client: reject malformed server public keys

Loop-in and loop-out responses carry compressed server public keys that are copied into fixed-size fields and later used for HTLC construction. Validate the length and parse each compressed key before storing it, and validate the MuSig2 loop-in receiver internal key as well.

This turns short or unparsable server keys into explicit errors instead of silently zero-padding short responses or accepting an invalid internal key. Update root test mocks to return size-correct MuSig2 signing data under the stricter checks.

server_mock_test.go

@@ -6,12 +6,14 @@ import (
 	"testing"
 	"time"
 
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightninglabs/lndclient"
 	"github.com/lightninglabs/loop/loopdb"
 	"github.com/lightninglabs/loop/test"
+	"github.com/lightningnetwork/lnd/input"
 	invpkg "github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -32,6 +34,13 @@ var (
 	testMaxSwapAmount              = btcutil.Amount(1000000)
 )
 
+// mockMuSig2SigningData returns size-correct placeholder data. Client tests
+// only assert response handling, not MuSig2 cryptographic validity.
+func mockMuSig2SigningData() ([]byte, []byte, error) {
+	return make([]byte, musig2.PubNonceSize),
+		make([]byte, input.MuSig2PartialSigSize), nil
+}
+
 // serverMock is used in client unit tests to simulate swap server behaviour.
 type serverMock struct {
 	expectedSwapAmt  btcutil.Amount
@@ -276,7 +285,7 @@ func (s *serverMock) MuSig2SignSweep(_ context.Context, _ loopdb.ProtocolVersion
 	_ lntypes.Hash, _ [32]byte, _ []byte, _ []byte) ([]byte,
 	[]byte, error) {
 
-	return nil, nil, nil
+	return mockMuSig2SigningData()
 }
 
 func (s *serverMock) MultiMuSig2SignSweep(ctx context.Context,
@@ -285,7 +294,7 @@ func (s *serverMock) MultiMuSig2SignSweep(ctx context.Context,
 	prevoutMap map[wire.OutPoint]*wire.TxOut) (
 	[]byte, []byte, error) {
 
-	return nil, nil, nil
+	return mockMuSig2SigningData()
 }
 
 func (s *serverMock) PushKey(_ context.Context, _ loopdb.ProtocolVersion,

swap_server_client.go

@@ -405,13 +405,9 @@ func (s *grpcSwapServerClient) NewLoopOutSwap(ctx context.Context,
 		return nil, err
 	}
 
-	var senderKey [33]byte
-	copy(senderKey[:], swapResp.SenderKey)
-
-	// Validate sender key.
-	_, err = btcec.ParsePubKey(senderKey[:])
+	senderKey, err := parseServerPubKey("sender key", swapResp.SenderKey)
 	if err != nil {
-		return nil, fmt.Errorf("invalid sender key: %v", err)
+		return nil, err
 	}
 
 	return &newLoopOutResponse{
@@ -470,14 +466,22 @@ func (s *grpcSwapServerClient) NewLoopInSwap(ctx context.Context,
 		return nil, err
 	}
 
-	var receiverKey, receiverInternalKey [33]byte
-	copy(receiverKey[:], swapResp.ReceiverKey)
-	copy(receiverInternalKey[:], swapResp.ReceiverInternalPubkey)
-
-	// Validate receiver key.
-	_, err = btcec.ParsePubKey(receiverKey[:])
+	receiverKey, err := parseServerPubKey(
+		"receiver key", swapResp.ReceiverKey,
+	)
 	if err != nil {
-		return nil, fmt.Errorf("invalid sender key: %v", err)
+		return nil, err
+	}
+
+	var receiverInternalKey [btcec.PubKeyBytesLenCompressed]byte
+	if loopdb.CurrentProtocolVersion() >= loopdb.ProtocolVersionMuSig2 {
+		receiverInternalKey, err = parseServerPubKey(
+			"receiver internal key",
+			swapResp.ReceiverInternalPubkey,
+		)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	return &newLoopInResponse{
@@ -488,6 +492,29 @@ func (s *grpcSwapServerClient) NewLoopInSwap(ctx context.Context,
 	}, nil
 }
 
+// parseServerPubKey validates that keyBytes is a well-formed compressed public
+// key received from the server and returns it as a fixed-size array. The name
+// argument is used to produce a descriptive error if validation fails.
+func parseServerPubKey(name string,
+	keyBytes []byte) ([btcec.PubKeyBytesLenCompressed]byte, error) {
+
+	var key [btcec.PubKeyBytesLenCompressed]byte
+
+	if len(keyBytes) != btcec.PubKeyBytesLenCompressed {
+		return key, fmt.Errorf("invalid %s length: got %d, want %d",
+			name, len(keyBytes), btcec.PubKeyBytesLenCompressed)
+	}
+
+	_, err := btcec.ParsePubKey(keyBytes)
+	if err != nil {
+		return key, fmt.Errorf("invalid %s: %v", name, err)
+	}
+
+	copy(key[:], keyBytes)
+
+	return key, nil
+}
+
 // ServerUpdate summarizes an update from the swap server.
 type ServerUpdate struct {
 	// State is the state that the server has sent us.

swap_server_client_test.go

@@ -0,0 +1,28 @@
+package loop
+
+import (
+	"testing"
+
+	looptest "github.com/lightninglabs/loop/test"
+	"github.com/stretchr/testify/require"
+)
+
+// TestParseServerPubKey ensures that parseServerPubKey accepts a valid
+// compressed public key and rejects keys with an invalid length or contents.
+func TestParseServerPubKey(t *testing.T) {
+	t.Parallel()
+
+	_, pubKey := looptest.CreateKey(1)
+	pubKeyBytes := pubKey.SerializeCompressed()
+
+	parsedKey, err := parseServerPubKey("test key", pubKeyBytes)
+	require.NoError(t, err)
+	require.Equal(t, pubKeyBytes, parsedKey[:])
+
+	_, err = parseServerPubKey("test key", pubKeyBytes[:32])
+	require.ErrorContains(t, err, "invalid test key length")
+
+	invalidKey := make([]byte, 33)
+	_, err = parseServerPubKey("test key", invalidKey)
+	require.ErrorContains(t, err, "invalid test key")
+}

testcontext_test.go

@@ -67,7 +67,7 @@ func mockMuSig2SignSweep(ctx context.Context,
 	prevoutMap map[wire.OutPoint]*wire.TxOut) (
 	[]byte, []byte, error) {
 
-	return nil, nil, nil
+	return mockMuSig2SigningData()
 }
 
 func newSwapClient(t *testing.T, config *clientConfig) *Client {