handshake: Add ix_hybrid_mlkem1024, DefinedNet/noise hybrid IX with ML-KEM-1024 over version 3 certificates

header/header.go

@@ -72,12 +72,17 @@ const (
 	// in through the psk2 slot. Its messages exceed the MTU and travel as
 	// fragments (see handshake/fragment).
 	HandshakeIXPSK2MLKEM1024 MessageSubType = 2
+
+	// HandshakeIXHybridMLKEM1024 is the hybrid IX of github.com/DefinedNet/noise:
+	// IX with X25519 or P256 and ML-KEM-1024 side by side, authenticated by
+	// version 3 certificates. It takes three messages and travels as fragments.
+	HandshakeIXHybridMLKEM1024 MessageSubType = 3
 )
 
 // IsFragmented reports whether handshake messages of this subtype travel as
 // fragments.
 func IsFragmented(st MessageSubType) bool {
-	return st == HandshakeIXPSK2MLKEM1024
+	return st == HandshakeIXPSK2MLKEM1024 || st == HandshakeIXHybridMLKEM1024
 }
 
 var ErrHeaderTooShort = errors.New("header is too short")
@@ -99,8 +104,9 @@ var subTypeMap = map[MessageType]*map[MessageSubType]string{
 	Test:        &subTypeTestMap,
 	CloseTunnel: &subTypeNoneMap,
 	Handshake: {
-		HandshakeIXPSK0:          "ix_psk0",
-		HandshakeIXPSK2MLKEM1024: "ix_psk2_mlkem1024",
+		HandshakeIXPSK0:            "ix_psk0",
+		HandshakeIXPSK2MLKEM1024:   "ix_psk2_mlkem1024",
+		HandshakeIXHybridMLKEM1024: "ix_hybrid_mlkem1024",
 	},
 	Control: &subTypeNoneMap,
 }
@@ -211,7 +217,7 @@ func IsValidSubType(t MessageType, s MessageSubType) bool {
 	case Message:
 		return s == MessageNone || s == MessageRelay
 	case Handshake:
-		return s == HandshakeIXPSK0 || s == HandshakeIXPSK2MLKEM1024
+		return s == HandshakeIXPSK0 || s == HandshakeIXPSK2MLKEM1024 || s == HandshakeIXHybridMLKEM1024
 	case Test:
 		return s == TestReply || s == TestRequest
 	case Control, CloseTunnel, RecvError, LightHouse: