text: /*
   * Copyright 2015-2018 The OpenSSL Project Authors. All Rights Reserved.
   *
   * Licensed under the Apache License 2.0 (the "License").  You may not use
   * this file except in compliance with the License.  You can obtain a copy
   * in the file LICENSE in the source distribution or at
   * https://www.openssl.org/source/license.html
   */
  
  #ifndef OSSL_CRYPTO_CHACHA_H
  #define OSSL_CRYPTO_CHACHA_H
  # pragma once
  
  #include <stddef.h>
  
  /*
   * ChaCha20_ctr32 encrypts |len| bytes from |inp| with the given key and
   * nonce and writes the result to |out|, which may be equal to |inp|.
   * The |key| is not 32 bytes of verbatim key material though, but the
   * said material collected into 8 32-bit elements array in host byte
   * order. Same approach applies to nonce: the |counter| argument is
   * pointer to concatenated nonce and counter values collected into 4
   * 32-bit elements. This, passing crypto material collected into 32-bit
   * elements as opposite to passing verbatim byte vectors, is chosen for
   * efficiency in multi-call scenarios.
   */
  void ChaCha20_ctr32(unsigned char *out, const unsigned char *inp,
                      size_t len, const unsigned int key[8],
                      const unsigned int counter[4]);
  /*
   * You can notice that there is no key setup procedure. Because it's
   * as trivial as collecting bytes into 32-bit elements, it's reckoned
   * that below macro is sufficient.
   */
  #define CHACHA_U8TOU32(p)  ( \
                  ((unsigned int)(p)[0])     | ((unsigned int)(p)[1]<<8) | \
                  ((unsigned int)(p)[2]<<16) | ((unsigned int)(p)[3]<<24)  )
  
  #define CHACHA_KEY_SIZE         32
  #define CHACHA_CTR_SIZE         16
  #define CHACHA_BLK_SIZE         64
  
  #endif
  