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

handshake/credential.go

@@ -14,39 +14,61 @@ type Credential struct {
 	Cert        cert.Certificate  // the certificate
 	Bytes       []byte            // pre-marshaled certificate bytes
 	privateKey  []byte            // static private key (public half lives in Cert)
+	publicKey   []byte            // static public key, the classical half for a v3 Cert
+	kemKey      noise.KEMKey      // static ML-KEM-1024 keypair, only for a v3 Cert
 	cipherSuite noise.CipherSuite // pre-built cipher suite (DH + cipher + hash)
 }
 
 // NewCredential creates a Credential with all material needed for handshake
 // participation. The cipherSuite should be pre-built by the caller with the
 // appropriate DH function, cipher, and hash.
+//
+// For a version 3 certificate privateKey is the composite private key,
+// classical || ML-KEM-1024 seed, matching the composite Cert.PublicKey().
 func NewCredential(
 	c cert.Certificate,
 	hsBytes []byte,
 	privateKey []byte,
 	cipherSuite noise.CipherSuite,
 ) *Credential {
-	return &Credential{
+	hc := &Credential{
 		Cert:        c,
 		Bytes:       hsBytes,
 		privateKey:  privateKey,
+		publicKey:   c.PublicKey(),
 		cipherSuite: cipherSuite,
 	}
+	if v2 := cert.V2Form(c); v2 != nil {
+		classical := v2.PublicKey()
+		hc.publicKey = classical
+		hc.kemKey = noise.KEMKey{
+			Private: privateKey[len(privateKey)-cert.MLKEM1024PrivateKeySize:],
+			Public:  c.PublicKey()[len(classical):],
+		}
+		hc.privateKey = privateKey[:len(privateKey)-cert.MLKEM1024PrivateKeySize]
+	}
+	return hc
 }
 
 // buildHandshakeState creates a noise.HandshakeState from this credential.
 // pskPlacement selects the noise psk modifier; 0 means none. A psk for
-// placements 2 and up is supplied later with SetPresharedKey.
-func (hc *Credential) buildHandshakeState(initiator bool, pattern noise.HandshakePattern, pskPlacement int) (*noise.HandshakeState, error) {
-	return noise.NewHandshakeState(noise.Config{
+// placements 2 and up is supplied later with SetPresharedKey. A hybrid
+// pattern also gets ML-KEM-1024 and the static ML-KEM key.
+func (hc *Credential) buildHandshakeState(initiator bool, info subtypeInfo) (*noise.HandshakeState, error) {
+	cfg := noise.Config{
 		CipherSuite:           hc.cipherSuite,
 		Random:                rand.Reader,
-		Pattern:               pattern,
+		Pattern:               info.pattern,
 		Initiator:             initiator,
-		StaticKeypair:         noise.DHKey{Private: hc.privateKey, Public: hc.Cert.PublicKey()},
+		StaticKeypair:         noise.DHKey{Private: hc.privateKey, Public: hc.publicKey},
 		PresharedKey:          []byte{},
-		PresharedKeyPlacement: pskPlacement,
-	})
+		PresharedKeyPlacement: info.pskPlacement,
+	}
+	if info.hybrid {
+		cfg.KEMFunc = KEMMLKEM1024
+		cfg.StaticKEMKeypair = hc.kemKey
+	}
+	return noise.NewHandshakeState(cfg)
 }
 
