Sign a composition with a trusted key and compose one by hand with mesh-compose (issue 1943, S5)

The node-engine applies only a composition signed by the controller or the operator, and when the controller cannot
compose its own fix the operator composes, signs and writes one at the terminal (ADR 0539 clauses 3 and 10).

go/delivery/composed_value.go

@@ -46,6 +46,9 @@ type Composed struct {
 	Resources []json.RawMessage
 	Filled    []string
 	Own       NodeOwn
+	// Signature is the controller's or, made by hand, the operator's (ADR 0539 clause 3, SignComposed); it signs the
+	// digest and is never part of it.
+	Signature *Signature
 }
 
 // composedWire is a composition as it is written.
@@ -58,6 +61,7 @@ type composedWire struct {
 	Resources []json.RawMessage `json:"resources"`
 	Filled    []string          `json:"filled_files,omitempty"`
 	Own       *NodeOwn          `json:"own,omitempty"`
+	Signature *Signature        `json:"signature,omitempty"`
 }
 
 // canonicalJSON is raw with its object keys sorted and its spacing dropped, its numbers kept as written.
@@ -126,6 +130,11 @@ func (c Composed) check() error {
 			}
 		}
 	}
+	if c.Signature != nil {
+		if err := c.Signature.check(); err != nil {
+			return err
+		}
+	}
 	return nil
 }
 
@@ -135,7 +144,7 @@ func (c Composed) wire() (composedWire, error) {
 		return composedWire{}, err
 	}
 	w := composedWire{V: Version, Node: c.Node, Module: c.Module, FP: c.FP, Data: c.Data, Filled: c.Filled,
-		Resources: make([]json.RawMessage, len(c.Resources))}
+		Signature: c.Signature, Resources: make([]json.RawMessage, len(c.Resources))}
 	for i, r := range c.Resources {
 		canon, err := canonicalJSON(r)
 		if err != nil {
@@ -174,7 +183,7 @@ func DecodeComposed(data []byte) (Composed, error) {
 		return Composed{}, fmt.Errorf("delivery: a composition does not fit its shape: %w", err)
 	}
 	c := Composed{V: w.V, Node: w.Node, Module: w.Module, FP: w.FP, Data: w.Data, Resources: w.Resources,
-		Filled: w.Filled}
+		Filled: w.Filled, Signature: w.Signature}
 	if w.Own != nil {
 		c.Own = *w.Own
 	}
@@ -184,9 +193,10 @@ func DecodeComposed(data []byte) (Composed, error) {
 	return c, nil
 }
 
-// ComposedDigestOf is a composition's digest, "sha256:<hex>", of its encoding. A want names it (ADR 0539 clause 1),
-// and the controller writes it beside the composition.
+// ComposedDigestOf is a composition's digest, "sha256:<hex>", of its encoding without its signature. A want names it
+// (ADR 0539 clause 1), the controller writes it beside the composition, and the signature signs it.
 func ComposedDigestOf(c Composed) (string, error) {
+	c.Signature = nil
 	raw, err := EncodeComposed(c)
 	if err != nil {
 		return "", err
@@ -195,7 +205,7 @@ func ComposedDigestOf(c Composed) (string, error) {
 	return "sha256:" + hex.EncodeToString(sum[:]), nil
 }
 
-// SameComposed says whether two compositions are the same value, so the controller writes
+// SameComposed says whether two compositions are the same value, their signatures included, so the controller writes
 // only when they differ.
 func SameComposed(a, b Composed) bool {
 	ra, errA := EncodeComposed(a)