spaces: leaving a room says so, so an owner stops publishing a seat nobody sits in

crates/persona-state/src/space.rs

@@ -255,6 +255,24 @@ pub fn join_proof_valid(
     crate::verify_sig_in(SIGN_DOMAIN, persona, &join_proof_payload(space_id, owner, room_key), proof)
 }
 
+/// What a member signs to give up their seat. Prefixed, so a join proof can
+/// never be replayed as a leave, and signed by the key that HOLDS the seat —
+/// holding it is the whole authority to vacate it, and the owner needs no
+/// second fact to act.
+pub fn leave_proof_payload(space_id: &[u8; 16], owner: &[u8; 32], worn: &[u8; 32]) -> Vec<u8> {
+    let mut v = Vec::with_capacity(5 + 16 + 32 + 32);
+    v.extend_from_slice(b"leave");
+    v.extend_from_slice(space_id);
+    v.extend_from_slice(owner);
+    v.extend_from_slice(worn);
+    blake3::hash(&v).as_bytes().to_vec()
+}
+
+/// True when whoever sits under `worn` really did ask to leave this Space.
+pub fn leave_proof_valid(space_id: &[u8; 16], owner: &[u8; 32], worn: &[u8; 32], proof: &[u8]) -> bool {
+    crate::verify_sig_in(SIGN_DOMAIN, worn, &leave_proof_payload(space_id, owner, worn), proof)
+}
+
 pub fn checkpoint_valid(owner: &[u8; 32], space_id: &[u8; 16], c: &Checkpoint) -> bool {
     c.schema_version == SCHEMA_VERSION
         && &c.space_id == space_id
@@ -453,6 +471,27 @@ mod tests {
         assert!(!join_proof_valid(&[4; 16], &ok, &room, &mallory, &proof), "anyone could claim it");
     }
 
+    /// A seat is given up by the key that holds it, and a join proof is not a
+    /// leave proof — otherwise the owner's own invitation answer could be
+    /// replayed to unseat the person who sent it.
+    #[test]
+    fn a_leave_proof_comes_from_the_seat_it_gives_up() {
+        let owner = SigningKey::from_bytes(&[1; 32]);
+        let ok = owner.verifying_key().to_bytes();
+        let room_secret = [9u8; 32];
+        let room = member_public(&room_secret);
+        let proof = sign_as_member(&room_secret, &leave_proof_payload(&[4; 16], &ok, &room));
+        assert!(leave_proof_valid(&[4; 16], &ok, &room, &proof));
+        // Not reusable in another Space, and not against another seat.
+        assert!(!leave_proof_valid(&[5; 16], &ok, &room, &proof), "replayed into another Space");
+        assert!(!leave_proof_valid(&[4; 16], &ok, &member_public(&[8; 32]), &proof), "bound to the wrong seat");
+        // A join proof carries a different payload, so it cannot stand in.
+        let bob_persona = SigningKey::from_bytes(&[2; 32]);
+        let bp = bob_persona.verifying_key().to_bytes();
+        let join = sign(&bob_persona, &join_proof_payload(&[4; 16], &ok, &bp));
+        assert!(!leave_proof_valid(&[4; 16], &ok, &bp, &join), "a join proof unseated its own sender");
+    }
+
     #[test]
     fn roster_is_the_authority_and_removal_is_an_epoch() {
         let owner = SigningKey::from_bytes(&[1; 32]);

src-tauri/src/inbox.rs

@@ -54,6 +54,7 @@ pub enum Cargo {
     SpaceKey(SpaceKeyCargo),
     ProposalPing(ProposalPing),
     SpaceJoin(SpaceJoin),
+    SpaceLeave(SpaceLeave),
 }
 
 /// The answer to an invitation: "here is the key I will wear in your Space."
@@ -68,6 +69,18 @@ pub struct SpaceJoin {
     pub proof: Vec<u8>,
 }
 