 // GetCredentialFunc returns the handshake credential for the given version,

handshake/errors.go

@@ -19,4 +19,5 @@ var (
 	ErrAsymmetricCipherKeys    = errors.New("noise produced only one cipher key")
 	ErrMultiMessageUnsupported = errors.New("multi-message handshake patterns are not yet supported by the manager")
 	ErrSubtypeMismatch         = errors.New("packet subtype does not match handshake machine subtype")
+	ErrCertVersionMismatch     = errors.New("handshake subtype does not allow this certificate version")
 )

handshake/hybrid_test.go

@@ -0,0 +1,259 @@
+package handshake
+
+import (
+	"crypto/mlkem"
+	"net/netip"
+	"testing"
+	"time"
+
+	"github.com/DefinedNet/noise"
+	"github.com/slackhq/nebula/cert"
+	ct "github.com/slackhq/nebula/cert_test"
+	"github.com/slackhq/nebula/header"
+	"github.com/slackhq/nebula/noiseutil"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func hybridCipherSuite(curve cert.Curve) noise.CipherSuite {
+	if curve == cert.Curve_P256 {
+		return noise.NewCipherSuite(noiseutil.DHP256, noise.CipherAESGCM, noise.HashSHA256)
+	}
+	return noise.NewCipherSuite(noise.DH25519, noise.CipherChaChaPoly, noise.HashSHA256)
+}
+
+// newHybridCertState builds a peer holding a v3 certificate, with credentials for it and for its v2 form, the way
+// the pki does.
+func newHybridCertState(t *testing.T, curve cert.Curve, ca cert.Certificate, caKey []byte, name string, network string) (*testCertState, []byte) {
+	t.Helper()
+	c, _, rawPrivKey, _ := ct.NewTestCert(
+		cert.Version3, curve, ca, caKey,
+		name, ca.NotBefore(), ca.NotAfter(), []netip.Prefix{netip.MustParsePrefix(network)}, nil, nil,
+	)
+	priv, _, _, err := cert.UnmarshalPrivateKeyFromPEM(rawPrivKey)
+	require.NoError(t, err)
+	return hybridCertStateFor(t, curve, c, priv), priv
+}
+
+func hybridCertStateFor(t *testing.T, curve cert.Curve, c cert.Certificate, priv []byte) *testCertState {
+	t.Helper()
+	hs3, err := c.MarshalForHandshakes()
+	require.NoError(t, err)
+	v2 := cert.V2Form(c)
+	hs2, err := v2.MarshalForHandshakes()
+	require.NoError(t, err)
+
+	ncs := hybridCipherSuite(curve)
+	return &testCertState{
+		version: cert.Version3,
+		creds: map[cert.Version]*Credential{
+			cert.Version3: NewCredential(c, hs3, priv, ncs),
+			cert.Version2: NewCredential(v2, hs2, priv[:len(priv)-cert.MLKEM1024PrivateKeySize], ncs),
+		},
+	}
+}
+
+func newHybridMachine(t *testing.T, cs *testCertState, verifier CertVerifier, initiator bool, localIndex uint32) *Machine {
+	t.Helper()
+	m, err := NewMachine(
+		cert.Version3, cs.getCredential,
+		verifier, func() (uint32, error) { return localIndex, nil },
+		initiator, header.HandshakeIXHybridMLKEM1024,
+	)
+	require.NoError(t, err)
+	return m
+}
+
+type hybridPeers struct {
+	pool               *cert.CAPool
+	ca                 cert.Certificate
+	caKey              []byte
+	initCS, respCS     *testCertState
+	initPriv, respPriv []byte
+}
+
+func newHybridPeers(t *testing.T, curve cert.Curve) hybridPeers {
+	t.Helper()
+	ca, _, caKey, _ := ct.NewTestCaCert(cert.Version3, curve, time.Now().Add(-time.Hour), time.Now().Add(time.Hour), nil, nil, nil)
+	pool := cert.NewCAPool()
+	require.NoError(t, pool.AddCA(ca))
+	initCS, initPriv := newHybridCertState(t, curve, ca, caKey, "initiator", "10.0.0.1/24")
+	respCS, respPriv := newHybridCertState(t, curve, ca, caKey, "responder", "10.0.0.2/24")
+	return hybridPeers{pool: pool, ca: ca, caKey: caKey, initCS: initCS, respCS: respCS, initPriv: initPriv, respPriv: respPriv}
+}
+
+func TestHybridIX(t *testing.T) {
+	for _, curve := range []cert.Curve{cert.Curve_CURVE25519, cert.Curve_P256} {
