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// Copyright 2019 Google Inc.
// 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.
#include <cstdint>
#include <memory>
#include <string>
#include <vector>
#include "openssl/aead.h"
#include "tink/subtle/common_enums.h"
#include "tink/subtle/stream_segment_decrypter.h"
#include "tink/util/secret_data.h"
#include "tink/util/statusor.h"
namespace crypto {
namespace tink {
namespace subtle {
// StreamSegmentDecrypter for streaming decryption using AES-GCM with HKDF as
// key derivation function.
// Each ciphertext uses a new AES-GCM key that is derived from the key
// derivation key, a randomly chosen salt of the same size as the key and a
// nonce prefix.
// The format of a ciphertext is
// other || header || segment_0 || segment_1 || ... || segment_k.
// where:
// - segment_i is the i-th segment of the ciphertext.
// - the size of segment_1 .. segment_{k-1} is get_ciphertext_segment_size()
// - segment_0 is shorter, so that segment_0, the header, and other information
// of size get_ciphertext_offset() align with get_ciphertext_segment_size().
// The format of the header is
// header_size || salt || nonce_prefix
// where
// - header_size is 1 byte determining the size of the header
// - salt is a salt used in the key derivation
// - nonce_prefix is the prefix of the nonce
// In principle header_size is redundant information, since the length of the
// header can be determined from the key size.
class AesGcmHkdfStreamSegmentDecrypter : public StreamSegmentDecrypter {
// All sizes are in bytes.
struct Params {
util::SecretData ikm;
HashType hkdf_hash;
int derived_key_size;
int ciphertext_offset;
int ciphertext_segment_size;
std::string associated_data;
// A factory.
static util::StatusOr<std::unique_ptr<StreamSegmentDecrypter>> New(
Params params);
// Overridden methods of StreamSegmentDecrypter.
util::Status Init(const std::vector<uint8_t>& header) override;
util::Status DecryptSegment(
const std::vector<uint8_t>& ciphertext,
int64_t segment_number,
bool is_last_segment,
std::vector<uint8_t>* plaintext_buffer) override;
int get_header_size() const override {
return header_size_;
int get_plaintext_segment_size() const override;
int get_ciphertext_segment_size() const override {
return ciphertext_segment_size_;
int get_ciphertext_offset() const override {
return ciphertext_offset_;
explicit AesGcmHkdfStreamSegmentDecrypter(Params params);
// Parameters set upon decrypter creation.
// All sizes are in bytes.
const util::SecretData ikm_;
const HashType hkdf_hash_;
const int derived_key_size_;
const int ciphertext_offset_;
const int ciphertext_segment_size_;
const std::string associated_data_;
const int header_size_;
// Parameters set when initializing with data from stream header.
bool is_initialized_ = false;
std::vector<uint8_t> salt_;
std::vector<uint8_t> nonce_prefix_;
bssl::UniquePtr<EVP_AEAD_CTX> ctx_;
} // namespace subtle
} // namespace tink
} // namespace crypto