components/webcrypto/algorithms/aes_ctr.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include <openssl/aes.h>
   6 #include <openssl/evp.h>
   7
   8 #include "base/logging.h"
   9 #include "base/macros.h"
  10 #include "base/numerics/safe_math.h"
  11 #include "base/stl_util.h"
  12 #include "components/webcrypto/algorithms/aes.h"
  13 #include "components/webcrypto/algorithms/util_openssl.h"
  14 #include "components/webcrypto/crypto_data.h"
  15 #include "components/webcrypto/key.h"
  16 #include "components/webcrypto/status.h"
  17 #include "components/webcrypto/webcrypto_util.h"
  18 #include "crypto/openssl_util.h"
  19 #include "crypto/scoped_openssl_types.h"
  20 #include "third_party/WebKit/public/platform/WebCryptoAlgorithmParams.h"
  21
  22 namespace webcrypto {
  23
  24 namespace {
  25
  26 const EVP_CIPHER* GetAESCipherByKeyLength(size_t key_length_bytes) {
  27   // BoringSSL does not support 192-bit AES keys.
  28   switch (key_length_bytes) {
  29     case 16:
  30       return EVP_aes_128_ctr();
  31     case 32:
  32       return EVP_aes_256_ctr();
  33     default:
  34       return NULL;
  35   }
  36 }
  37
  38 // Encrypts/decrypts given a 128-bit counter.
  39 //
  40 // |output| must be a pointer to a buffer which has a length of at least
  41 // |input.byte_length()|.
  42 Status AesCtrEncrypt128BitCounter(const EVP_CIPHER* cipher,
  43                                   const CryptoData& raw_key,
  44                                   const CryptoData& input,
  45                                   const CryptoData& counter,
  46                                   uint8_t* output) {
  47   DCHECK(cipher);
  48   DCHECK_EQ(16u, counter.byte_length());
  49
  50   crypto::OpenSSLErrStackTracer err_tracer(FROM_HERE);
  51   crypto::ScopedOpenSSL<EVP_CIPHER_CTX, EVP_CIPHER_CTX_free> context(
  52       EVP_CIPHER_CTX_new());
  53
  54   if (!context.get())
  55     return Status::OperationError();
  56
  57   if (!EVP_CipherInit_ex(context.get(), cipher, NULL, raw_key.bytes(),
  58                          counter.bytes(), ENCRYPT)) {
  59     return Status::OperationError();
  60   }
  61
  62   int output_len = 0;
  63   if (!EVP_CipherUpdate(context.get(), output, &output_len, input.bytes(),
  64                         input.byte_length())) {
  65     return Status::OperationError();
  66   }
  67   int final_output_chunk_len = 0;
  68   if (!EVP_CipherFinal_ex(context.get(), output + output_len,
  69                           &final_output_chunk_len)) {
  70     return Status::OperationError();
  71   }
  72
  73   output_len += final_output_chunk_len;
  74   if (static_cast<unsigned int>(output_len) != input.byte_length())
  75     return Status::ErrorUnexpected();
  76
  77   return Status::Success();
  78 }
  79
  80 // Returns ceil(a/b), where a and b are integers.
  81 template <typename T>
  82 T CeilDiv(T a, T b) {
  83   return a == 0 ? 0 : 1 + (a - 1) / b;
  84 }
  85
  86 // Extracts the counter as a BIGNUM. The counter is the rightmost
  87 // "counter_length_bits" of the block, interpreted as a big-endian number.
  88 crypto::ScopedBIGNUM GetCounter(const CryptoData& counter_block,
  89                                 unsigned int counter_length_bits) {
  90   unsigned int counter_length_remainder_bits = (counter_length_bits % 8);
  91
  92   // If the counter is a multiple of 8 bits then can call BN_bin2bn() directly.
  93   if (counter_length_remainder_bits == 0) {
  94     unsigned int byte_length = counter_length_bits / 8;
  95     return crypto::ScopedBIGNUM(BN_bin2bn(
  96         counter_block.bytes() + counter_block.byte_length() - byte_length,
  97         byte_length, NULL));
  98   }
  99
 100   // Otherwise make a copy of the counter and zero out the topmost bits so
 101   // BN_bin2bn() can be called with a byte stream.