+		t.Run(curve.String(), func(t *testing.T) {
+			p := newHybridPeers(t, curve)
+			v := testVerifier(p.pool)
+			initM := newHybridMachine(t, p.initCS, v, true, 1000)
+			respM := newHybridMachine(t, p.respCS, v, false, 2000)
+
+			msg1, err := initM.Initiate(nil)
+			require.NoError(t, err)
+
+			// The responder answers msg1 but can not complete until msg3.
+			msg2, respResult, err := respM.ProcessPacket(nil, msg1)
+			require.NoError(t, err)
+			require.Nil(t, respResult)
+			require.NotEmpty(t, msg2)
+
+			// The initiator completes by writing msg3.
+			msg3, initResult, err := initM.ProcessPacket(nil, msg2)
+			require.NoError(t, err)
+			require.NotNil(t, initResult)
+			require.NotEmpty(t, msg3)
+
+			out, respResult, err := respM.ProcessPacket(nil, msg3)
+			require.NoError(t, err)
+			require.Nil(t, out)
+			require.NotNil(t, respResult)
+
+			t.Logf("msg1 %d B, msg2 %d B, msg3 %d B", len(msg1), len(msg2), len(msg3))
+			for i, msg := range [][]byte{msg1, msg2, msg3} {
+				h := &header.H{}
+				require.NoError(t, h.Parse(msg))
+				assert.Equal(t, header.HandshakeIXHybridMLKEM1024, h.Subtype)
+				assert.Equal(t, uint64(i+1), h.MessageCounter)
+			}
+
+			assert.Equal(t, uint32(2000), initResult.RemoteIndex)
+			assert.Equal(t, uint32(1000), respResult.RemoteIndex)
+			assert.Equal(t, cert.Version3, initResult.RemoteCert.Certificate.Version())
+			assert.Equal(t, cert.Version3, respResult.RemoteCert.Certificate.Version())
+			assert.Equal(t, "responder", initResult.RemoteCert.Certificate.Name())
+			assert.Equal(t, "initiator", respResult.RemoteCert.Certificate.Name())
+			assert.Equal(t, cert.Version3, initResult.MyCert.Version())
+			assert.Equal(t, uint64(3), initResult.MessageIndex)
+			assert.Equal(t, uint64(3), respResult.MessageIndex)
+
+			// The keys pair up in both directions.
+			for _, pair := range [][2]*Result{{initResult, respResult}, {respResult, initResult}} {
+				ct, err := pair[0].EKey.Encrypt(nil, nil, []byte("hello"))
+				require.NoError(t, err)
+				pt, err := pair[1].DKey.Decrypt(nil, nil, ct)
+				require.NoError(t, err)
+				assert.Equal(t, []byte("hello"), pt)
+			}
+		})
+	}
+}
+
+// A hybrid handshake needs v3 certificates, and a v3 certificate needs a hybrid handshake.
+func TestHybridIX_CertVersions(t *testing.T) {
+	p := newHybridPeers(t, cert.Curve_CURVE25519)
+	v := testVerifier(p.pool)
+
+	_, err := NewMachine(cert.Version2, p.initCS.getCredential, v, func() (uint32, error) { return 1, nil }, true, header.HandshakeIXHybridMLKEM1024)
+	require.ErrorIs(t, err, ErrCertVersionMismatch)
+	_, err = NewMachine(cert.Version3, p.initCS.getCredential, v, func() (uint32, error) { return 1, nil }, true, header.HandshakeIXPSK0)
+	require.ErrorIs(t, err, ErrCertVersionMismatch)
+	_, err = NewMachine(cert.Version3, p.initCS.getCredential, v, func() (uint32, error) { return 1, nil }, true, header.HandshakeIXPSK2MLKEM1024)
+	require.ErrorIs(t, err, ErrCertVersionMismatch)
+
+	// The v2 form still does a classical handshake, here with a peer that only has v2.
+	ca2 := cert.V2Form(p.ca)
+	pool2 := cert.NewCAPool()
+	require.NoError(t, pool2.AddCA(ca2))
+	classicalCAKey := p.caKey[:len(p.caKey)-cert.MLDSA87PrivateKeySize]
+	v2Peer := newTestCertState(t, ca2, classicalCAKey, "v2peer", []netip.Prefix{netip.MustParsePrefix("10.0.0.3/24")})
+	p.initCS.version = cert.Version2
+	initResult, respResult := doFullHandshake(t, p.initCS, v2Peer, pool2)
+	assert.Equal(t, cert.Version2, initResult.RemoteCert.Certificate.Version())
+	assert.Equal(t, cert.Version2, respResult.RemoteCert.Certificate.Version())
+	assert.Equal(t, cert.Version2, initResult.MyCert.Version())
+}
+
