xwing: HPKE integration

hpke/otherkem.go

@@ -0,0 +1,77 @@
+package hpke
+
+// Shim to use generic KEM (kem.Scheme) as HPKE KEM.
+
+import (
+	"github.com/cloudflare/circl/internal/sha3"
+	"github.com/cloudflare/circl/kem"
+)
+
+type otherKEM struct {
+	kem  kem.Scheme
+	name string
+}
+
+func (h otherKEM) PrivateKeySize() int        { return h.kem.PrivateKeySize() }
+func (h otherKEM) SeedSize() int              { return h.kem.SeedSize() }
+func (h otherKEM) CiphertextSize() int        { return h.kem.CiphertextSize() }
+func (h otherKEM) PublicKeySize() int         { return h.kem.PublicKeySize() }
+func (h otherKEM) EncapsulationSeedSize() int { return h.kem.EncapsulationSeedSize() }
+func (h otherKEM) SharedKeySize() int         { return h.kem.SharedKeySize() }
+func (h otherKEM) Name() string               { return h.name }
+
+func (h otherKEM) AuthDecapsulate(skR kem.PrivateKey,
+	ct []byte,
+	pkS kem.PublicKey,
+) ([]byte, error) {
+	panic("AuthDecapsulate is not supported for this KEM")
+}
+
+func (h otherKEM) AuthEncapsulate(pkr kem.PublicKey, sks kem.PrivateKey) (
+	ct []byte, ss []byte, err error,
+) {
+	panic("AuthEncapsulate is not supported for this KEM")
+}
+
+func (h otherKEM) AuthEncapsulateDeterministically(pkr kem.PublicKey, sks kem.PrivateKey, seed []byte) (ct, ss []byte, err error) {
+	panic("AuthEncapsulateDeterministically is not supported for this KEM")
+}
+
+func (h otherKEM) Encapsulate(pkr kem.PublicKey) (
+	ct []byte, ss []byte, err error,
+) {
+	return h.kem.Encapsulate(pkr)
+}
+
+func (h otherKEM) Decapsulate(skr kem.PrivateKey, ct []byte) ([]byte, error) {
+	return h.kem.Decapsulate(skr, ct)
+}
+
+func (h otherKEM) EncapsulateDeterministically(
+	pkr kem.PublicKey, seed []byte,
+) (ct, ss []byte, err error) {
+	return h.kem.EncapsulateDeterministically(pkr, seed)
+}
+
+// HPKE requires DeriveKeyPair() to take any seed larger than the private key
+// size, whereas typical KEMs expect a specific seed size. We'll just use
+// SHAKE256 to hash it to the right size as in X-Wing.
+func (h otherKEM) DeriveKeyPair(seed []byte) (kem.PublicKey, kem.PrivateKey) {
+	seed2 := make([]byte, h.kem.SeedSize())
+	hh := sha3.NewShake256()
+	_, _ = hh.Write(seed)
+	_, _ = hh.Read(seed2)
+	return h.kem.DeriveKeyPair(seed2)
+}
+
+func (h otherKEM) GenerateKeyPair() (kem.PublicKey, kem.PrivateKey, error) {
+	return h.kem.GenerateKeyPair()
+}
+
+func (h otherKEM) UnmarshalBinaryPrivateKey(data []byte) (kem.PrivateKey, error) {
+	return h.kem.UnmarshalBinaryPrivateKey(data)
+}
+
+func (h otherKEM) UnmarshalBinaryPublicKey(data []byte) (kem.PublicKey, error) {
+	return h.kem.UnmarshalBinaryPublicKey(data)
+}