frombus gives the composing node's core credentials as the vault's boxes sealed to it, from the same own secrets (issue 1126, ADR 0349 §16, ADR 0550 §5)

go/compose/frombus/frombus.go

@@ -210,6 +210,14 @@ func Input(ctx context.Context, r Reader, mesh Mesh, s Scope) (compose.Input, er
 		if in.OwnSecrets, err = r.OwnSecrets(ctx, s.Node); err != nil {
 			return compose.Input{}, fmt.Errorf("frombus: the own secrets of %s's modules: %w", s.Node, err)
 		}
+		// The core's credentials travel as the vault's box sealed to the machine (ADR 0349 §16): the same values, which
+		// the composition reads for a core module as its kept boxes.
+		for _, o := range in.OwnSecrets {
+			if json.Valid([]byte(o.Sealed)) {
+				in.KeptCoreBoxes = append(in.KeptCoreBoxes, compose.CoreBox{Node: o.Node, Module: o.Module, Name: o.Name,
+					SealedTo: o.SealedTo, Box: json.RawMessage(o.Sealed)})
+			}
+		}
 	}
 	return in, nil
 }

go/compose/frombus/frombus_test.go

@@ -122,8 +122,10 @@ func TestInput(t *testing.T) {
 		},
 		Holdings:   []catalogue.Held{{Claim: "mesh-private-network", Scope: "mesh", Node: "anchor", Module: overlay.Name}},
 		OwnSecrets: []compose.OwnSecretRow{{Node: "laptop", Module: "xterm", Name: "token", Origin: "made", SealedTo: "sk", Sealed: "{}"}},
-		Tunnels:    map[string]compose.FoundTunnel{"anchor": {Tunnel: tun}},
-		Adopted:    &compose.AdoptedTunnel{Tunnel: tun, Hub: "anchor", Adopted: true},
+		// The same value as a core credential's box, for a core module (ADR 0349 §16).
+		KeptCoreBoxes: []compose.CoreBox{{Node: "laptop", Module: "xterm", Name: "token", SealedTo: "sk", Box: json.RawMessage("{}")}},
+		Tunnels:       map[string]compose.FoundTunnel{"anchor": {Tunnel: tun}},
+		Adopted:       &compose.AdoptedTunnel{Tunnel: tun, Hub: "anchor", Adopted: true},
 		// The controller's order is by address; laptop enrolled under its peer's key, and a person named the other.
 		CarriedPeers: []compose.CarriedPeer{{PublicKey: keys["laptop"], Address: "10.77.0.2", EnrolledAs: "laptop"},
 			{PublicKey: "peer", Address: "10.77.0.9", Named: "phone"}},