+// A responder that only trusts the v2 form of the CA can not verify a v3 certificate.
+func TestHybridIX_V2TrustOnly(t *testing.T) {
+	p := newHybridPeers(t, cert.Curve_CURVE25519)
+	pool2 := cert.NewCAPool()
+	require.NoError(t, pool2.AddCA(cert.V2Form(p.ca)))
+
+	initM := newHybridMachine(t, p.initCS, testVerifier(p.pool), true, 1000)
+	respM := newHybridMachine(t, p.respCS, testVerifier(pool2), false, 2000)
+	msg1, err := initM.Initiate(nil)
+	require.NoError(t, err)
+	_, _, err = respM.ProcessPacket(nil, msg1)
+	require.ErrorIs(t, err, cert.ErrCaNotFound)
+}
+
+// A corrupted msg3 is dropped without killing the responder, and the real one still completes.
+func TestHybridIX_CorruptMsg3(t *testing.T) {
+	p := newHybridPeers(t, cert.Curve_CURVE25519)
+	v := testVerifier(p.pool)
+	initM := newHybridMachine(t, p.initCS, v, true, 1000)
+	respM := newHybridMachine(t, p.respCS, v, false, 2000)
+
+	msg1, err := initM.Initiate(nil)
+	require.NoError(t, err)
+	msg2, _, err := respM.ProcessPacket(nil, msg1)
+	require.NoError(t, err)
+	msg3, _, err := initM.ProcessPacket(nil, msg2)
+	require.NoError(t, err)
+
+	bad := append([]byte(nil), msg3...)
+	bad[header.Len+10] ^= 1
+	_, res, err := respM.ProcessPacket(nil, bad)
+	require.Error(t, err)
+	require.Nil(t, res)
+	require.False(t, respM.Failed())
+
+	_, res, err = respM.ProcessPacket(nil, msg3)
+	require.NoError(t, err)
+	require.NotNil(t, res)
+}
+
+// withKEMKey returns a peer holding c and its classical private key, but some other ML-KEM key: someone who broke
+// the classical key, say with a quantum computer, and wants to be that peer.
+func withKEMKey(t *testing.T, curve cert.Curve, c cert.Certificate, priv []byte) *testCertState {
+	t.Helper()
+	dk, err := mlkem.GenerateKey1024()
+	require.NoError(t, err)
+	forged := append(append([]byte(nil), priv[:len(priv)-cert.MLKEM1024PrivateKeySize]...), dk.Bytes()...)
+	return hybridCertStateFor(t, curve, c, forged)
+}
+
+// The classical private key alone does not get anyone through as the initiator: msg2 is unreadable without the
+// initiator's ML-KEM key.
+func TestHybridIX_ImpersonateInitiator(t *testing.T) {
+	p := newHybridPeers(t, cert.Curve_CURVE25519)
+	v := testVerifier(p.pool)
+	victim := p.initCS.creds[cert.Version3].Cert
+	attacker := withKEMKey(t, cert.Curve_CURVE25519, victim, p.initPriv)
+
+	initM := newHybridMachine(t, attacker, v, true, 1000)
+	respM := newHybridMachine(t, p.respCS, v, false, 2000)
+	msg1, err := initM.Initiate(nil)
+	require.NoError(t, err)
+	msg2, _, err := respM.ProcessPacket(nil, msg1)
+	require.NoError(t, err)
+
+	_, res, err := initM.ProcessPacket(nil, msg2)
+	require.Error(t, err)
+	require.Nil(t, res)
+}
+
+// The classical private key alone does not get anyone through as the responder either: the initiator completes,
+// but msg3 is unreadable without the responder's ML-KEM key, so no packet under the new keys can ever come back.
+func TestHybridIX_ImpersonateResponder(t *testing.T) {
+	p := newHybridPeers(t, cert.Curve_CURVE25519)
+	v := testVerifier(p.pool)
+	victim := p.respCS.creds[cert.Version3].Cert
+	attacker := withKEMKey(t, cert.Curve_CURVE25519, victim, p.respPriv)
+
+	initM := newHybridMachine(t, p.initCS, v, true, 1000)
+	respM := newHybridMachine(t, attacker, v, false, 2000)
+	msg1, err := initM.Initiate(nil)
+	require.NoError(t, err)
+	msg2, _, err := respM.ProcessPacket(nil, msg1)
+	require.NoError(t, err)
+	msg3, initResult, err := initM.ProcessPacket(nil, msg2)
+	require.NoError(t, err)
+	require.NotNil(t, initResult)
+
+	_, res, err := respM.ProcessPacket(nil, msg3)
+	require.Error(t, err)
+	require.Nil(t, res)
+}

