/*

* ChaCha_RNG

* (C) 2017 Jack Lloyd

*

* Botan is released under the Simplified BSD License (see license.txt)

*/

#ifndef BOTAN_CHACHA_RNG_H_

#define BOTAN_CHACHA_RNG_H_

#include <botan/stateful_rng.h>

#include <botan/stream_cipher.h>

#include <botan/mac.h>

namespace Botan {

class Entropy_Sources;

/**

* ChaCha_RNG is a very fast but completely ad-hoc RNG created by

* creating a 256-bit random value and using it as a key for ChaCha20.

*

* The RNG maintains two 256-bit keys, one for HMAC_SHA256 (HK) and the

* other for ChaCha20 (CK). To compute a new key in response to

* reseeding request or add_entropy calls, ChaCha_RNG computes

*   CK' = HMAC_SHA256(HK, input_material)

* Then a new HK' is computed by running ChaCha20 with the new key to

* output 32 bytes:

*   HK' = ChaCha20(CK')

*

* Now output can be produced by continuing to produce output with ChaCha20

* under CK'

*

* The first HK (before seeding occurs) is taken as the all zero value.

*

* @warning This RNG construction is probably fine but is non-standard.

* The primary reason to use it is in cases where the other RNGs are

* not fast enough.

*/

class BOTAN_PUBLIC_API(2,3) ChaCha_RNG final : public Stateful_RNG

   {

   public:

      /**

      * Automatic reseeding is disabled completely, as it has no access to

      * any source for seed material.

      *

      * If a fork is detected, the RNG will be unable to reseed itself

      * in response. In this case, an exception will be thrown rather

      * than generating duplicated output.

      */

      ChaCha_RNG();

      /**

      * Provide an initial seed to the RNG, without providing an

      * underlying RNG or entropy source. Automatic reseeding is

      * disabled completely, as it has no access to any source for

      * seed material.

      *

      * If a fork is detected, the RNG will be unable to reseed itself

      * in response. In this case, an exception will be thrown rather

      * than generating duplicated output.

      *

      * @param seed the seed material, should be at least 256 bits

      */

      ChaCha_RNG(const secure_vector<uint8_t>& seed);

      /**

      * Automatic reseeding from @p underlying_rng will take place after

      * @p reseed_interval many requests or after a fork was detected.

      *

      * @param underlying_rng is a reference to some RNG which will be used

      * to perform the periodic reseeding

      * @param reseed_interval specifies a limit of how many times

      * the RNG will be called before automatic reseeding is performed

      */

      ChaCha_RNG(RandomNumberGenerator& underlying_rng,

                 size_t reseed_interval = BOTAN_RNG_DEFAULT_RESEED_INTERVAL);

      /**

      * Automatic reseeding from @p entropy_sources will take place after

      * @p reseed_interval many requests or after a fork was detected.

      *

      * @param entropy_sources will be polled to perform reseeding periodically

      * @param reseed_interval specifies a limit of how many times

      * the RNG will be called before automatic reseeding is performed.

      */

      ChaCha_RNG(Entropy_Sources& entropy_sources,

                 size_t reseed_interval = BOTAN_RNG_DEFAULT_RESEED_INTERVAL);

      /**

      * Automatic reseeding from @p underlying_rng and @p entropy_sources

      * will take place after @p reseed_interval many requests or after

      * a fork was detected.

      *

      * @param underlying_rng is a reference to some RNG which will be used

      * to perform the periodic reseeding

      * @param entropy_sources will be polled to perform reseeding periodically

      * @param reseed_interval specifies a limit of how many times

      * the RNG will be called before automatic reseeding is performed.

      */

      ChaCha_RNG(RandomNumberGenerator& underlying_rng,

                 Entropy_Sources& entropy_sources,

                 size_t reseed_interval = BOTAN_RNG_DEFAULT_RESEED_INTERVAL);

      std::string name() const override { return "ChaCha_RNG"; }

      void clear() override;

      void randomize(uint8_t output[], size_t output_len) override;

      void randomize_with_input(uint8_t output[], size_t output_len,

                                const uint8_t input[], size_t input_len) override;

      void add_entropy(const uint8_t input[], size_t input_len) override;

      size_t security_level() const override;

      size_t max_number_of_bytes_per_request() const override { return 0; }

   private:

      void update(const uint8_t input[], size_t input_len);

      std::unique_ptr<MessageAuthenticationCode> m_hmac;

      std::unique_ptr<StreamCipher> m_chacha;

   };

}

#endif