+/// "I am giving up my seat." The owner cannot see a member walk away — the
+/// roster is theirs to publish — so leaving says so, sealed. `worn` is the key
+/// on the roster, and `proof` is signed BY that key: holding the seat is the
+/// whole authority to vacate it.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SpaceLeave {
+    pub space_id: [u8; 16],
+    pub owner: [u8; 32],
+    pub worn: [u8; 32],
+    pub proof: Vec<u8>,
+}
+
 /// "Someone put a proposal on your playlist's board." Carries only enough to
 /// name it in a toast — the board itself stays the truth, and this is a
 /// best-effort nudge: the proposal stands whether or not the ping lands.
@@ -507,6 +520,7 @@ async fn dispatch(app: &tauri::AppHandle, sender: [u8; 32], cargo: Cargo) {
         Cargo::SpaceKey(k) => crate::spaces::on_inbox_space_key(app, sender, k).await,
         Cargo::ProposalPing(p) => crate::proposals::on_inbox_proposal(app, sender, p),
         Cargo::SpaceJoin(j) => crate::spaces::on_inbox_join(app, sender, j).await,
+        Cargo::SpaceLeave(l) => crate::spaces::on_inbox_leave(app, sender, l).await,
     }
 }
 
@@ -572,8 +586,15 @@ mod tests {
             room_key: [10u8; 32],
             proof: vec![1, 2, 3],
         });
+        let leave = Cargo::SpaceLeave(SpaceLeave {
+            space_id: [11u8; 16],
+            owner: [12u8; 32],
+            worn: [13u8; 32],
+            proof: vec![4, 5, 6],
+        });
         assert_eq!(index(&ping), 6, "a new variant must be appended, never inserted");
         assert_eq!(index(&join), 7, "a new variant must be appended, never inserted");
+        assert_eq!(index(&leave), 8, "a new variant must be appended, never inserted");
 
         let back: Cargo = bincode::deserialize(&bincode::serialize(&ping).unwrap()).unwrap();
         match back {

src-tauri/src/spaces.rs

@@ -24,7 +24,7 @@ use persona_state::space as sp;
 use serde::{Deserialize, Serialize};
 use tauri::{Emitter, Manager};
 
-use crate::inbox::{self, Cargo, SpaceInvite, SpaceJoin, SpaceKeyCargo};
+use crate::inbox::{self, Cargo, SpaceInvite, SpaceJoin, SpaceKeyCargo, SpaceLeave};
 use crate::ledger;
 use crate::persona;
 use crate::slsk_link;
@@ -635,12 +635,32 @@ pub async fn remove_member(app: &tauri::AppHandle, owner_b58: &str, space_b58: &
             .and_then(|(room, _)| key_of(room).ok())
             .unwrap_or(asked)
     };
+    unseat(app, owner_b58, space_b58, me, owner, space_id, gone).await?;
+    open(app, owner_b58, space_b58).await
+}
+
+/// Take one key off the roster and re-key everyone who stays. The owner does
+/// this both when they remove someone and when someone leaves of their own
+/// accord, and it must look identical from the outside: a member who walks
+/// away is only trusted to have DELETED the epoch key, never proven to.
+async fn unseat(
+    app: &tauri::AppHandle,
+    owner_b58: &str,
+    space_b58: &str,
+    me: [u8; 32],
+    owner: [u8; 32],
+    space_id: [u8; 16],
+    gone: [u8; 32],
+) -> Result<sp::Checkpoint, String> {
     let c = bump(app, me, owner, space_id, |c| c.members.retain(|m| m.key != gone)).await?;
     let rid = record_id(owner_b58, space_b58);
     let ledger = ledger::hold(LEDGER_FILE).await;
     let mut l = load(app);
     let rec = l.spaces.get_mut(&rid).ok_or("not a Space this device knows")?;
     rec.last_epoch = c.epoch;
+    // Nobody sits under that key any more, so neither does the private
+    // mapping. Keeping it would hold a persona this device no longer needs.
+    rec.roster.remove(&b58(&gone));
     if c.private {
         // A removal rotates: everyone who stays gets the new epoch's key.
         let k = rand32();
@@ -665,9 +685,9 @@ pub async fn remove_member(app: &tauri::AppHandle, owner_b58: &str, space_b58: &
     } else {
         save(app, &l)?;
     }
-    // open() takes this same ledger; the write is done, so let go first.
+    // open() takes this same ledger; callers go straight back into it.
     drop(ledger);
-    open(app, owner_b58, space_b58).await
+    Ok(c)
 }
 
 pub async fn set_pinned(app: &tauri::AppHandle, owner_b58: &str, space_b58: &str, pl_owner: Option<String>, pl_uuid: Option<String>) -> Result<SpaceView, String> {
@@ -683,12 +703,34 @@ pub async fn set_pinned(app: &tauri::AppHandle, owner_b58: &str, space_b58: &str
 }
 