handshake/kem.go

@@ -6,6 +6,9 @@ import (
 	"crypto/sha256"
 	"errors"
 	"fmt"
+	"io"
+
+	"github.com/DefinedNet/noise"
 )
 
 // kemPSKSalt separates the psk derivation from any other use of the secret.
@@ -66,3 +69,39 @@ func (m *Machine) kemDecapsulate(msg []byte) ([]byte, error) {
 	}
 	return msg[mlkem.CiphertextSize1024:], nil
 }
+
+// KEMMLKEM1024 is ML-KEM-1024 for the noise library's hybrid patterns. Private
+// keys are the 64 byte seed, the same form v3 private keys store.
+var KEMMLKEM1024 noise.KEMFunc = kemMLKEM1024{}
+
+type kemMLKEM1024 struct{}
+
+// GenerateKeypair ignores random, crypto/mlkem always draws from crypto/rand.
+func (kemMLKEM1024) GenerateKeypair(_ io.Reader) (noise.KEMKey, error) {
+	dk, err := mlkem.GenerateKey1024()
+	if err != nil {
+		return noise.KEMKey{}, fmt.Errorf("mlkem1024: GenerateKey: %w", err)
+	}
+	return noise.KEMKey{Private: dk.Bytes(), Public: dk.EncapsulationKey().Bytes()}, nil
+}
+
+func (kemMLKEM1024) Encapsulate(pubkey []byte) ([]byte, []byte, error) {
+	ek, err := mlkem.NewEncapsulationKey1024(pubkey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("%w: %w", ErrInvalidKEMKey, err)
+	}
+	ss, ct := ek.Encapsulate()
+	return ct, ss, nil
+}
+
+func (kemMLKEM1024) Decapsulate(privkey, ciphertext []byte) ([]byte, error) {
+	dk, err := mlkem.NewDecapsulationKey1024(privkey)
+	if err != nil {
+		return nil, fmt.Errorf("mlkem1024: NewDecapsulationKey: %w", err)
+	}
+	return dk.Decapsulate(ciphertext)
+}
+
+func (kemMLKEM1024) PublicKeyLen() int  { return mlkem.EncapsulationKeySize1024 }
+func (kemMLKEM1024) CiphertextLen() int { return mlkem.CiphertextSize1024 }
+func (kemMLKEM1024) KEMName() string    { return "MLKEM1024" }

handshake/machine.go

@@ -65,6 +65,7 @@ type Machine struct {
 	msgs           []msgFlags
 	myVersion      cert.Version
 	subtype        header.MessageSubType
+	hybrid         bool
 	indexAllocated bool
 	remoteCertSet  bool
 	payloadSet     bool
@@ -91,12 +92,18 @@ func NewMachine(
 		return nil, err
 	}
 
