Use plain X{25519,448} for PQ hybrids instead of HPKE

kem/hybrid/hybrid.go

@@ -33,7 +33,6 @@ package hybrid
 import (
 	"errors"
 
-	"github.com/cloudflare/circl/hpke"
 	"github.com/cloudflare/circl/internal/sha3"
 	"github.com/cloudflare/circl/kem"
 	"github.com/cloudflare/circl/kem/kyber/kyber1024"
@@ -58,25 +57,25 @@ func Kyber1024X448() kem.Scheme { return kyber1024X }
 var kyber512X kem.Scheme = &scheme{
 	"Kyber512-X25519",
 	kyber512.Scheme(),
-	hpke.KEM_X25519_HKDF_SHA256.Scheme(),
+	x25519Kem,
 }
 
 var kyber768X kem.Scheme = &scheme{
 	"Kyber768-X25519",
 	kyber768.Scheme(),
-	hpke.KEM_X25519_HKDF_SHA256.Scheme(),
+	x25519Kem,
 }
 
 var kyber768X4 kem.Scheme = &scheme{
 	"Kyber768-X448",
 	kyber768.Scheme(),
-	hpke.KEM_X448_HKDF_SHA512.Scheme(),
+	x448Kem,
 }
 
 var kyber1024X kem.Scheme = &scheme{
 	"Kyber1024-X448",
 	kyber1024.Scheme(),
-	hpke.KEM_X448_HKDF_SHA512.Scheme(),
+	x448Kem,
 }
 
 // Public key of a hybrid KEM.

