Sign a composition through a signer that keeps its private half (issue 1943, S5)

The controller's identity store signs a message and never hands its key out, as it signed declarations.

go/delivery/composed_signature.go

@@ -36,13 +36,33 @@ func SignComposed(c Composed, key ed25519.PrivateKey) (Composed, error) {
 	if len(key) != ed25519.PrivateKeySize {
 		return Composed{}, fmt.Errorf("delivery: the key to sign a composition with is not an ed25519 private key")
 	}
+	return SignComposedBy(c, key.Public().(ed25519.PublicKey), func(message []byte) ([]byte, error) {
+		return ed25519.Sign(key, message), nil
+	})
+}
+
+// SignComposedBy is SignComposed for a signer that keeps its private half to itself (the controller's identity store
+// signs a message and hands back the signature): sign signs the message it is given with the key whose public half is
+// signer. A signature that does not verify against signer is refused, so a composition is never written signed by a
+// key other than the one it names.
+func SignComposedBy(c Composed, signer ed25519.PublicKey, sign func(message []byte) ([]byte, error)) (Composed, error) {
+	if len(signer) != ed25519.PublicKeySize {
+		return Composed{}, fmt.Errorf("delivery: the signer of a composition is not an ed25519 public key")
+	}
+	c.Signature = nil
 	digest, err := ComposedDigestOf(c)
 	if err != nil {
 		return Composed{}, err
 	}
-	sig := ed25519.Sign(key, []byte(composedSigningContext+digest))
-	c.Signature = &Signature{Signer: base64.StdEncoding.EncodeToString(key.Public().(ed25519.PublicKey)),
-		Sig: base64.StdEncoding.EncodeToString(sig)}
+	message := []byte(composedSigningContext + digest)
+	sig, err := sign(message)
+	if err != nil {
+		return Composed{}, err
+	}
+	if !ed25519.Verify(signer, message, sig) {
+		return Composed{}, fmt.Errorf("delivery: the signature made for the composition of %q on %s does not verify against its signer", c.Module, c.Node)
+	}
+	c.Signature = &Signature{Signer: base64.StdEncoding.EncodeToString(signer), Sig: base64.StdEncoding.EncodeToString(sig)}
 	return c, nil
 }
 

go/delivery/composed_signature_test.go

@@ -83,3 +83,21 @@ func TestACompositionIsSignedByATrustedKey(t *testing.T) {
 		t.Errorf("a signature naming no key was encoded")
 	}
 }
+
+// 1943/AC5: a signer that keeps its private half signs through a function, and the composition verifies as one signed
+// with the key; a function that signs with another key than the one named is refused.
+func TestACompositionIsSignedByAKeptKey(t *testing.T) {
+	pub, priv := aSigningKey(t)
+	_, other := aSigningKey(t)
+	c := aComposition()
+	signed, err := SignComposedBy(c, pub, func(m []byte) ([]byte, error) { return ed25519.Sign(priv, m), nil })
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := VerifyComposed(signed, []ed25519.PublicKey{pub}); err != nil {
+		t.Errorf("a composition signed through the function does not verify: %v", err)
+	}
+	if _, err := SignComposedBy(c, pub, func(m []byte) ([]byte, error) { return ed25519.Sign(other, m), nil }); err == nil {
+		t.Errorf("a signature by another key than the one named was accepted")
+	}
+}