+	// Version 3 certificates go with hybrid patterns and nothing else, the
+	// classical and phase 1 subtypes use the v2 form instead.
+	if info.hybrid != (version == cert.Version3) {
+		return nil, fmt.Errorf("%w: %v", ErrCertVersionMismatch, version)
+	}
+
 	cred := getCred(version)
 	if cred == nil {
 		return nil, fmt.Errorf("%w: %v", ErrNoCredential, version)
 	}
 
-	hs, err := cred.buildHandshakeState(initiator, info.pattern, info.pskPlacement)
+	hs, err := cred.buildHandshakeState(initiator, info)
 	if err != nil {
 		return nil, fmt.Errorf("build noise state: %w", err)
 	}
@@ -105,6 +112,7 @@ func NewMachine(
 		hs:         hs,
 		subtype:    subtype,
 		msgs:       info.msgs,
+		hybrid:     info.hybrid,
 		getCred:    getCred,
 		allocIndex: allocIndex,
 		verifier:   verifier,
@@ -226,11 +234,6 @@ func (m *Machine) ProcessPacket(out, packet []byte) ([]byte, *Result, error) {
 		return nil, nil, ErrInitiateNotCalled
 	}
 
-	// The (eKey, dKey) ordering here is correct for IX, where the initiator
-	// completes the handshake by reading the responder's stage-2 message.
-	// noise returns (cs1, cs2) where cs1 is the initiator->responder cipher.
-	// For 3-message patterns where a responder finishes by reading the final
-	// message, this ordering would be wrong; revisit when XX/pqIX lands.
 	noiseMsg := packet[header.Len:]
 	if m.myMsgFlags().kemCiphertext {
 		var err error
@@ -242,7 +245,8 @@ func (m *Machine) ProcessPacket(out, packet []byte) ([]byte, *Result, error) {
 		}
 	}
 
-	msg, eKey, dKey, err := m.hs.ReadMessage(nil, noiseMsg)
+	msg, cs1, cs2, err := m.hs.ReadMessage(nil, noiseMsg)
+	eKey, dKey := m.cipherStates(cs1, cs2)
 	if err != nil {
 		// Noise ReadMessage failed. The noise library checkpoints and rolls back
 		// on failure, so the Machine is still alive. The caller can retry with
@@ -272,7 +276,7 @@ func (m *Machine) ProcessPacket(out, packet []byte) ([]byte, *Result, error) {
 	}
 
 	// ReadMessage didn't complete, produce the next outgoing message
-	out, dk, ek, err := m.buildResponse(out)
+	out, ek, dk, err := m.buildResponse(out)
 	if err != nil {
 		m.failed = true
 		return nil, nil, err
@@ -292,6 +296,15 @@ func (m *Machine) ProcessPacket(out, packet []byte) ([]byte, *Result, error) {
 	return out, nil, nil
 }
 
