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* Copyright 2018 gRPC authors.
* 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
package conn
import (
core ""
const (
// Overflow length n in bytes, never encrypt more than 2^(n*8) frames (in
// each direction).
overflowLenAES128GCM = 5
// aes128gcm is the struct that holds necessary information for ALTS record.
// The counter value is NOT included in the payload during the encryption and
// decryption operations.
type aes128gcm struct {
// inCounter is used in ALTS record to check that incoming counters are
// as expected, since ALTS record guarantees that messages are unwrapped
// in the same order that the peer wrapped them.
inCounter Counter
outCounter Counter
aead cipher.AEAD
// NewAES128GCM creates an instance that uses aes128gcm for ALTS record.
func NewAES128GCM(side core.Side, key []byte) (ALTSRecordCrypto, error) {
c, err := aes.NewCipher(key)
if err != nil {
return nil, err
a, err := cipher.NewGCM(c)
if err != nil {
return nil, err
return &aes128gcm{
inCounter: NewInCounter(side, overflowLenAES128GCM),
outCounter: NewOutCounter(side, overflowLenAES128GCM),
aead: a,
}, nil
// Encrypt is the encryption function. dst can contain bytes at the beginning of
// the ciphertext that will not be encrypted but will be authenticated. If dst
// has enough capacity to hold these bytes, the ciphertext and the tag, no
// allocation and copy operations will be performed. dst and plaintext do not
// overlap.
func (s *aes128gcm) Encrypt(dst, plaintext []byte) ([]byte, error) {
// If we need to allocate an output buffer, we want to include space for
// GCM tag to avoid forcing ALTS record to reallocate as well.
dlen := len(dst)
dst, out := SliceForAppend(dst, len(plaintext)+GcmTagSize)
seq, err := s.outCounter.Value()
if err != nil {
return nil, err
data := out[:len(plaintext)]
copy(data, plaintext) // data may alias plaintext
// Seal appends the ciphertext and the tag to its first argument and
// returns the updated slice. However, SliceForAppend above ensures that
// dst has enough capacity to avoid a reallocation and copy due to the
// append.
dst = s.aead.Seal(dst[:dlen], seq, data, nil)
return dst, nil
func (s *aes128gcm) EncryptionOverhead() int {
return GcmTagSize
func (s *aes128gcm) Decrypt(dst, ciphertext []byte) ([]byte, error) {
seq, err := s.inCounter.Value()
if err != nil {
return nil, err
// If dst is equal to ciphertext[:0], ciphertext storage is reused.
plaintext, err := s.aead.Open(dst, seq, ciphertext, nil)
if err != nil {
return nil, ErrAuth
return plaintext, nil
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