components/gcm_driver/crypto/gcm_message_cryptographer.cc

// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "components/gcm_driver/crypto/gcm_message_cryptographer.h"

#include <stddef.h>
#include <stdint.h>

#include <algorithm>
#include <sstream>

#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/ptr_util.h"
#include "base/numerics/safe_math.h"
#include "base/strings/string_util.h"
#include "base/sys_byteorder.h"
#include "crypto/hkdf.h"
#include "third_party/boringssl/src/include/openssl/aead.h"

namespace gcm {

namespace {

// Size, in bytes, of the nonce for a record. This must be at least the size
// of a uint64_t, which is used to indicate the record sequence number.
const uint64_t kNonceSize = 12;

// The default record size as defined by httpbis-encryption-encoding-06.
const size_t kDefaultRecordSize = 4096;

// Key size, in bytes, of a valid AEAD_AES_128_GCM key.
const size_t kContentEncryptionKeySize = 16;

// The BoringSSL functions used to seal (encrypt) and open (decrypt) a payload
// follow the same prototype, declared as follows.
using EVP_AEAD_CTX_TransformFunction =
    int(const EVP_AEAD_CTX *ctx, uint8_t *out, size_t *out_len,
        size_t max_out_len, const uint8_t *nonce, size_t nonce_len,
        const uint8_t *in, size_t in_len, const uint8_t *ad, size_t ad_len);

// Implementation of draft 03 of the Web Push Encryption standard:
// https://tools.ietf.org/html/draft-ietf-webpush-encryption-03
// https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-02
class WebPushEncryptionDraft03
    : public GCMMessageCryptographer::EncryptionScheme {
 public:
  WebPushEncryptionDraft03() = default;
  ~WebPushEncryptionDraft03() override = default;

  // GCMMessageCryptographer::EncryptionScheme implementation.
  std::string DerivePseudoRandomKey(
      const base::StringPiece& ecdh_shared_secret,
      const base::StringPiece& auth_secret) override {
    std::stringstream info_stream;
    info_stream << "Content-Encoding: auth" << '\x00';

    crypto::HKDF hkdf(ecdh_shared_secret, auth_secret, info_stream.str(),
                      32, /* key_bytes_to_generate */
                      0,  /* iv_bytes_to_generate */
                      0 /* subkey_secret_bytes_to_generate */);

    return hkdf.client_write_key().as_string();
  }

  // Creates the info parameter for an HKDF value for the given
  // |content_encoding| in accordance with draft-ietf-webpush-encryption-03.
  //
  // cek_info = "Content-Encoding: aesgcm" || 0x00 || context
  // nonce_info = "Content-Encoding: nonce" || 0x00 || context
  //
  // context = "P-256" || 0x00 ||
  //           length(recipient_public) || recipient_public ||
  //           length(sender_public) || sender_public
  //
  // The length of the public keys must be written as a two octet unsigned
  // integer in network byte order (big endian).
  std::string GenerateInfoForContentEncoding(
      EncodingType type,
      const base::StringPiece& recipient_public_key,
      const base::StringPiece& sender_public_key) override {
    std::stringstream info_stream;
    info_stream << "Content-Encoding: ";

    switch (type) {
      case EncodingType::CONTENT_ENCRYPTION_KEY:
        info_stream << "aesgcm";
        break;
      case EncodingType::NONCE:
        info_stream << "nonce";
        break;
    }

    info_stream << '\x00' << "P-256" << '\x00';

    uint16_t local_len =
        base::HostToNet16(static_cast<uint16_t>(recipient_public_key.size()));
    info_stream.write(reinterpret_cast<char*>(&local_len), sizeof(local_len));
    info_stream << recipient_public_key;

    uint16_t peer_len =
        base::HostToNet16(static_cast<uint16_t>(sender_public_key.size()));
    info_stream.write(reinterpret_cast<char*>(&peer_len), sizeof(peer_len));
    info_stream << sender_public_key;

    return info_stream.str();
  }

  // draft-ietf-webpush-encryption-03 defines that the padding is included at
  // the beginning of the message. The first two bytes, in network byte order,
  // contain the length of the included padding. Then that exact number of bytes
  // must follow as padding, all of which must have a zero value.
  //
  // TODO(peter): Add support for message padding if the GCMMessageCryptographer
  // starts encrypting payloads for reasons other than testing.
  std::string CreateRecord(const base::StringPiece& plaintext) override {
    std::string record;
    record.reserve(sizeof(uint16_t) + plaintext.size());
    record.append(sizeof(uint16_t), '\x00');

    plaintext.AppendToString(&record);
    return record;
  }

  // The record padding in draft-ietf-webpush-encryption-03 is included at the
  // beginning of the record. The first two bytes indicate the length of the
  // padding. All padding bytes immediately follow, and must be set to zero.
  bool ValidateAndRemovePadding(base::StringPiece& record) override {
    // Records must be at least two octets in size (to hold the padding).
    // Records that are smaller, i.e. a single octet, are invalid.
    if (record.size() < sizeof(uint16_t))
      return false;

