fix: reject malformed network input in parsers (#1340)

## Summary
- validate handshake metadata field lengths before fixed-size reads
- reject truncated multicast advertisements before slicing the hash
payload
- add regression tests for malformed and truncated input

## Why
Both parsers currently trust length information from the incoming
payload a bit too much. Malformed network input can reach fixed-size
reads/slices and panic the process instead of being rejected cleanly.

## Testing
- go test ./...

src/multicast/advertisement.go

@@ -26,14 +26,18 @@ func (m *multicastAdvertisement) MarshalBinary() ([]byte, error) {
 }
 
 func (m *multicastAdvertisement) UnmarshalBinary(b []byte) error {
-	if len(b) < ed25519.PublicKeySize+8 {
+	const headerLen = ed25519.PublicKeySize + 8
+	if len(b) < headerLen {
 		return fmt.Errorf("invalid multicast beacon")
 	}
 	m.MajorVersion = binary.BigEndian.Uint16(b[0:2])
 	m.MinorVersion = binary.BigEndian.Uint16(b[2:4])
 	m.PublicKey = append(m.PublicKey[:0], b[4:4+ed25519.PublicKeySize]...)
 	m.Port = binary.BigEndian.Uint16(b[4+ed25519.PublicKeySize : 6+ed25519.PublicKeySize])
-	dl := binary.BigEndian.Uint16(b[6+ed25519.PublicKeySize : 8+ed25519.PublicKeySize])
-	m.Hash = append(m.Hash[:0], b[8+ed25519.PublicKeySize:8+ed25519.PublicKeySize+dl]...)
+	dl := int(binary.BigEndian.Uint16(b[6+ed25519.PublicKeySize : 8+ed25519.PublicKeySize]))
+	if len(b) < headerLen+dl {
+		return fmt.Errorf("invalid multicast beacon")
+	}
+	m.Hash = append(m.Hash[:0], b[headerLen:headerLen+dl]...)
 	return nil
 }

src/multicast/advertisement_test.go

@@ -2,6 +2,7 @@ package multicast
 
 import (
 	"crypto/ed25519"
+	"encoding/binary"
 	"reflect"
 	"testing"
 )
@@ -36,3 +37,20 @@ func TestMulticastAdvertisementRoundTrip(t *testing.T) {
 		t.Fatalf("differences found after round-trip")
 	}
 }
+
+func TestMulticastAdvertisementRejectsTruncatedHash(t *testing.T) {
+	pk, _, err := ed25519.GenerateKey(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b := make([]byte, ed25519.PublicKeySize+8)
+	copy(b[4:], pk)
+	binary.BigEndian.PutUint16(b[4+ed25519.PublicKeySize:6+ed25519.PublicKeySize], 9001)
+	binary.BigEndian.PutUint16(b[6+ed25519.PublicKeySize:8+ed25519.PublicKeySize], 32)
+
+	var adv multicastAdvertisement
+	if err := adv.UnmarshalBinary(b); err == nil {
+		t.Fatal("expected truncated beacon to be rejected")
+	}
+}