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double_ratchet.go
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double_ratchet.go
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package e2ee
import (
"bytes"
"crypto/hmac"
"crypto/sha256"
"encoding/binary"
"errors"
"io"
"golang.org/x/crypto/hkdf"
)
type ratchetHeader struct {
DH [32]byte
PN uint32
N uint32
raw []byte
}
type ratchetKeyPair struct {
privateKey x25519PrivateKey
publicKey x25519PublicKey
}
/// skipped 向け
type messageKey struct {
key []byte
nonce []byte
}
type mkskippedKey struct {
DH [32]byte
N uint32
}
type ratchetState struct {
selfDH ratchetKeyPair
remoteDH [32]byte
rootKey []byte
selfChainKey []byte
remoteChainKey []byte
selfN uint32
remoteN uint32
PN uint32
mkskipped map[mkskippedKey]messageKey
}
func generateRatchetKeyPair() (*ratchetKeyPair, error) {
x25519KeyPair, err := generateX25519KeyPair()
if err != nil {
return nil, err
}
return &ratchetKeyPair{
privateKey: x25519KeyPair.privateKey,
publicKey: x25519KeyPair.publicKey,
}, nil
}
func kdfRk(previousRootKey []byte, senderRatchetKeyPrivate x25519PrivateKey, receiverRatchetKeyPublic x25519PublicKey) ([]byte, []byte, error) {
a := dh(senderRatchetKeyPrivate, receiverRatchetKeyPublic)
hash := sha256.New
info := []byte("SoraRatchet")
hkdf := hkdf.New(hash, a[:], previousRootKey, info)
rootKey := make([]byte, 32)
chainKey := make([]byte, 32)
if _, err := io.ReadFull(hkdf, rootKey); err != nil {
return nil, nil, err
}
if _, err := io.ReadFull(hkdf, chainKey); err != nil {
return nil, nil, err
}
return rootKey, chainKey, nil
}
func senderRatchetInit(sk []byte, preKeyBundle preKeyBundle) (*ratchetState, error) {
ratchetKeyPair, err := generateRatchetKeyPair()
if err != nil {
return nil, err
}
rootKey, chainKey, err := kdfRk(sk, ratchetKeyPair.privateKey, preKeyBundle.signedPreKey)
if err != nil {
return nil, err
}
return &ratchetState{
selfDH: *ratchetKeyPair,
remoteDH: preKeyBundle.signedPreKey,
rootKey: rootKey,
selfChainKey: chainKey,
selfN: 0,
remoteN: 0,
PN: 0,
mkskipped: make(map[mkskippedKey]messageKey),
}, nil
}
func receiverRatchetInit(sk []byte, signedPreKeyPublic x25519PublicKey, signedPreKeyPrivate x25519PrivateKey) *ratchetState {
return &ratchetState{
// 初回は receiver の signedPreKey を利用する
// A = ECDH-X25519(Sender_RatchetKey, Receiver_SignedPreKey)
selfDH: ratchetKeyPair{
publicKey: signedPreKeyPublic,
privateKey: signedPreKeyPrivate,
},
rootKey: sk,
selfN: 0,
remoteN: 0,
PN: 0,
mkskipped: make(map[mkskippedKey]messageKey),
}
}
func parseHeader(header []byte) (*ratchetHeader, error) {
ratchetHeader := &ratchetHeader{raw: header}
buf := bytes.NewReader(header)
if err := binary.Read(buf, binary.BigEndian, &ratchetHeader.DH); err != nil {
return nil, err
}
if err := binary.Read(buf, binary.BigEndian, &ratchetHeader.PN); err != nil {
return nil, err
}
if err := binary.Read(buf, binary.BigEndian, &ratchetHeader.N); err != nil {
return nil, err
}
return ratchetHeader, nil
}
// 送られてきた header.dh を引数にとる
func (rs *ratchetState) ratchet(remoteDH [32]byte) error {
rs.PN = rs.selfN
rs.selfN = 0
rs.remoteN = 0
rs.remoteDH = remoteDH
rootKey, remoteChainKey, err := kdfRk(rs.rootKey, rs.selfDH.privateKey, rs.remoteDH)
if err != nil {
return err
}
rs.rootKey = rootKey
rs.remoteChainKey = remoteChainKey
ratchetKeyPair, err := generateRatchetKeyPair()
if err != nil {
return err
}
rs.selfDH = *ratchetKeyPair
rootKey, selfChainKey, err := kdfRk(rs.rootKey, rs.selfDH.privateKey, rs.remoteDH)
if err != nil {
return err
}
rs.rootKey = rootKey
rs.selfChainKey = selfChainKey
return nil
}
func (rs *ratchetState) ratchetDecrypt(header []byte, ciphertext []byte, ad []byte) ([]byte, error) {
ratchetHeader, err := parseHeader(header)
if err != nil {
return nil, err
}
plaintext, err := rs.trySkippedMessageKeys(ratchetHeader, ciphertext, ad)
if err != nil {
return nil, err
}
if plaintext != nil {
return plaintext, nil
}
remoteDH := ratchetHeader.DH
if rs.remoteDH != remoteDH {
if err := rs.skipMessageKeys(ratchetHeader.PN); err != nil {
return nil, err
}
if err := rs.ratchet(remoteDH); err != nil {
return nil, err
}
}
if err := rs.skipMessageKeys(ratchetHeader.N); err != nil {
return nil, err
}
messageKey, nonce, err := rs.newReceiverMessageKey()
if err != nil {
return nil, err
}
rs.newReceiverChainKey()
rs.remoteN++
plaintext, err = decrypt(messageKey, nonce, ciphertext, append(ad, header...))