    // Records contain a two-byte, big-endian padding length followed by zero to
    // 65535 bytes of padding. Padding bytes must be zero but, since AES-GCM
    // authenticates the plaintext, checking and removing padding need not be
    // done in constant-time.
    uint16_t padding_length = (static_cast<uint8_t>(record[0]) << 8) |
                              static_cast<uint8_t>(record[1]);
    record.remove_prefix(sizeof(uint16_t));

    if (padding_length > record.size()) {
      return false;
    }

    for (size_t i = 0; i < padding_length; ++i) {
      if (record[i] != 0)
        return false;
    }

    record.remove_prefix(padding_length);
    return true;
  }

 private:
  DISALLOW_COPY_AND_ASSIGN(WebPushEncryptionDraft03);
};

}  // namespace

const size_t GCMMessageCryptographer::kAuthenticationTagBytes = 16;
const size_t GCMMessageCryptographer::kSaltSize = 16;

GCMMessageCryptographer::GCMMessageCryptographer(Version version) {
  switch (version) {
    case Version::DRAFT_03:
      encryption_scheme_ = base::MakeUnique<WebPushEncryptionDraft03>();
      return;
  }

  NOTREACHED();
}

GCMMessageCryptographer::~GCMMessageCryptographer() = default;

bool GCMMessageCryptographer::Encrypt(
    const base::StringPiece& recipient_public_key,
    const base::StringPiece& sender_public_key,
    const base::StringPiece& ecdh_shared_secret,
    const base::StringPiece& auth_secret,
    const base::StringPiece& salt,
    const base::StringPiece& plaintext,
    size_t* record_size,
    std::string* ciphertext) const {
  DCHECK_EQ(recipient_public_key.size(), 65u);
  DCHECK_EQ(sender_public_key.size(), 65u);
  DCHECK(record_size);
  DCHECK(ciphertext);

  // TODO(peter): DCHECK the lengths of |ecdh_shared_secret|, |auth_secret| and
  // |salt|.

  if (salt.size() != kSaltSize)
    return false;

  std::string prk = encryption_scheme_->DerivePseudoRandomKey(
      ecdh_shared_secret, auth_secret);

  std::string content_encryption_key = DeriveContentEncryptionKey(
      recipient_public_key, sender_public_key, prk, salt);
  std::string nonce =
      DeriveNonce(recipient_public_key, sender_public_key, prk, salt);

  std::string record = encryption_scheme_->CreateRecord(plaintext);
  std::string encrypted_record;

  if (!TransformRecord(Direction::ENCRYPT, record, content_encryption_key,
                       nonce, &encrypted_record)) {
    return false;
  }

  // The advertised record size must be at least one more than the padded
  // plaintext to ensure only one record.
  *record_size = std::max(kDefaultRecordSize, record.size() + 1);

  ciphertext->swap(encrypted_record);
  return true;
}

bool GCMMessageCryptographer::Decrypt(
    const base::StringPiece& recipient_public_key,
    const base::StringPiece& sender_public_key,
    const base::StringPiece& ecdh_shared_secret,
    const base::StringPiece& auth_secret,
    const base::StringPiece& salt,
    const base::StringPiece& ciphertext,
    size_t record_size,
    std::string* plaintext) const {
  DCHECK_EQ(recipient_public_key.size(), 65u);
  DCHECK_EQ(sender_public_key.size(), 65u);
  DCHECK(plaintext);

  // TODO(peter): DCHECK the lengths of |ecdh_shared_secret|, |auth_secret| and
  // |salt|.

  if (record_size <= 1)
    return false;

  std::string prk = encryption_scheme_->DerivePseudoRandomKey(
      ecdh_shared_secret, auth_secret);

  std::string content_encryption_key = DeriveContentEncryptionKey(
      recipient_public_key, sender_public_key, prk, salt);

  std::string nonce =
      DeriveNonce(recipient_public_key, sender_public_key, prk, salt);

