efd..5983c7c 100644
--- a/docs/access.md
+++ b/docs/access.md
@@ -31,7 +31,12 @@ password hashes: every member reads the document).
   doesn't read is left out, and anything but `false` disables.
 * **`keys`**: the user's SSH keys, each by a name of its choosing, as an
   `authorized_keys` line has it (`"laptop": "ssh-ed25519 AAAA…"`; Ed25519 only): what
-  it signs values with (Values signed by their users, below).
+  it signs values with (Values signed by their users, below) and logs in with. A user
+  manages its own, whatever else it may do: `GET /auth/keys`, `PUT /auth/keys/<name>`
+  with the line, `DELETE /auth/keys/<name>`; under a strict policy, by signing its
+  entry with only its keys changed (`continuum-signed key-add laptop
+  ~/.ssh/id_ed25519.pub`, `key-remove laptop`), with a key it holds already. Every
+  node takes that from the user itself, as from one who may manage users.
 
 ## A new swarm
 
diff --git a/docs/openapi.yaml b/docs/openapi.yaml
index 2ebf7fc..de62b59 100644
--- a/docs/openapi.yaml
+++ b/docs/openapi.yaml
@@ -977,6 +977,49 @@ paths:
         '204': { description: Done. }
         '409': { description: No user registry; tokens are taken until they expire. }
 
+  /auth/keys:
+    get:
+      tags: [Auth]
+      summary: Your keys
+      description: Your keys, your registry entry as recorded, and under a strict policy the envelope holding it (`signed`), which signing it anew names (`prev`).
+      responses:
+        '200':
+          description: Yours.
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  keys: { type: object, additionalProperties: { type: string } }
+                  entry: { type: object }
+                  signed: { type: string, nullable: true }
+        '404': { description: No entry of yours. }
+
+  /auth/keys/{id}:
+    parameters:
+      - { name: id, in: path, required: true, schema: { type: string } }
+    put:
+      tags: [Auth]
+      summary: Add or replace one of your keys
+      description: Whatever else you may do. Under a strict policy `409`; sign your entry with the keys changed instead (`continuum-signed key-add`).
+      requestBody:
+        content:
+          text/plain:
+            schema: { type: string }
+            example: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAA… laptop
+      responses:
+        '204': { description: Done. }
+        '400': { description: Not an Ed25519 public key, or not a key's name. }
+        '404': { description: No entry of yours. }
+        '409': { description: Strict policy. }
+    delete:
+      tags: [Auth]
+      summary: Remove one of your keys
+      responses:
+        '204': { description: Done. }
+        '404': { description: No entry of yours, or no such key. }
+        '409': { description: Strict policy. }
+
   /auth/whoami:
     get:
       tags: [Auth]
diff --git a/scripts/continuum-signed b/scripts/continuum-signed
index 6dd1f4c..1523094 100755
--- a/scripts/continuum-signed
+++ b/scripts/continuum-signed
@@ -14,6 +14,9 @@
 #   continuum-signed seal PATH JSON        writes JSON at PATH sealed for its readers,
 #                                          signed
 #   continuum-signed open PATH             the JSON sealed at PATH, opened
+#   continuum-signed key-add ID FILE       adds (or replaces) key ID of the user's own,
+#                                          the public key in FILE (.pub), signed
+#   continuum-signed key-remove ID         removes key ID of the user's own, signed
 #
 # PATH is the store's (`config/mode`), its segments without `/` or characters that need
 # escaping in a URL. Settings, from the environment:
@@ -88,10 +91,11 @@ raw() {
         "$url/store/$1?raw=true" || true
 }
 
-# Writes JSON $2 at $1, signed: replacing the envelope there, if any (`prev`).
+# Writes JSON $2 at $1, signed: replacing the envelope there, if any (`prev`; $3, if
+# given, instead of reading it).
 write() {
     [ -n "$user" ] || die "who signs? set CONTINUUM_USER"
-    current=$(raw "$1")
+    if [ $# -ge 3 ]; then current=$3; else current=$(raw "$1"); fi
     case $current in
         cv1.*) prev=$(printf %s "$current" | sha256sum | cut -d' ' -f1) ;;
         *) prev= ;;
@@ -186,10 +190,26 @@ verify() {
     echo "$1: signed by $signer at $(jq -r .at "$work/payload")"
 }
 
