// Copyright 2019 Google LLC. All Rights Reserved. This file and proprietary
// source code may only be used and distributed under the Widevine License
// Agreement.

#ifndef WVCDM_UTIL_ADVANCE_IV_CTR_H_
#define WVCDM_UTIL_ADVANCE_IV_CTR_H_

#include <stdint.h>
#include <string.h>

#include "string_conversions.h"

namespace wvutil {

// Advance an IV according to ISO-CENC's CTR modes. The lower half of the IV is
// split off and treated as an unsigned 64-bit integer, then incremented by the
// number of complete crypto blocks decrypted. The resulting value is then
// copied back into the IV over the previous lower half.
inline void AdvanceIvCtr(uint8_t (*subsample_iv)[16], size_t bytes) {
  constexpr size_t kAesBlockSize = 16;
  constexpr size_t kIvSize = kAesBlockSize;
  constexpr size_t kCounterIndex = kIvSize / 2;
  constexpr size_t kCounterSize = kIvSize / 2;

  uint64_t counter;

  static_assert(
      sizeof(*subsample_iv) == kIvSize,
      "The subsample_iv field is no longer the length of an AES-128 IV.");
  static_assert(sizeof(counter) == kCounterSize,
                "A uint64_t failed to be half the size of an AES-128 IV.");

  // Defensive copy because the elements of the array may not be properly
  // aligned
  memcpy(&counter, &(*subsample_iv)[kCounterIndex], kCounterSize);

  const size_t increment =
      bytes / kAesBlockSize;  // The truncation here is intentional
  counter = htonll64(ntohll64(counter) + increment);

  memcpy(&(*subsample_iv)[kCounterIndex], &counter, kCounterSize);
}

}  // namespace wvutil

#endif  // WVCDM_UTIL_ADVANCE_IV_CTR_H_