message: Clarify the need for `ifdef`s in internal/aes_util and sort header includes.
diff: #include "absl/strings/string_view.h"
  #include "absl/types/span.h"
  #include "openssl/aes.h"
  -#include "tink/internal/util.h"
  -#include "tink/util/status.h"
  -#if !defined(OPENSSL_IS_BORINGSSL)
  +#ifndef OPENSSL_IS_BORINGSSL
  +// This is needed to use block128_f, which is necessary when OpenSSL is used.
  #include "openssl/modes.h"
  #endif
  +#include "tink/internal/util.h"
  #include "tink/util/secret_data.h"
  +#include "tink/util/status.h"
  namespace crypto {
  namespace tink {
  @@ -52,6 +53,10 @@ util::Status AesCtr128Crypt(absl::string_view data, uint8_t iv[AesBlockSize()],
  unsigned int num = 0;
  std::vector<uint8_t> ecount_buf(AesBlockSize(), 0);
  + // OpenSSL >= v1.1.0 public APIs no longer exposes an AES_ctr128_encrypt
  + // function; as an alternative we use CRYPTO_ctr128_encrypt when OpenSSL is
  + // used as a backend. The latter is not part of the public API of BoringSSL,
  + // so we must selectively compile using either of them.
  #ifdef OPENSSL_IS_BORINGSSL
  AES_ctr128_encrypt(reinterpret_cast<const uint8_t*>(data.data()),
  reinterpret_cast<uint8_t*>(out.data()), data.size(), key,
  