musig2: add combinedNonce getter

Author: sputn1ck

btcec/schnorr/musig2/context.go

@@ -548,6 +548,20 @@ func (s *Session) RegisterPubNonce(nonce [PubNonceSize]byte) (bool, error) {
 	return haveAllNonces, nil
 }
 
+// CombinedNonce returns the combined public nonce for the signing session.
+// This will be available after either:
+//  - All individual nonces have been registered via RegisterPubNonce, or
+//  - A combined nonce has been registered via RegisterCombinedNonce
+//
+// If the combined nonce is not yet available, this method returns an error.
+func (s *Session) CombinedNonce() ([PubNonceSize]byte, error) {
+	if s.combinedNonce == nil {
+		return [PubNonceSize]byte{}, ErrCombinedNonceUnavailable
+	}
+
+	return *s.combinedNonce, nil
+}
+
 // RegisterCombinedNonce allows a caller to directly register a combined nonce
 // that was generated externally. This is useful in coordinator-based
 // protocols where the coordinator aggregates all nonces and distributes the

btcec/schnorr/musig2/musig2_test.go

@@ -746,4 +746,84 @@ func TestSigningWithAggregatedNonce(t *testing.T) {
 			t.Fatalf("final signature is invalid")
 		}
 	})
+
+	t.Run("get combined nonce after RegisterCombinedNonce", func(t *testing.T) {
+		privKey, _ := btcec.NewPrivateKey()
+		privKey2, _ := btcec.NewPrivateKey()
+		signSet := []*btcec.PublicKey{privKey.PubKey(), privKey2.PubKey()}
+
+		signCtx, _ := NewContext(privKey, false, WithKnownSigners(signSet))
+		session, _ := signCtx.NewSession()
+
+		// Should fail before registering combined nonce.
+		_, err := session.CombinedNonce()
+		if err != ErrCombinedNonceUnavailable {
+			t.Fatalf("expected ErrCombinedNonceUnavailable, got: %v", err)
+		}
+
+		// Register combined nonce.
+		expectedNonce := [PubNonceSize]byte{1, 2, 3}
+		err = session.RegisterCombinedNonce(expectedNonce)
+		if err != nil {
+			t.Fatalf("RegisterCombinedNonce failed: %v", err)
+		}
+
+		// Should succeed after registering.
+		gotNonce, err := session.CombinedNonce()
+		if err != nil {
+			t.Fatalf("CombinedNonce failed: %v", err)
+		}
+
+		if gotNonce != expectedNonce {
+			t.Fatalf("expected nonce %x, got %x", expectedNonce, gotNonce)
+		}
+	})
+
+	t.Run("get combined nonce after RegisterPubNonce", func(t *testing.T) {
+		const numSigners = 3
+
+		signerKeys := make([]*btcec.PrivateKey, numSigners)
+		signSet := make([]*btcec.PublicKey, numSigners)
+		for i := 0; i < numSigners; i++ {
+			privKey, _ := btcec.NewPrivateKey()
+			signerKeys[i] = privKey
+			signSet[i] = privKey.PubKey()
+		}
+
+		sessions := make([]*Session, numSigners)
+		for i, signerKey := range signerKeys {
+			signCtx, _ := NewContext(signerKey, false, WithKnownSigners(signSet))
+			session, _ := signCtx.NewSession()
+			sessions[i] = session
+		}
+
+		pubNonces := make([][PubNonceSize]byte, numSigners)
+		for i, session := range sessions {
+			pubNonces[i] = session.PublicNonce()
+		}
+
+		// Should fail before all nonces are registered.
+		_, err := sessions[0].CombinedNonce()
+		if err != ErrCombinedNonceUnavailable {
+			t.Fatalf("expected ErrCombinedNonceUnavailable before all nonces, got: %v", err)
+		}
+
+		// Register all nonces via RegisterPubNonce.
+		for i := 1; i < numSigners; i++ {
+			sessions[0].RegisterPubNonce(pubNonces[i])
+		}
+
+		// Should succeed after all nonces are registered.
+		gotNonce, err := sessions[0].CombinedNonce()
+		if err != nil {
+			t.Fatalf("CombinedNonce failed: %v", err)
+		}
+
+		// Verify it matches what AggregateNonces produces.
+		expectedNonce, _ := AggregateNonces(pubNonces)
+		if gotNonce != expectedNonce {
+			t.Fatalf("combined nonce mismatch: expected %x, got %x",
+				expectedNonce[:8], gotNonce[:8])
+		}
+	})
 }