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// Copyright 2022 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package subtle
import (
"bytes"
"errors"
"fmt"
"google.golang.org/protobuf/proto"
"github.com/tink-crypto/tink-go/v2/keyset"
hpkepb "github.com/tink-crypto/tink-go/v2/proto/hpke_go_proto"
tinkpb "github.com/tink-crypto/tink-go/v2/proto/tink_go_proto"
)
const (
// HPKE public key length from
// https://www.rfc-editor.org/rfc/rfc9180.html#section-7.1.
hpkeX25519HKDFSHA256PubKeyLen = 32
hpkePublicKeyTypeURL = "type.googleapis.com/google.crypto.tink.HpkePublicKey"
hpkePrivateKeyTypeURL = "type.googleapis.com/google.crypto.tink.HpkePrivateKey"
)
// SerializePrimaryPublicKey serializes a public keyset handle's primary key if
// the primary key is a public key and matches both the template argument and a
// supported template.
//
// Supported templates are the same as KeysetHandleFromSerializedPublicKey's:
// - DHKEM_X25519_HKDF_SHA256_HKDF_SHA256_CHACHA20_POLY1305_Raw_Key_Template,
// which returns the KEM-encoding of the public key, i.e. SerializePublicKey
// in https://www.rfc-editor.org/rfc/rfc9180.html#section-7.1.1.
func SerializePrimaryPublicKey(handle *keyset.Handle, template *tinkpb.KeyTemplate) ([]byte, error) {
templateParams, err := hpkeParamsFromTemplate(template)
if err != nil {
return nil, fmt.Errorf("failed to verify key template: %v", err)
}
// Create keyset from handle.
w := new(bytes.Buffer)
if err := handle.WriteWithNoSecrets(keyset.NewBinaryWriter(w)); err != nil {
return nil, fmt.Errorf("failed to write key: %v", err)
}
ks := &tinkpb.Keyset{}
if err := proto.Unmarshal(w.Bytes(), ks); err != nil {
return nil, fmt.Errorf("failed to unmarshal Keyset %v: %v", ks, err)
}
if len(ks.GetKey()) < 1 {
return nil, errors.New("empty keyset")
}
// Verify and return handle's primary key.
for _, key := range ks.GetKey() {
if key.GetStatus() != tinkpb.KeyStatusType_ENABLED ||
key.GetOutputPrefixType() != tinkpb.OutputPrefixType_RAW ||
key.GetKeyId() != ks.GetPrimaryKeyId() {
continue
}
keyData := key.GetKeyData()
if keyData.GetKeyMaterialType() != tinkpb.KeyData_ASYMMETRIC_PUBLIC {
return nil, errors.New("primary key is not asymmetric public")
}
if keyData.GetTypeUrl() != hpkePublicKeyTypeURL {
return nil, fmt.Errorf("primary key does not have key type URL %s", hpkePublicKeyTypeURL)
}
hpkeKey := &hpkepb.HpkePublicKey{}
if err := proto.Unmarshal(keyData.GetValue(), hpkeKey); err != nil {
return nil, fmt.Errorf("failed to unmarshal HpkePublicKey %v: %v", hpkeKey, err)
}
// Check equality between HPKE params in handle's primary key and in
// template, as template's params have already been verified.
if !proto.Equal(templateParams, hpkeKey.GetParams()) {
return nil, errors.New("HPKE params in handle and template are not equal")
}
return hpkeKey.GetPublicKey(), nil
}
return nil, errors.New("no valid primary HPKE public key in keyset")
}
// KeysetHandleFromSerializedPublicKey returns a keyset handle containing a
// primary key that has the specified pubKeyBytes and matches template.
//
// Supported templates are the same as PublicKeyFromPrimaryKey's:
// - DHKEM_X25519_HKDF_SHA256_HKDF_SHA256_CHACHA20_POLY1305_Raw_Key_Template,
// which requires pubKeyBytes to be the KEM-encoding of the public key, i.e.
// SerializePublicKey in
// https://www.rfc-editor.org/rfc/rfc9180.html#section-7.1.1.
func KeysetHandleFromSerializedPublicKey(pubKeyBytes []byte, template *tinkpb.KeyTemplate) (*keyset.Handle, error) {
params, err := hpkeParamsFromTemplate(template)
if err != nil {
return nil, fmt.Errorf("failed to verify key template: %v", err)
}
if len(pubKeyBytes) != hpkeX25519HKDFSHA256PubKeyLen {
return nil, fmt.Errorf("pubKeyBytes length is %d but should be %d", len(pubKeyBytes), hpkeX25519HKDFSHA256PubKeyLen)
}
pubKey := &hpkepb.HpkePublicKey{
Version: 0,
Params: params,
PublicKey: pubKeyBytes,
}
serializedPubKey, err := proto.Marshal(pubKey)
if err != nil {
return nil, fmt.Errorf("failed to marshal HpkePublicKey %v: %v", pubKey, err)
}
ks := &tinkpb.Keyset{
PrimaryKeyId: 1,
Key: []*tinkpb.Keyset_Key{
{
KeyData: &tinkpb.KeyData{
TypeUrl: hpkePublicKeyTypeURL,
Value: serializedPubKey,
KeyMaterialType: tinkpb.KeyData_ASYMMETRIC_PUBLIC,
},
Status: tinkpb.KeyStatusType_ENABLED,
KeyId: 1,
OutputPrefixType: tinkpb.OutputPrefixType_RAW,
},
},
}
return keyset.NewHandleWithNoSecrets(ks)
}
// hpkeParamsFromTemplate returns HPKE params after verifying that template is
// supported.
//
// Supported templates include:
// - DHKEM_X25519_HKDF_SHA256_HKDF_SHA256_CHACHA20_POLY1305_Raw_Key_Template.
func hpkeParamsFromTemplate(template *tinkpb.KeyTemplate) (*hpkepb.HpkeParams, error) {
if template.GetTypeUrl() != hpkePrivateKeyTypeURL {
return nil, fmt.Errorf("not key type URL %s", hpkePrivateKeyTypeURL)
}
if template.GetOutputPrefixType() != tinkpb.OutputPrefixType_RAW {
return nil, errors.New("not raw output prefix type")
}
keyFormat := &hpkepb.HpkeKeyFormat{}
if err := proto.Unmarshal(template.GetValue(), keyFormat); err != nil {
return nil, fmt.Errorf("failed to unmarshal HpkeKeyFormat(%v): %v", template.GetValue(), err)
}
params := keyFormat.GetParams()
if kem := params.GetKem(); kem != hpkepb.HpkeKem_DHKEM_X25519_HKDF_SHA256 {
return nil, fmt.Errorf("HPKE KEM %s not supported", kem)
}
if kdf := params.GetKdf(); kdf != hpkepb.HpkeKdf_HKDF_SHA256 {
return nil, fmt.Errorf("HPKE KDF %s not supported", kdf)
}
if aead := params.GetAead(); aead != hpkepb.HpkeAead_CHACHA20_POLY1305 {
return nil, fmt.Errorf("HPKE AEAD %s not supported", aead)
}
return params, nil
}
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