A node's record carries the mesh's own certificate as the machine holds it, and frombus composes with the composing node's (issue 1126, ADR 0555 §3)

go/compose/frombus/frombus.go

@@ -106,6 +106,20 @@ func Input(ctx context.Context, r Reader, mesh Mesh, s Scope) (compose.Input, er
 
 	in := compose.Input{OverlayCIDR: mesh.OverlayCIDR, InternalSuffix: mesh.InternalSuffix, Authority: mesh.Authority,
 		Catalogue: map[string]catalogue.Manifest{}}
+	if in.Authority == "" {
+		// The mesh's own certificate, public, as the composing node holds it; for the check of every node, as the first
+		// node holding one does.
+		if rec, ok := records[s.Node]; ok && rec.Authority != "" {
+			in.Authority = rec.Authority
+		} else {
+			for _, node := range sortedKeys(records) {
+				if a := records[node].Authority; a != "" {
+					in.Authority = a
+					break
+				}
+			}
+		}
+	}
 	read := func(module, digest string) (catalogue.Manifest, error) {
 		raw, err := r.Manifest(ctx, digest)
 		if err != nil {

go/compose/frombus/frombus_test.go

@@ -88,7 +88,7 @@ func TestInput(t *testing.T) {
 			},
 			records: map[string]delivery.NodeRecord{
 				"anchor": {Node: "anchor", TunnelKey: keys["anchor"], Capabilities: caps, Contract: 2},
-				"laptop": {Node: "laptop", TunnelKey: keys["laptop"], Capabilities: caps},
+				"laptop": {Node: "laptop", TunnelKey: keys["laptop"], Capabilities: caps, Authority: "the mesh's certificate"},
 			},
 			modules:   map[string]delivery.Module{"xterm": {Manifest: digest}, "unread": {}},
 			manifests: map[string][]byte{digest: xterm},
@@ -127,6 +127,7 @@ func TestInput(t *testing.T) {
 		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.
+		Authority: "the mesh's certificate",
 		CarriedPeers: []compose.CarriedPeer{{PublicKey: keys["laptop"], Address: "10.77.0.2", EnrolledAs: "laptop"},
 			{PublicKey: "peer", Address: "10.77.0.9", Named: "phone"}},
 	}

go/delivery/noderecord.go

@@ -52,6 +52,9 @@ type NodeRecord struct {
 	Networks     []catalogue.AttachedNetwork `json:"networks,omitempty"`
 	NetworksSaid bool                        `json:"networks-said,omitempty"`
 	Contract     int                         `json:"contract,omitempty"`
+	// Authority is the mesh's own certificate as this machine holds it (public): the one its declaration last wrote where
+	// a module asked for it. A node composing its own composes a certificate with it (compose.Input.Authority).
+	Authority string `json:"authority,omitempty"`
 }
 
 func (r NodeRecord) check() error {