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.

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")
+}