How to Robustly Seed the mt19937 PRNG for Better Random Number Generation?

How to Seed the mt19937 PRNG Robustly

While commonly suggested, using std::random_device to seed an mt19937 PRNG has numerous shortcomings, including insufficient entropy and potential non-uniformity. This article presents a solution that overcomes these issues:

Portable and Thorough Seeding

The following approach utilizes a platform-specific source of cryptographic-strength random bytes (sysrandom) to seed the PRNG:

std::uint_least32_t seed;
sysrandom(&seed, sizeof(seed));
std::mt19937 gen(seed);

sysrandom is a wrapper function that retrieves random bytes from a suitable source, depending on the platform:

Windows:

• CryptGenRandom (provder: PROV_RSA_FULL)

Unix-Like:

• /dev/urandom (fallback: std::random_device if unavailable)

Other:

• std::random_device (recommended to avoid if possible)

By relying on cryptographic-quality sources, this solution ensures sufficient entropy and thorough seeding.

Comparison to Boost

Boost's random_device employs similar techniques (e.g., MS_DEF_PROV on Windows, /dev/urandom on *Nix), making the presented approach comparable in portability and quality.

Linux Specialization

For added security on Linux 3.17 , getrandom (which blocks if CSPRNG is not initialized) can be used instead of /dev/urandom:

#if defined(__linux__)...
size_t sysrandom(void* dst, size_t dstlen)
{
    int bytes = syscall(SYS_getrandom, dst, dstlen, 0);
    if (bytes != dstlen) { throw std::runtime_error(...); }
    return dstlen;
}
#elif defined(_WIN32)...
#else...
#endif

OpenBSD Caveat

Modern OpenBSD lacks /dev/urandom. Utilize getentropy instead:

#if defined(__OpenBSD__)...
size_t sysrandom(void* dst, size_t dstlen)
{
    int bytes = getentropy(dst, dstlen);
    if (bytes != dstlen) { throw std::runtime_error(...); }
    return dstlen;
}
#endif

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