 pub async fn leave(app: &tauri::AppHandle, owner_b58: &str, space_b58: &str) -> Result<(), String> {
+    let me = my_key(app).await?;
+    let owner = key_of(owner_b58)?;
+    let space_id = id_of(space_b58)?;
+    let rid = record_id(owner_b58, space_b58);
+    // Signed BEFORE the record goes: the room secret is the only thing that
+    // can prove this seat asked to be vacated, and leaving destroys it.
+    let (worn, room) = {
+        let l = load(app);
+        let rec = l.spaces.get(&rid).ok_or("not a Space this device knows")?;
+        if rec.my_role == "owner" {
+            return Err("an owner keeps the Space; remove members instead".into());
+        }
+        (worn_key(rec, &me), room_secret_of(rec))
+    };
+    let payload = sp::leave_proof_payload(&space_id, &owner, &worn);
+    let proof = match room {
+        Some(secret) => sp::sign_as_member(&secret, &payload),
+        None => sign(app, payload).await?,
+    };
+    // Best-effort, and deliberately so: the person asked to go, so they go
+    // whether or not the owner is reachable. A lost notice leaves a seat on
+    // the roster that can no longer post — the owner can still remove it.
+    let cargo = Cargo::SpaceLeave(SpaceLeave { space_id, owner, worn, proof });
+    if let Err(e) = inbox::send(app, &owner, &cargo).await {
+        log::warn!("spaces: left {space_b58} but could not tell its owner: {e}");
+    }
     let _ledger = ledger::hold(LEDGER_FILE).await;
     let mut l = load(app);
-    let rid = record_id(owner_b58, space_b58);
-    if l.spaces.get(&rid).is_some_and(|r| r.my_role == "owner") {
-        return Err("an owner keeps the Space; remove members instead".into());
-    }
     l.spaces.remove(&rid);
     save(app, &l)
 }
@@ -771,6 +813,44 @@ async fn send_join(app: &tauri::AppHandle, owner: &[u8; 32], space_id: &[u8; 16]
     }
 }
 
+/// Sealed cargo: a member gave up their seat. Three things have to hold —
+/// the seat exists, its holder signed the request, and the sender is the
+/// person this device seated under it. The last one is what stops anyone who
+/// merely READ the roster (the keys are published) from unseating a stranger.
+pub(crate) async fn on_inbox_leave(app: &tauri::AppHandle, sender: [u8; 32], lv: SpaceLeave) {
+    let owner_b58 = b58(&lv.owner);
+    let space_b58 = b58(&lv.space_id);
+    let rid = record_id(&owner_b58, &space_b58);
+    {
+        let _ledger = ledger::hold(LEDGER_FILE).await;
+        let l = load(app);
+        let Some(rec) = l.spaces.get(&rid) else { return };
+        if rec.my_role != "owner" {
+            return;
+        }
+        // Room-local seats map back to a persona here and nowhere else; a
+        // Space seated before room keys holds the persona on the roster
+        // itself, and then the sender IS the seat.
+        let behind = rec.roster.get(&b58(&lv.worn)).map(String::as_str);
+        if behind.unwrap_or(&b58(&lv.worn)) != b58(&sender) {
+            log::warn!("spaces: a leave came from someone other than the member seated under that key");
+            return;
+        }
+    }
+    if !sp::leave_proof_valid(&lv.space_id, &lv.owner, &lv.worn, &lv.proof) {
+        log::warn!("spaces: a leave did not prove it came from the seat it gives up");
+        return;
+    }
+    let me = match my_key(app).await {
+        Ok(k) => k,
+        Err(_) => return,
+    };
+    match unseat(app, &owner_b58, &space_b58, me, lv.owner, lv.space_id, lv.worn).await {
+        Ok(c) => log::info!("spaces: a member left {:?}, roster at epoch {}", c.name, c.epoch),
+        Err(e) => log::warn!("spaces: could not unseat a member who left: {e}"),
+    }
+}
+
 /// A member who holds a room key but is not on the roster never got seated.
 /// Say so again, spaced out, rather than leaving them stranded.
 const JOIN_RETRY_SECS: u64 = 120;