+# The user's own entry with key $1 set to the line in file $2, or removed (no $2),
+# signed: a user manages its own keys, whatever else it may do (GET /auth/keys gives
+# its entry and the envelope holding it).
+own_key() {
+    curl -sf -H "Authorization: Bearer $CONTINUUM_TOKEN" "$url/auth/keys" >"$work/own" ||
+        die "no entry of $user's"
+    if [ $# -ge 2 ]; then
+        line=$(head -n 1 "$2") || die "no key in $2"
+        entry=$(jq -c --arg id "$1" --arg line "$line" '.entry | .keys[$id] = $line' "$work/own")
+    else
+        jq -e --arg id "$1" '.keys | has($id)' "$work/own" >/dev/null || die "no key $1"
+        entry=$(jq -c --arg id "$1" '.entry | del(.keys[$id])' "$work/own")
+    fi
+    write "_continuum/users/$user" "$entry" "$(jq -r '.signed // empty' "$work/own")"
+}
+
 command=${1:-}
 [ $# -gt 0 ] && shift
 case $command in
-    put | delete | get | verify | seal | open)
+    put | delete | get | verify | seal | open | key-add | key-remove)
         # One login for all the command's requests, unless given a token.
         CONTINUUM_TOKEN=${CONTINUUM_TOKEN:-$(login)}
         [ -n "$CONTINUUM_TOKEN" ] || die "couldn't log in as $user"
@@ -203,5 +223,7 @@ case $command in
     verify) [ $# -eq 1 ] || die "verify PATH"; verify "$1" ;;
     seal) [ $# -eq 2 ] || die "seal PATH JSON"; seal "$1" "$2" ;;
     open) [ $# -eq 1 ] || die "open PATH"; open_sealed "$1" ;;
+    key-add) [ $# -eq 2 ] || die "key-add ID FILE"; own_key "$1" "$2" ;;
+    key-remove) [ $# -eq 1 ] || die "key-remove ID"; own_key "$1" ;;
     *) sed -n '2,/^$/s/^# \{0,1\}//p' "$0" >&2; exit 2 ;;
 esac
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() {
diff --git a/src/api/mod.rs b/src/api/mod.rs
index 0e7fc21..43d3c20 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -74,6 +74,11 @@ pub(crate) fn router(state: AppState, auth_state: AuthState, debug_api: bool) ->
         )
         .route("/auth/logout", axum::routing::post(login::logout))
         .route("/auth/whoami", get(login::whoami))
+        .route("/auth/keys", get(users::own_keys))
+        .route(
+            "/auth/keys/{id}",
+            axum::routing::put(users::put_own_key).delete(users::delete_own_key),
+        )
         .route("/admin/users", get(users::list).post(users::add_several))
         .route("/admin/users/{name}/tokens", delete(users::revoke_tokens))
         .route(
diff --git a/src/api/store.rs b/src/api/store.rs
index 81d0046..16dd966 100644
--- a/src/api/store.rs
+++ b/src/api/store.rs
@@ -464,7 +464,16 @@ pub(crate) async fn put_store_entry(
         return Err(StatusCode::SERVICE_UNAVAILABLE.into());
     }
 
-    let path = parse_write_path(&state, &user, &access, &key, "PUT")?;
+    // A user's own entry, signed by it: its keys are its own to change, whatever else it
+    // may do (`envelope::check` takes nothing else changed).
+    let own_entry = media_type(&headers) == SIGNED
+        && parse_key_path(&key)
+            .is_ok_and(|path| path == crate::access::registry::user_path(&user.0));
+    let path = if own_entry {
+        crate::access::registry::user_path(&user.0)
+    } else {
+        parse_write_path(&state, &user, &access, &key, "PUT")?
+    };
     let sign = Sign::Write {
         path: path.clone(),
         asked: query.signed,
diff --git a/src/api/users.rs b/src/api/users.rs
index 8972566..122fc4b 100644
--- a/src/api/users.rs
+++ b/src/api/users.rs
@@ -18,6 +18,15 @@
 //!   one exists. How a swarm without a registry gets one, the caller among those who
 //!   may manage users.
 //!
+//! A user manages its own keys (`keys`, `authorized_keys` lines), whatever it may do
+//! otherwise:
+//!
+//! * `GET /auth/keys`: the caller's keys, its entry, and the envelope holding it under a
+//!   strict policy (`signed`), to sign the next one against.
+//! * `PUT /auth/keys/{id}`: one added or replaced (the line as the body);
+//!   `DELETE /auth/keys/{id}` removes one. Under a strict policy `409`: the user signs
+//!   its entry with the keys changed, nothing else (`continuum-signed key-add`).
+//!
 //! Looking needs `read` on `_continuum/users`, changing `write` on it (in a swarm
 //! without a registry, being in `--protocol-writers`). Every change is one change to
 //! the document, logged, named by who made it, and refused (`409`) where it would lock
@@ -129,6 +138,100 @@ fn shown(doc: &impl crate::document::ReadDoc, name: &str) -> Option<JsonValue> {
     Some(entry)
 }
 
