rv_16_random_bits is not deterministic and uses /dev/urandom

[Timmmm]

This function is used to provide random data to the seed CSR:

// Provides entropy for the scalar cryptography extension.
uint64_t rv_16_random_bits(void)
{
  // This function can be changed to support deterministic sequences of
  // pseudo-random bytes. This is useful for testing.
  const char *name = "/dev/urandom";
  FILE *f = fopen(name, "rb");
  uint16_t val;
  if (fread(&val, 2, 1, f) != 1) {
    fprintf(stderr, "Unable to read 2 bytes from %s\n", name);
  }
  fclose(f);
  return (uint64_t)val;
}

It seems like the author was aware that just using /dev/urandom was a bad idea. Since we are using C++ now a better implementation is trivial:

// Provides entropy for the scalar cryptography extension.
uint64_t rv_16_random_bits()
{
  static std::mt19937_64 rng(0);
  return static_cast<uint16_t>(rng());
}

This is deterministic and should give identical results across different compilers/C++ libraries (weirdly only some C++ PRNGs are guarnateed to do that). The global static is unfortunate but we can solve that later.