kem/hybrid/xkem.go

@@ -0,0 +1,208 @@
+package hybrid
+
+import (
+	"bytes"
+	cryptoRand "crypto/rand"
+	"crypto/subtle"
+
+	"github.com/cloudflare/circl/dh/x25519"
+	"github.com/cloudflare/circl/dh/x448"
+	"github.com/cloudflare/circl/internal/sha3"
+	"github.com/cloudflare/circl/kem"
+)
+
+type xPublicKey struct {
+	scheme *xScheme
+	key    []byte
+}
+type xPrivateKey struct {
+	scheme *xScheme
+	key    []byte
+}
+type xScheme struct {
+	size int
+}
+
+var (
+	x25519Kem = &xScheme{x25519.Size}
+	x448Kem   = &xScheme{x448.Size}
+)
+
+func (sch *xScheme) Name() string {
+	switch sch.size {
+	case x25519.Size:
+		return "X25519"
+	case x448.Size:
+		return "X448"
+	}
+	panic(kem.ErrTypeMismatch)
+}
+
+func (sch *xScheme) PublicKeySize() int         { return sch.size }
+func (sch *xScheme) PrivateKeySize() int        { return sch.size }
+func (sch *xScheme) SeedSize() int              { return sch.size }
+func (sch *xScheme) SharedKeySize() int         { return sch.size }
+func (sch *xScheme) CiphertextSize() int        { return sch.size }
+func (sch *xScheme) EncapsulationSeedSize() int { return sch.size }
+
+func (sk *xPrivateKey) Scheme() kem.Scheme { return sk.scheme }
+func (pk *xPublicKey) Scheme() kem.Scheme  { return pk.scheme }
+
+func (sk *xPrivateKey) MarshalBinary() ([]byte, error) {
+	ret := make([]byte, len(sk.key))
+	copy(ret, sk.key)
+	return ret, nil
+}
+
+func (sk *xPrivateKey) Equal(other kem.PrivateKey) bool {
+	oth, ok := other.(*xPrivateKey)
+	if !ok {
+		return false
+	}
+	if oth.scheme != sk.scheme {
+		return false
+	}
+	return subtle.ConstantTimeCompare(oth.key, sk.key) == 1
+}
+
+func (sk *xPrivateKey) Public() kem.PublicKey {
+	pk := xPublicKey{sk.scheme, make([]byte, sk.scheme.size)}
+	switch sk.scheme.size {
+	case x25519.Size:
+		var sk2, pk2 x25519.Key
+		copy(sk2[:], sk.key)
+		x25519.KeyGen(&pk2, &sk2)
+		copy(pk.key, pk2[:])
+	case x448.Size:
+		var sk2, pk2 x448.Key
+		copy(sk2[:], sk.key)
+		x448.KeyGen(&pk2, &sk2)
+		copy(pk.key, pk2[:])
+	}
+	return &pk
+}
+
+func (pk *xPublicKey) Equal(other kem.PublicKey) bool {
+	oth, ok := other.(*xPublicKey)
+	if !ok {
+		return false
+	}
+	if oth.scheme != pk.scheme {
+		return false
+	}
+	return bytes.Equal(oth.key, pk.key)
+}
+
+func (pk *xPublicKey) MarshalBinary() ([]byte, error) {
+	ret := make([]byte, pk.scheme.size)
+	copy(ret, pk.key)
+	return ret, nil
+}
+
+func (sch *xScheme) GenerateKeyPair() (kem.PublicKey, kem.PrivateKey, error) {
+	seed := make([]byte, sch.SeedSize())
+	_, err := cryptoRand.Read(seed)
+	if err != nil {
+		return nil, nil, err
+	}
+	pk, sk := sch.DeriveKeyPair(seed)
+	return pk, sk, nil
+}
+
+func (sch *xScheme) DeriveKeyPair(seed []byte) (kem.PublicKey, kem.PrivateKey) {
+	if len(seed) != sch.SeedSize() {
+		panic(kem.ErrSeedSize)
+	}
+	sk := xPrivateKey{scheme: sch, key: make([]byte, sch.size)}
+
+	h := sha3.NewShake256()
+	_, _ = h.Write(seed)
+	_, _ = h.Read(sk.key)
+
+	return sk.Public(), &sk
+}
+
+func (sch *xScheme) Encapsulate(pk kem.PublicKey) (ct, ss []byte, err error) {
+	seed := make([]byte, sch.EncapsulationSeedSize())
+	_, err = cryptoRand.Read(seed)
+	if err != nil {
+		return
+	}
+	return sch.EncapsulateDeterministically(pk, seed)
+}
+
+func (pk *xPublicKey) X(sk *xPrivateKey) []byte {
+	if pk.scheme != sk.scheme {
+		panic(kem.ErrTypeMismatch)
+	}
+
+	switch pk.scheme.size {
+	case x25519.Size:
+		var ss2, pk2, sk2 x25519.Key
+		copy(pk2[:], pk.key)
+		copy(sk2[:], sk.key)
+		x25519.Shared(&ss2, &sk2, &pk2)
+		return ss2[:]
+	case x448.Size:
+		var ss2, pk2, sk2 x448.Key
+		copy(pk2[:], pk.key)
+		copy(sk2[:], sk.key)
+		x448.Shared(&ss2, &sk2, &pk2)
+		return ss2[:]
+	}
+	panic(kem.ErrTypeMismatch)
+}
+
+func (sch *xScheme) EncapsulateDeterministically(
+	pk kem.PublicKey, seed []byte,
+) (ct, ss []byte, err error) {
+	if len(seed) != sch.EncapsulationSeedSize() {
+		return nil, nil, kem.ErrSeedSize
+	}
+	pub, ok := pk.(*xPublicKey)
+	if !ok || pub.scheme != sch {
+		return nil, nil, kem.ErrTypeMismatch
+	}
+
+	pk2, sk2 := sch.DeriveKeyPair(seed)
+	ss = pub.X(sk2.(*xPrivateKey))
+	ct, _ = pk2.MarshalBinary()
+	return
+}
+
+func (sch *xScheme) Decapsulate(sk kem.PrivateKey, ct []byte) ([]byte, error) {
+	if len(ct) != sch.CiphertextSize() {
+		return nil, kem.ErrCiphertextSize
+	}
+
+	priv, ok := sk.(*xPrivateKey)
+	if !ok || priv.scheme != sch {
+		return nil, kem.ErrTypeMismatch
+	}
+
+	pk, err := sch.UnmarshalBinaryPublicKey(ct)
+	if err != nil {
+		return nil, err
+	}
+
+	ss := pk.(*xPublicKey).X(priv)
+	return ss, nil
+}
+
+func (sch *xScheme) UnmarshalBinaryPublicKey(buf []byte) (kem.PublicKey, error) {
+	if len(buf) != sch.PublicKeySize() {
+		return nil, kem.ErrPubKeySize
+	}
+	ret := xPublicKey{sch, make([]byte, sch.size)}
+	copy(ret.key, buf)
+	return &ret, nil
+}
+
+func (sch *xScheme) UnmarshalBinaryPrivateKey(buf []byte) (kem.PrivateKey, error) {
+	if len(buf) != sch.PrivateKeySize() {
+		return nil, kem.ErrPrivKeySize
+	}
+	ret := xPrivateKey{sch, make([]byte, sch.size)}
+	copy(ret.key, buf)
+	return &ret, nil
+}