+/// `GET /auth/keys`.
+pub(crate) async fn own_keys(
+    State(state): State<AppState>,
+    Extension(user): Extension<AuthenticatedUser>,
+) -> Result<Json<JsonValue>, ApiError> {
+    let replica = state.doc.read().await;
+    let path = registry::user_path(&user.0);
+    let entry = store::read_plain(&replica.doc, &path)
+        .ok()
+        .flatten()
+        .ok_or_else(|| ApiError(StatusCode::NOT_FOUND, Some("no entry of yours".to_owned())))?;
+    let signed = match store::read_path(&replica.doc, &path) {
+        Ok(Some(JsonValue::String(text))) if text.starts_with("cv1.") => Some(text),
+        _ => None,
+    };
+    Ok(Json(json!({
+        "keys": entry.get("keys").cloned().unwrap_or_else(|| json!({})),
+        "entry": entry,
+        "signed": signed,
+    })))
+}
+
+/// `PUT /auth/keys/{id}`.
+pub(crate) async fn put_own_key(
+    State(state): State<AppState>,
+    Extension(user): Extension<AuthenticatedUser>,
+    Path(id): Path<String>,
+    body: String,
+) -> Result<StatusCode, ApiError> {
+    let key = crate::access::sshsig::PublicKey::parse(body.trim()).map_err(|why| bad(&why))?;
+    own_key(&state, &user.0, id, Some(key.line())).await
+}
+
+/// `DELETE /auth/keys/{id}`.
+pub(crate) async fn delete_own_key(
+    State(state): State<AppState>,
+    Extension(user): Extension<AuthenticatedUser>,
+    Path(id): Path<String>,
+) -> Result<StatusCode, ApiError> {
+    own_key(&state, &user.0, id, None).await
+}
+
+/// `user`'s key `id` set to `line`, or removed: one change, as `user`.
+async fn own_key(
+    state: &AppState,
+    user: &str,
+    id: String,
+    line: Option<String>,
+) -> Result<StatusCode, ApiError> {
+    if !registry::valid_name(&id) {
+        return Err(bad("not a key's name: 1 to 64 of A-Z a-z 0-9 . _ @ -"));
+    }
+    if !state.is_ready().await {
+        return Err(StatusCode::SERVICE_UNAVAILABLE.into());
+    }
+    let entry = registry::user_path(user);
+    let mut path = entry.clone();
+    path.extend(["keys".to_owned(), id.clone()]);
+    let adding = line.is_some();
+    state
+        .mutate_doc_for(Some(user.to_owned()), Sign::Protocol, move |doc| {
+            if crate::access::policy::of(&*doc).strict() {
+                return Err(ApiError(
+                    StatusCode::CONFLICT,
+                    Some(
+                        "users sign their entries (strict policy): sign yours with the keys \
+                         changed (continuum-signed key-add, key-remove)"
+                            .to_owned(),
+                    ),
+                ));
+            }
+            if store::read_path(&*doc, &entry)?.is_none() {
+                return Err(ApiError(
+                    StatusCode::NOT_FOUND,
+                    Some("no entry of yours".to_owned()),
+                ));
+            }
+            match line {
+                Some(line) => store::write_path(doc, &path, &json!(line))?,
+                None => match store::delete_path(doc, &path, false) {
+                    Ok(()) => {}
+                    Err(crate::document::store::StoreError::NotFound) => {
+                        return Err(ApiError(StatusCode::NOT_FOUND, None));
+                    }
+                    Err(error) => return Err(error.into()),
+                },
+            }
+            Ok(())
+        })
+        .await?;
+    info!(user = %user, key = %id, "{}", if adding { "Set own key" } else { "Removed own key" });
+    Ok(StatusCode::NO_CONTENT)
+}
+
 /// `GET /admin/users`.
 pub(crate) async fn list(State(state): State<AppState>) -> Json<JsonValue> {
     let replica = state.doc.read().await;
diff --git a/src/swarm/refusals.rs b/src/swarm/refusals.rs
index 2ba7908..c1949eb 100644
--- a/src/swarm/refusals.rs
+++ b/src/swarm/refusals.rs
@@ -18,8 +18,8 @@ use std::time::Duration;
 
 use tracing::{debug, warn};
 
-use crate::document::replica::Epoch;
 use crate::document::ChangeHash;
+use crate::document::replica::Epoch;
 use crate::state::AppState;
 use crate::transport::{ParsedMessage, RefusalsMessage};