key_schedule.go

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package tls

import (
	"crypto/ecdh"
	"crypto/hmac"
	"errors"
	"hash"
	"io"

	"github.com/metacubex/mlkem"
)

// This file contains the functions necessary to compute the TLS 1.3 key
// schedule. See RFC 8446, Section 7.

// nextTrafficSecret generates the next traffic secret, given the current one,
// according to RFC 8446, Section 7.2.
func (c *cipherSuiteTLS13) nextTrafficSecret(trafficSecret []byte) []byte {
	return tls13ExpandLabel(c.hash.New, trafficSecret, "traffic upd", nil, c.hash.Size())
}

// trafficKey generates traffic keys according to RFC 8446, Section 7.3.
func (c *cipherSuiteTLS13) trafficKey(trafficSecret []byte) (key, iv []byte) {
	key = tls13ExpandLabel(c.hash.New, trafficSecret, "key", nil, c.keyLen)
	iv = tls13ExpandLabel(c.hash.New, trafficSecret, "iv", nil, aeadNonceLength)
	return
}

// finishedHash generates the Finished verify_data or PskBinderEntry according
// to RFC 8446, Section 4.4.4. See sections 4.4 and 4.2.11.2 for the baseKey
// selection.
func (c *cipherSuiteTLS13) finishedHash(baseKey []byte, transcript hash.Hash) []byte {
	finishedKey := tls13ExpandLabel(c.hash.New, baseKey, "finished", nil, c.hash.Size())
	verifyData := hmac.New(c.hash.New, finishedKey)
	verifyData.Write(transcript.Sum(nil))
	return verifyData.Sum(nil)
}

// exportKeyingMaterial implements RFC5705 exporters for TLS 1.3 according to
// RFC 8446, Section 7.5.
func (c *cipherSuiteTLS13) exportKeyingMaterial(s *tls13MasterSecret, transcript hash.Hash) func(string, []byte, int) ([]byte, error) {
	expMasterSecret := s.ExporterMasterSecret(transcript)
	return func(label string, context []byte, length int) ([]byte, error) {
		return expMasterSecret.Exporter(label, context, length), nil
	}
}

type keySharePrivateKeys struct {
	ecdhe *ecdh.PrivateKey
	mlkem mlkem.Decapsulator
}

// A keyExchange implements a TLS 1.3 KEM.
type keyExchange interface {
	// keyShares generates one or two key shares.
	//
	// The first one will match the id, the second (if present) reuses the
	// traditional component of the requested hybrid, as allowed by
	// draft-ietf-tls-hybrid-design-09, Section 3.2.
	keyShares(rand io.Reader) (*keySharePrivateKeys, []keyShare, error)

	// serverSharedSecret computes the shared secret and the server's key share.
	serverSharedSecret(rand io.Reader, clientKeyShare []byte) ([]byte, keyShare, error)

	// clientSharedSecret computes the shared secret given the server's key
	// share and the keys generated by keyShares.
	clientSharedSecret(priv *keySharePrivateKeys, serverKeyShare []byte) ([]byte, error)
}

func keyExchangeForCurveID(id CurveID) (keyExchange, error) {
	newMLKEMPrivateKey768 := func(b []byte) (mlkem.Decapsulator, error) {
		return mlkem.NewDecapsulationKey768(b)
	}
	newMLKEMPrivateKey1024 := func(b []byte) (mlkem.Decapsulator, error) {
		return mlkem.NewDecapsulationKey1024(b)
	}
	newMLKEMPublicKey768 := func(b []byte) (mlkem.Encapsulator, error) {
		return mlkem.NewEncapsulationKey768(b)
	}
	newMLKEMPublicKey1024 := func(b []byte) (mlkem.Encapsulator, error) {
		return mlkem.NewEncapsulationKey1024(b)
	}
	switch id {
	case X25519:
		return &ecdhKeyExchange{id, ecdh.X25519()}, nil
	case CurveP256:
		return &ecdhKeyExchange{id, ecdh.P256()}, nil
	case CurveP384:
		return &ecdhKeyExchange{id, ecdh.P384()}, nil
	case CurveP521:
		return &ecdhKeyExchange{id, ecdh.P521()}, nil
	case X25519MLKEM768:
		return &hybridKeyExchange{id, ecdhKeyExchange{X25519, ecdh.X25519()},
			32, mlkem.EncapsulationKeySize768, mlkem.CiphertextSize768,
			newMLKEMPrivateKey768, newMLKEMPublicKey768}, nil
	case SecP256r1MLKEM768:
		return &hybridKeyExchange{id, ecdhKeyExchange{CurveP256, ecdh.P256()},
			65, mlkem.EncapsulationKeySize768, mlkem.CiphertextSize768,
			newMLKEMPrivateKey768, newMLKEMPublicKey768}, nil
	case SecP384r1MLKEM1024:
		return &hybridKeyExchange{id, ecdhKeyExchange{CurveP384, ecdh.P384()},
			97, mlkem.EncapsulationKeySize1024, mlkem.CiphertextSize1024,
			newMLKEMPrivateKey1024, newMLKEMPublicKey1024}, nil
	default:
		return nil, errors.New("tls: unsupported key exchange")
	}
}

type ecdhKeyExchange struct {
	id    CurveID
	curve ecdh.Curve
}

func (ke *ecdhKeyExchange) keyShares(rand io.Reader) (*keySharePrivateKeys, []keyShare, error) {
	priv, err := ke.curve.GenerateKey(rand)
	if err != nil {
		return nil, nil, err
	}
	return &keySharePrivateKeys{ecdhe: priv}, []keyShare{{ke.id, priv.PublicKey().Bytes()}}, nil
}

