A token can be issued for a machine's tunnel key, and it joins through the tunnel

token issue --overlay-key records the key the machine made, binds the
token to it, gives the machine its address and makes it a peer of the
hub, pushing the hub before the token is shown. The token carries the
hub's tunnel and the bus at its address on the private network, and
enrolment refuses any other key (novox/hq ADR 0169). Tokens without a
key enrol as before until the bus is closed. Also a token verb.

internal/inventory/migrations/0055-a-token-is-issued-for-a-tunnel-key.sql

@@ -0,0 +1,7 @@
+-- A token issued for a tunnel key (novox/hq ADR 0169).
+--
+-- A machine that joins through the tunnel makes its key first, and the token is issued for it: the
+-- hub is told the key before the token is shown. So enrolment must take that key and no other — a
+-- different one is a machine the hub does not know, offering a tunnel that would never answer. Null
+-- for a token issued without one, which enrols as before.
+alter table enrolment_token add column overlay_key text;

internal/inventory/nodes.go

@@ -356,6 +356,33 @@ func (i *Inventory) Claim(ctx context.Context, secret, by string, again bool) (N
 	return scanNode(i.store.Pool().QueryRow(ctx, `select `+nodeColumns+` from node where id = $1`, id))
 }
 
+// BindTokenToKey records the tunnel key a node's live token was issued for (novox/hq ADR 0169), so
+// enrolment takes that key and no other. Refused when the node has no live token to bind: a key
+// recorded against nothing would be a promise nothing keeps.
+func (i *Inventory) BindTokenToKey(ctx context.Context, node, key string) error {
+	tag, err := i.store.Pool().Exec(ctx,
+		`update enrolment_token set overlay_key = $2
+		  where node = $1 and redeemed is null and expires > now()`, node, key)
+	if err != nil {
+		return err
+	}
+	if tag.RowsAffected() == 0 {
+		return fmt.Errorf("no live token to issue for the tunnel key")
+	}
+	return nil
+}
+
+// TokenKey is the tunnel key a token was issued for, or empty when it was issued without one.
+func (i *Inventory) TokenKey(ctx context.Context, secret string) (string, error) {
+	var key *string
+	err := i.store.Pool().QueryRow(ctx,
+		`select overlay_key from enrolment_token where secret = $1`, hashSecret(secret)).Scan(&key)
+	if err != nil || key == nil {
+		return "", err
+	}
+	return *key, nil
+}
+
 // Spend makes a claimed token used, only for the presenter holding the claim. The last write to the
 // store in an enrolment, so a token is spent exactly when the node it enrolled is complete. Spent
 // again by the same presenter is not an error: an answer lost after the first spend.