  // The |ciphertext| must be at least of size kAuthenticationTagBytes, which
  // is the case when an empty message with a zero padding length has been
  // received. The |record_size| must be large enough to use only one record.
  // https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-03#section-2
  if (ciphertext.size() < sizeof(uint16_t) + kAuthenticationTagBytes ||
      ciphertext.size() > record_size + kAuthenticationTagBytes) {
    return false;
  }

  std::string decrypted_record_string;
  if (!TransformRecord(Direction::DECRYPT, ciphertext, content_encryption_key,
                       nonce, &decrypted_record_string)) {
    return false;
  }

  DCHECK(!decrypted_record_string.empty());

  base::StringPiece decrypted_record(decrypted_record_string);
  if (!encryption_scheme_->ValidateAndRemovePadding(decrypted_record))
    return false;

  decrypted_record.CopyToString(plaintext);
  return true;
}

bool GCMMessageCryptographer::TransformRecord(Direction direction,
                                              const base::StringPiece& input,
                                              const base::StringPiece& key,
                                              const base::StringPiece& nonce,
                                              std::string* output) const {
  DCHECK(output);

  const EVP_AEAD* aead = EVP_aead_aes_128_gcm();

  EVP_AEAD_CTX context;
  if (!EVP_AEAD_CTX_init(&context, aead,
                         reinterpret_cast<const uint8_t*>(key.data()),
                         key.size(), EVP_AEAD_DEFAULT_TAG_LENGTH, nullptr)) {
    return false;
  }

  base::CheckedNumeric<size_t> maximum_output_length(input.size());
  if (direction == Direction::ENCRYPT)
    maximum_output_length += kAuthenticationTagBytes;

  // WriteInto requires the buffer to finish with a NULL-byte.
  maximum_output_length += 1;

  size_t output_length = 0;
  uint8_t* raw_output = reinterpret_cast<uint8_t*>(
      base::WriteInto(output, maximum_output_length.ValueOrDie()));

  EVP_AEAD_CTX_TransformFunction* transform_function =
      direction == Direction::ENCRYPT ? EVP_AEAD_CTX_seal : EVP_AEAD_CTX_open;

  if (!transform_function(
          &context, raw_output, &output_length, output->size(),
          reinterpret_cast<const uint8_t*>(nonce.data()), nonce.size(),
          reinterpret_cast<const uint8_t*>(input.data()), input.size(),
          nullptr, 0)) {
    EVP_AEAD_CTX_cleanup(&context);
    return false;
  }

  EVP_AEAD_CTX_cleanup(&context);

  base::CheckedNumeric<size_t> expected_output_length(input.size());
  if (direction == Direction::ENCRYPT)
    expected_output_length += kAuthenticationTagBytes;
  else
    expected_output_length -= kAuthenticationTagBytes;

  DCHECK_EQ(expected_output_length.ValueOrDie(), output_length);

  output->resize(output_length);
  return true;
}

std::string GCMMessageCryptographer::DeriveContentEncryptionKey(
    const base::StringPiece& recipient_public_key,
    const base::StringPiece& sender_public_key,
    const base::StringPiece& ecdh_shared_secret,
    const base::StringPiece& salt) const {
  std::string content_encryption_key_info =
      encryption_scheme_->GenerateInfoForContentEncoding(
          EncryptionScheme::EncodingType::CONTENT_ENCRYPTION_KEY,
          recipient_public_key, sender_public_key);

  crypto::HKDF hkdf(ecdh_shared_secret, salt, content_encryption_key_info,
                    kContentEncryptionKeySize, 0, /* iv_bytes_to_generate */
                    0 /* subkey_secret_bytes_to_generate */);

  return hkdf.client_write_key().as_string();
}

std::string GCMMessageCryptographer::DeriveNonce(
    const base::StringPiece& recipient_public_key,
    const base::StringPiece& sender_public_key,
    const base::StringPiece& ecdh_shared_secret,
    const base::StringPiece& salt) const {
  std::string nonce_info = encryption_scheme_->GenerateInfoForContentEncoding(
      EncryptionScheme::EncodingType::NONCE, recipient_public_key,
      sender_public_key);

  crypto::HKDF hkdf(ecdh_shared_secret, salt, nonce_info, kNonceSize,
                    0, /* iv_bytes_to_generate */
                    0 /* subkey_secret_bytes_to_generate */);

  // https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-02
  // defines that the result should be XOR'ed with the record's sequence number,
  // however, Web Push encryption is limited to a single record per
  // https://tools.ietf.org/html/draft-ietf-webpush-encryption-03.

  return hkdf.client_write_key().as_string();
}

}  // namespace gcm