+// cipherStates orders the cipher states noise returns, (cs1, cs2) with cs1 the
+// initiator to responder direction, into this side's (eKey, dKey).
+func (m *Machine) cipherStates(cs1, cs2 *noise.CipherState) (*noise.CipherState, *noise.CipherState) {
+	if m.result.Initiator {
+		return cs1, cs2
+	}
+	return cs2, cs1
+}
+
 func (m *Machine) completed(eKey, dKey *noise.CipherState) *Result {
 	m.kemKey = nil
 	m.kemCiphertext = nil
@@ -372,6 +385,14 @@ func (m *Machine) processPayload(msg []byte, flags msgFlags) error {
 }
 
 func (m *Machine) validateCert(payload Payload) error {
+	// A v3 certificate only ever comes with a hybrid pattern, and a hybrid
+	// pattern only ever with one. The composite static key would not recombine
+	// with anything else anyway, but say so plainly.
+	if m.hybrid != (cert.Version(payload.CertVersion) == cert.Version3) {
+		m.failed = true
+		return fmt.Errorf("%w: %v", ErrCertVersionMismatch, payload.CertVersion)
+	}
+
 	cred := m.getCred(m.myVersion)
 	if cred == nil {
 		m.failed = true
@@ -487,17 +508,11 @@ func (m *Machine) buildResponse(out []byte) ([]byte, *noise.CipherState, *noise.
 
 	// noise.WriteMessage appends the encrypted handshake message to out,
 	// reusing capacity when present.
-	//
-	// The (dKey, eKey) ordering here is correct for IX, where the responder
-	// completes the handshake by writing the stage-2 message. noise returns
-	// (cs1, cs2) where cs1 is the initiator->responder cipher (which is the
-	// responder's decrypt key). For 3-message patterns where an initiator
-	// finishes by writing the final message, this ordering would be wrong;
-	// revisit when XX/pqIX lands.
-	out, dKey, eKey, err := m.hs.WriteMessage(out, hsBytes)
+	out, cs1, cs2, err := m.hs.WriteMessage(out, hsBytes)
 	if err != nil {
 		return nil, nil, nil, fmt.Errorf("noise WriteMessage: %w", err)
 	}
 
-	return out, dKey, eKey, nil
+	eKey, dKey := m.cipherStates(cs1, cs2)
+	return out, eKey, dKey, nil
 }

handshake/patterns.go

@@ -21,6 +21,10 @@ type subtypeInfo struct {
 	pattern      noise.HandshakePattern
 	pskPlacement int // noise psk modifier position, 0 means no psk token
 	msgs         []msgFlags
+
+	// hybrid patterns carry composite static keys and need version 3
+	// certificates on both sides; every other pattern refuses them.
+	hybrid bool
 }
 
 // subtypeInfos defines the noise pattern and message content layout for each
@@ -51,6 +55,29 @@ var subtypeInfos = map[header.MessageSubType]subtypeInfo{
 		},
 	},
 
+	// Hybrid IX (github.com/DefinedNet/noise, see its PQ_RESEARCH.md):
+	//   msg1 (I->R): e, s                          payload + cert
+	//   msg2 (R->I): ekem, skem, e, ee, se, s, es  payload + cert
+	//   msg3 (I->R): skem
+	// e and s are composite, classical || ML-KEM-1024. The static ML-KEM key is
+	// the post-quantum half of the v3 certificate's public key, so it travels in
+	// s and the certificate leaves it out. The responder authenticates the
+	// initiator when msg3 decrypts, and the initiator the responder when the
+	// first packet under the new keys does.
+	//
+	// TODO: the responder's e also carries an ephemeral ML-KEM key that no token
+	// ever encapsulates to, 1568 bytes of msg2 and a key generation for nothing.
+	// Dropping it needs a change to the noise library.
+	header.HandshakeIXHybridMLKEM1024: {
+		pattern: noise.HandshakeHybridIX,
+		hybrid:  true,
+		msgs: []msgFlags{
+			{expectsPayload: true, expectsCert: true},
+			{expectsPayload: true, expectsCert: true},
+			{},
+		},
+	},
+
 	// XX: 3 messages
 	//   msg1 (I->R): payload only
 	//   msg2 (R->I): payload + cert

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:

header/header_test.go

@@ -96,8 +96,9 @@ func TestTypeMap(t *testing.T) {
 		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,
 	}, subTypeMap)