if err != nil {
return nil, err
}
return plaintext, nil
}
// チェインキーは生成済みとする
func (rs *ratchetState) ratchetEncrypt(plaintext []byte, ad []byte) ([]byte, []byte, error) {
messageKey, nonce, err := rs.newSenderMessageKey()
if err != nil {
return nil, nil, err
}
// chainkey の更新
rs.newSenderChainKey()
header, err := rs.header()
if err != nil {
return nil, nil, err
}
rs.selfN++
ciphertext, err := encrypt(messageKey, nonce, plaintext, append(ad, header...))
if err != nil {
return nil, nil, err
}
return header, ciphertext, nil
}
func (rs *ratchetState) header() ([]byte, error) {
buf := new(bytes.Buffer)
if err := binary.Write(buf, binary.BigEndian, rs.selfDH.publicKey); err != nil {
return nil, err
}
if err := binary.Write(buf, binary.BigEndian, rs.PN); err != nil {
return nil, err
}
if err := binary.Write(buf, binary.BigEndian, rs.selfN); err != nil {
return nil, err
}
return buf.Bytes(), nil
}
func newChainKey(chainKey []byte) []byte {
mac := hmac.New(sha256.New, chainKey)
mac.Write([]byte{2})
return mac.Sum(nil)
}
func (rs *ratchetState) newReceiverChainKey() {
rs.remoteChainKey = newChainKey(rs.remoteChainKey)
}
func (rs *ratchetState) newSenderChainKey() {
rs.selfChainKey = newChainKey(rs.selfChainKey)
}
// KDF_CK
func newMessageKey(chainKey []byte) ([]byte, []byte, error) {
h := hmac.New(sha256.New, chainKey)
h.Write([]byte{1})
seed := h.Sum(nil)
hash := sha256.New
salt := make([]byte, 44)
info := []byte("SoraMessageKeys")
hkdf := hkdf.New(hash, seed, salt, info)
// AES-GCM は 32 でよし
messageKey := make([]byte, 32)
// 12 ほしい
nonce := make([]byte, 12)
if _, err := io.ReadFull(hkdf, messageKey); err != nil {
return nil, nil, err
}
if _, err := io.ReadFull(hkdf, nonce); err != nil {
return nil, nil, err
}
return messageKey, nonce, nil
}
func (rs *ratchetState) newSenderMessageKey() ([]byte, []byte, error) {
return newMessageKey(rs.selfChainKey)
}
func (rs *ratchetState) newReceiverMessageKey() ([]byte, []byte, error) {
return newMessageKey(rs.remoteChainKey)
}
func (rs *ratchetState) trySkippedMessageKeys(header *ratchetHeader, ciphertext []byte, AD []byte) ([]byte, error) {
mkskippedKey := &mkskippedKey{
DH: header.DH,
N: header.N,
}
messageKey, ok := rs.mkskipped[*mkskippedKey]
if ok {
delete(rs.mkskipped, *mkskippedKey)
plaintext, err := decrypt(messageKey.key, messageKey.nonce, ciphertext, append(AD, header.raw...))
if err != nil {
return nil, err
}
return plaintext, nil
}
return nil, nil
}
// MAX_SKIP定数も定義する必要がある。
// これは、1つのチェーンでスキップできるメッセージキーの最大数を指定します。
// ルーチンでのメッセージの紛失や遅延を許容するのに十分な高さに設定しなければならないが、
// 悪意のある送信者が過剰な受信者の計算を引き起こすことができないように十分に低い値に設定しなければならない。
var maxSkip uint32 = 10
func (rs *ratchetState) skipMessageKeys(until uint32) error {
if rs.remoteN+maxSkip < until {
// TODO(v): 切断を促すエラー処理
return errors.New("NotImplementedError")
}
if rs.remoteChainKey != nil {
for rs.remoteN < until {
var mkskippedKey = &mkskippedKey{
DH: rs.remoteDH,
N: rs.remoteN,
}
key, nonce, err := rs.newReceiverMessageKey()
if err != nil {
return err
}
rs.newReceiverChainKey()
var messageKey = &messageKey{
key: key,
nonce: nonce,
}
rs.mkskipped[*mkskippedKey] = *messageKey
rs.remoteN++
}
}
return nil
}