/*
* (C) 2016 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#ifndef BOTAN_STATEFUL_RNG_H_
#define BOTAN_STATEFUL_RNG_H_

#include <botan/rng.h>

namespace Botan {

/**
* Inherited by RNGs which maintain in-process state, like HMAC_DRBG.
* On Unix these RNGs are vulnerable to problems with fork, where the
* RNG state is duplicated, and the parent and child process RNGs will
* produce identical output until one of them reseeds. Stateful_RNG
* reseeds itself whenever a fork is detected, or after a set number of
* bytes have been output.
*
* Not implemented by RNGs which access an external RNG, such as the
* system PRNG or a hardware RNG.
*/
class BOTAN_PUBLIC_API(2,0) Stateful_RNG : public RandomNumberGenerator
   {
   public:
      /**
      * @param 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
      */
      Stateful_RNG(RandomNumberGenerator& rng,
                   Entropy_Sources& entropy_sources,
                   size_t reseed_interval) :
         m_underlying_rng(&rng),
         m_entropy_sources(&entropy_sources),
         m_reseed_interval(reseed_interval)
         {}

      /**
      * @param 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
      */
      Stateful_RNG(RandomNumberGenerator& rng, size_t reseed_interval) :
         m_underlying_rng(&rng),
         m_reseed_interval(reseed_interval)
         {}

      /**
      * @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
      */
      Stateful_RNG(Entropy_Sources& entropy_sources, size_t reseed_interval) :
         m_entropy_sources(&entropy_sources),
         m_reseed_interval(reseed_interval)
         {}

      /**
      * In this case, automatic reseeding is impossible
      */
      Stateful_RNG() : m_reseed_interval(0) {}

      /**
      * Consume this input and mark the RNG as initialized regardless
      * of the length of the input or the current seeded state of
      * the RNG.
      */
      void initialize_with(const uint8_t input[], size_t length);

      bool is_seeded() const override final;

      bool accepts_input() const override final { return true; }

      /**
      * Mark state as requiring a reseed on next use
      */
      void force_reseed();

      void reseed_from_rng(RandomNumberGenerator& rng,
                           size_t poll_bits = BOTAN_RNG_RESEED_POLL_BITS) override final;

      /**
      * Overrides default implementation and also includes the current
      * process ID and the reseed counter.
      */
      void randomize_with_ts_input(uint8_t output[], size_t output_len) override final;

      /**
      * Poll provided sources for up to poll_bits bits of entropy
      * or until the timeout expires. Returns estimate of the number
      * of bits collected.
      */
      size_t reseed(Entropy_Sources& srcs,
                    size_t poll_bits = BOTAN_RNG_RESEED_POLL_BITS,
                    std::chrono::milliseconds poll_timeout = BOTAN_RNG_RESEED_DEFAULT_TIMEOUT) override;

      /**
      * @return intended security level of this DRBG
      */
      virtual size_t security_level() const = 0;

      /**
      * Some DRBGs have a notion of the maximum number of bytes per
      * request.  Longer requests (to randomize) will be treated as
      * multiple requests, and may initiate reseeding multiple times,
      * depending on the values of max_number_of_bytes_per_request and
      * reseed_interval(). This function returns zero if the RNG in
      * question does not have such a notion.
      *
      * @return max number of bytes per request (or zero)
      */
      virtual size_t max_number_of_bytes_per_request() const = 0;

      size_t reseed_interval() const { return m_reseed_interval; }

      void clear() override;

   protected:
      void reseed_check();

      /**
      * Called by a subclass to notify that a reseed has been
      * successfully performed.
      */
      void reset_reseed_counter() { m_reseed_counter = 1; }

   private:
      // A non-owned and possibly null pointer to shared RNG
      RandomNumberGenerator* m_underlying_rng = nullptr;

      // A non-owned and possibly null pointer to a shared Entropy_Source
      Entropy_Sources* m_entropy_sources = nullptr;

      const size_t m_reseed_interval;
      uint32_t m_last_pid = 0;

      /*
      * Set to 1 after a successful seeding, then incremented.  Reset
      * to 0 by clear() or a fork. This logic is used even if
      * automatic reseeding is disabled (via m_reseed_interval = 0)
      */
      size_t m_reseed_counter = 0;
   };

}

#endif