func (ke *ecdhKeyExchange) serverSharedSecret(rand io.Reader, clientKeyShare []byte) ([]byte, keyShare, error) {
	key, err := ke.curve.GenerateKey(rand)
	if err != nil {
		return nil, keyShare{}, err
	}
	peerKey, err := ke.curve.NewPublicKey(clientKeyShare)
	if err != nil {
		return nil, keyShare{}, err
	}
	sharedKey, err := key.ECDH(peerKey)
	if err != nil {
		return nil, keyShare{}, err
	}
	return sharedKey, keyShare{ke.id, key.PublicKey().Bytes()}, nil
}

func (ke *ecdhKeyExchange) clientSharedSecret(priv *keySharePrivateKeys, serverKeyShare []byte) ([]byte, error) {
	peerKey, err := ke.curve.NewPublicKey(serverKeyShare)
	if err != nil {
		return nil, err
	}
	sharedKey, err := priv.ecdhe.ECDH(peerKey)
	if err != nil {
		return nil, err
	}
	return sharedKey, nil
}

type hybridKeyExchange struct {
	id   CurveID
	ecdh ecdhKeyExchange

	ecdhElementSize     int
	mlkemPublicKeySize  int
	mlkemCiphertextSize int

	newMLKEMPrivateKey func([]byte) (mlkem.Decapsulator, error)
	newMLKEMPublicKey  func([]byte) (mlkem.Encapsulator, error)
}

func (ke *hybridKeyExchange) keyShares(rand io.Reader) (*keySharePrivateKeys, []keyShare, error) {
	priv, ecdhShares, err := ke.ecdh.keyShares(rand)
	if err != nil {
		return nil, nil, err
	}
	seed := make([]byte, mlkem.SeedSize)
	if _, err := io.ReadFull(rand, seed); err != nil {
		return nil, nil, err
	}
	priv.mlkem, err = ke.newMLKEMPrivateKey(seed)
	if err != nil {
		return nil, nil, err
	}
	var shareData []byte
	// For X25519MLKEM768, the ML-KEM-768 encapsulation key comes first.
	// For SecP256r1MLKEM768 and SecP384r1MLKEM1024, the ECDH share comes first.
	// See draft-ietf-tls-ecdhe-mlkem-02, Section 4.1.
	if ke.id == X25519MLKEM768 {
		shareData = append(priv.mlkem.Encapsulator().Bytes(), ecdhShares[0].data...)
	} else {
		shareData = append(ecdhShares[0].data, priv.mlkem.Encapsulator().Bytes()...)
	}
	return priv, []keyShare{{ke.id, shareData}, ecdhShares[0]}, nil
}

func (ke *hybridKeyExchange) serverSharedSecret(rand io.Reader, clientKeyShare []byte) ([]byte, keyShare, error) {
	if len(clientKeyShare) != ke.ecdhElementSize+ke.mlkemPublicKeySize {
		return nil, keyShare{}, errors.New("tls: invalid client key share length for hybrid key exchange")
	}
	var ecdhShareData, mlkemShareData []byte
	if ke.id == X25519MLKEM768 {
		mlkemShareData = clientKeyShare[:ke.mlkemPublicKeySize]
		ecdhShareData = clientKeyShare[ke.mlkemPublicKeySize:]
	} else {
		ecdhShareData = clientKeyShare[:ke.ecdhElementSize]
		mlkemShareData = clientKeyShare[ke.ecdhElementSize:]
	}
	ecdhSharedSecret, ks, err := ke.ecdh.serverSharedSecret(rand, ecdhShareData)
	if err != nil {
		return nil, keyShare{}, err
	}
	mlkemPeerKey, err := ke.newMLKEMPublicKey(mlkemShareData)
	if err != nil {
		return nil, keyShare{}, err
	}
	mlkemSharedSecret, mlkemKeyShare := mlkemPeerKey.Encapsulate()
	var sharedKey []byte
	if ke.id == X25519MLKEM768 {
		sharedKey = append(mlkemSharedSecret, ecdhSharedSecret...)
		ks.data = append(mlkemKeyShare, ks.data...)
	} else {
		sharedKey = append(ecdhSharedSecret, mlkemSharedSecret...)
		ks.data = append(ks.data, mlkemKeyShare...)
	}
	ks.group = ke.id
	return sharedKey, ks, nil
}

func (ke *hybridKeyExchange) clientSharedSecret(priv *keySharePrivateKeys, serverKeyShare []byte) ([]byte, error) {
	if len(serverKeyShare) != ke.ecdhElementSize+ke.mlkemCiphertextSize {
		return nil, errors.New("tls: invalid server key share length for hybrid key exchange")
	}
	var ecdhShareData, mlkemShareData []byte
	if ke.id == X25519MLKEM768 {
		mlkemShareData = serverKeyShare[:ke.mlkemCiphertextSize]
		ecdhShareData = serverKeyShare[ke.mlkemCiphertextSize:]
	} else {
		ecdhShareData = serverKeyShare[:ke.ecdhElementSize]
		mlkemShareData = serverKeyShare[ke.ecdhElementSize:]
	}
	ecdhSharedSecret, err := ke.ecdh.clientSharedSecret(priv, ecdhShareData)
	if err != nil {
		return nil, err
	}
	mlkemSharedSecret, err := priv.mlkem.Decapsulate(mlkemShareData)
	if err != nil {
		return nil, err
	}
	var sharedKey []byte
	if ke.id == X25519MLKEM768 {
		sharedKey = append(mlkemSharedSecret, ecdhSharedSecret...)
	} else {
		sharedKey = append(ecdhSharedSecret, mlkemSharedSecret...)
	}
	return sharedKey, nil
}