 102   unsigned int byte_length = CeilDiv(counter_length_bits, 8u);
 103   std::vector<uint8_t> counter(
 104       counter_block.bytes() + counter_block.byte_length() - byte_length,
 105       counter_block.bytes() + counter_block.byte_length());
 106   counter[0] &= ~(0xFF << counter_length_remainder_bits);
 107
 108   return crypto::ScopedBIGNUM(
 109       BN_bin2bn(vector_as_array(&counter), counter.size(), NULL));
 110 }
 111
 112 // Returns a counter block with the counter bits all set all zero.
 113 std::vector<uint8_t> BlockWithZeroedCounter(const CryptoData& counter_block,
 114                                             unsigned int counter_length_bits) {
 115   unsigned int counter_length_bytes = counter_length_bits / 8;
 116   unsigned int counter_length_bits_remainder = counter_length_bits % 8;
 117
 118   std::vector<uint8_t> new_counter_block(
 119       counter_block.bytes(),
 120       counter_block.bytes() + counter_block.byte_length());
 121
 122   size_t index = new_counter_block.size() - counter_length_bytes;
 123   memset(&new_counter_block.front() + index, 0, counter_length_bytes);
 124
 125   if (counter_length_bits_remainder) {
 126     new_counter_block[index - 1] &= 0xFF << counter_length_bits_remainder;
 127   }
 128
 129   return new_counter_block;
 130 }
 131
 132 // This function does encryption/decryption for AES-CTR (encryption and
 133 // decryption are the same).
 134 //
 135 // BoringSSL's interface for AES-CTR differs from that of WebCrypto. In
 136 // WebCrypto the caller specifies a 16-byte counter block and designates how
 137 // many of the right-most X bits to use as a big-endian counter. Whereas in
 138 // BoringSSL the entire counter block is interpreted as a 128-bit counter.
 139 //
 140 // In AES-CTR, the counter block MUST be unique across all messages that are
 141 // encrypted/decrypted. WebCrypto expects that the counter can start at any
 142 // value, and is therefore permitted to wrap around to zero on overflow.
 143 //
 144 // Some care is taken to fail if the counter wraps back to an earlier value.
 145 // However this protection is only enforced during a *single* call to
 146 // encrypt/decrypt.
 147 Status AesCtrEncryptDecrypt(const blink::WebCryptoAlgorithm& algorithm,
 148                             const blink::WebCryptoKey& key,
 149                             const CryptoData& data,
 150                             std::vector<uint8_t>* buffer) {
 151   const blink::WebCryptoAesCtrParams* params = algorithm.aesCtrParams();
 152   const std::vector<uint8_t>& raw_key = GetSymmetricKeyData(key);
 153
 154   if (params->counter().size() != 16)
 155     return Status::ErrorIncorrectSizeAesCtrCounter();
 156
 157   unsigned int counter_length_bits = params->lengthBits();
 158   if (counter_length_bits < 1 || counter_length_bits > 128)
 159     return Status::ErrorInvalidAesCtrCounterLength();
 160
 161   // The output of AES-CTR is the same size as the input. However BoringSSL
 162   // expects buffer sizes as an "int".
 163   base::CheckedNumeric<int> output_max_len = data.byte_length();
 164   if (!output_max_len.IsValid())
 165     return Status::ErrorDataTooLarge();
 166
 167   const EVP_CIPHER* const cipher = GetAESCipherByKeyLength(raw_key.size());
 168   if (!cipher)
 169     return Status::ErrorUnexpected();
 170
 171   const CryptoData counter_block(params->counter());
 172   buffer->resize(output_max_len.ValueOrDie());
 173
 174   // The total number of possible counter values is pow(2, counter_length_bits)
 175   crypto::ScopedBIGNUM num_counter_values(BN_new());
 176   if (!BN_lshift(num_counter_values.get(), BN_value_one(), counter_length_bits))
 177     return Status::ErrorUnexpected();
 178
 179   crypto::ScopedBIGNUM current_counter =
 180       GetCounter(counter_block, counter_length_bits);
 181
 182   // The number of AES blocks needed for encryption/decryption. The counter is
 183   // incremented this many times.
 184   crypto::ScopedBIGNUM num_output_blocks(BN_new());
 185   if (!BN_set_word(
 186           num_output_blocks.get(),
 187           CeilDiv(buffer->size(), static_cast<size_t>(AES_BLOCK_SIZE)))) {
 188     return Status::ErrorUnexpected();
 189   }
 190
 191   // If the counter is going to be incremented more times than there are counter
 192   // values, fail. (Repeating values of the counter block is bad).
 193   if (BN_cmp(num_output_blocks.get(), num_counter_values.get()) > 0)
 194     return Status::ErrorAesCtrInputTooLongCounterRepeated();
 195
 196   // This is the number of blocks that can be successfully encrypted without
 197   // overflowing the counter. Encrypting the subsequent block will need to
 198   // reset the counter to zero.
 199   crypto::ScopedBIGNUM num_blocks_until_reset(BN_new());
 200
 201   if (!BN_sub(num_blocks_until_reset.get(), num_counter_values.get(),
 202               current_counter.get())) {
 203     return Status::ErrorUnexpected();
 204   }
 205
 206   // If the counter can be incremented for the entire input without
 207   // wrapping-around, do it as a single call into BoringSSL.
 208   if (BN_cmp(num_blocks_until_reset.get(), num_output_blocks.get()) >= 0) {
 209     return AesCtrEncrypt128BitCounter(cipher, CryptoData(raw_key), data,
 210                                       counter_block, vector_as_array(buffer));
 211   }
 212
 213   // Otherwise the encryption needs to be done in 2 parts. The first part using
 214   // the current counter_block, and the next part resetting the counter portion
 215   // of the block to zero.
 216
 217   // This is guaranteed to fit in an "unsigned int" because input size in bytes
 218   // fits in an "unsigned int".
 219   BN_ULONG num_blocks_part1 = BN_get_word(num_blocks_until_reset.get());
 220   BN_ULONG input_size_part1 = num_blocks_part1 * AES_BLOCK_SIZE;
 221   DCHECK_LT(input_size_part1, data.byte_length());
 222
 223   // Encrypt the first part (before wrap-around).
 224   Status status = AesCtrEncrypt128BitCounter(
 225       cipher, CryptoData(raw_key), CryptoData(data.bytes(), input_size_part1),
 226       counter_block, vector_as_array(buffer));
 227   if (status.IsError())
 228     return status;
 229
 230   // Encrypt the second part (after wrap-around).
 231   std::vector<uint8_t> counter_block_part2 =
 232       BlockWithZeroedCounter(counter_block, counter_length_bits);
 233
 234   return AesCtrEncrypt128BitCounter(
 235       cipher, CryptoData(raw_key),
 236       CryptoData(data.bytes() + input_size_part1,
 237                  data.byte_length() - input_size_part1),
 238       CryptoData(counter_block_part2),
 239       vector_as_array(buffer) + input_size_part1);
 240 }
 241
 242 class AesCtrImplementation : public AesAlgorithm {
 243  public:
 244   AesCtrImplementation() : AesAlgorithm("CTR") {}
 245
 246   Status Encrypt(const blink::WebCryptoAlgorithm& algorithm,
 247                  const blink::WebCryptoKey& key,
 248                  const CryptoData& data,
 249                  std::vector<uint8_t>* buffer) const override {
 250     return AesCtrEncryptDecrypt(algorithm, key, data, buffer);
 251   }
 252
 253   Status Decrypt(const blink::WebCryptoAlgorithm& algorithm,
 254                  const blink::WebCryptoKey& key,
 255                  const CryptoData& data,
 256                  std::vector<uint8_t>* buffer) const override {
 257     return AesCtrEncryptDecrypt(algorithm, key, data, buffer);
 258   }
 259 };
 260
 261 }  // namespace
 262
 263 scoped_ptr<AlgorithmImplementation> CreateAesCtrImplementation() {
 264   return make_scoped_ptr(new AesCtrImplementation);
 265 }
 266
 267 }  // namespace webcrypto