random.h File Reference

#include <crypto/chacha20.h>
#include <crypto/common.h>
#include <uint256.h>
#include <stdint.h>
#include <limits>

Go to the source code of this file.

Classes
class	FastRandomContext

Functions
void	GetRandBytes (unsigned char *buf, int num) noexcept
uint64_t	GetRand (uint64_t nMax) noexcept
int	GetRandInt (int nMax) noexcept
uint256	GetRandHash () noexcept
void	GetStrongRandBytes (unsigned char *buf, int num) noexcept
void	RandAddSeedSleep ()
template<typename I , typename R >
void	Shuffle (I first, I last, R &&rng)
void	GetOSRand (unsigned char *ent32)
bool	Random_SanityCheck ()
void	RandomInit ()

Variables
static const int	NUM_OS_RANDOM_BYTES = 32

Function Documentation

GetOSRand()

void GetOSRand

(

unsigned char *

ent32

)

Get 32 bytes of system entropy. Do not use this in application code: use GetStrongRandBytes instead.

Get 32 bytes of system entropy.

GetRand()

uint64_t GetRand ( uint64_t  nMax )

noexcept

GetRandBytes()

void GetRandBytes ( unsigned char *  buf, int  num  )

noexcept

Overall design of the RNG and entropy sources.

We maintain a single global 256-bit RNG state for all high-quality randomness. The following (classes of) functions interact with that state by mixing in new entropy, and optionally extracting random output from it:

• The GetRand*() class of functions, as well as construction of FastRandomContext objects, perform 'fast' seeding, consisting of mixing in:
  • A stack pointer (indirectly committing to calling thread and call stack)
  • A high-precision timestamp (rdtsc when available, c++ high_resolution_clock otherwise)
  • 64 bits from the hardware RNG (rdrand) when available. These entropy sources are very fast, and only designed to protect against situations where a VM state restore/copy results in multiple systems with the same randomness. FastRandomContext on the other hand does not protect against this once created, but is even faster (and acceptable to use inside tight loops).
• The GetStrongRand*() class of function perform 'slow' seeding, including everything that fast seeding includes, but additionally:
  • OS entropy (/dev/urandom, getrandom(), ...). The application will terminate if this entropy source fails.
  • Bytes from OpenSSL's RNG (which itself may be seeded from various sources)
  • Another high-precision timestamp (indirectly committing to a benchmark of all the previous sources). These entropy sources are slower, but designed to make sure the RNG state contains fresh data that is unpredictable to attackers.
• RandAddSeedSleep() seeds everything that fast seeding includes, but additionally:
  • A high-precision timestamp before and after sleeping 1ms.
  • (On Windows) Once every 10 minutes, performance monitoring data from the OS.

- Once every minute, strengthen the entropy for 10 ms using repeated SHA512. These just exploit the fact the system is idle to improve the quality of the RNG slightly.

On first use of the RNG (regardless of what function is called first), all entropy sources used in the 'slow' seeder are included, but also:

• 256 bits from the hardware RNG (rdseed or rdrand) when available.
• (On Windows) Performance monitoring data from the OS.
• (On Windows) Through OpenSSL, the screen contents.
• Strengthen the entropy for 100 ms using repeated SHA512.

When mixing in new entropy, H = SHA512(entropy || old_rng_state) is computed, and (up to) the first 32 bytes of H are produced as output, while the last 32 bytes become the new RNG state.Generate random data via the internal PRNG.

These functions are designed to be fast (sub microsecond), but do not necessarily meaningfully add entropy to the PRNG state.

Thread-safe.

GetRandHash()

uint256 GetRandHash ( )

noexcept

GetRandInt()

int GetRandInt ( int  nMax )

noexcept

GetStrongRandBytes()

void GetStrongRandBytes ( unsigned char *  buf, int  num  )

noexcept

Gather entropy from various sources, feed it into the internal PRNG, and generate random data using it.

This function will cause failure whenever the OS RNG fails.

Thread-safe.

RandAddSeedSleep()

void RandAddSeedSleep

(

)

Sleep for 1ms, gather entropy from various sources, and feed them to the PRNG state.

Thread-safe.

Random_SanityCheck()

bool Random_SanityCheck

(

)

Check that OS randomness is available and returning the requested number of bytes.

RandomInit()

void RandomInit

(

)

Initialize global RNG state and log any CPU features that are used.

Calling this function is optional. RNG state will be initialized when first needed if it is not called.

Shuffle()

template<typename I , typename R >

void Shuffle	(	I	first,
		I	last,
		R &&	rng
	)

More efficient than using std::shuffle on a FastRandomContext.

This is more efficient as std::shuffle will consume entropy in groups of 64 bits at the time and throw away most.

This also works around a bug in libstdc++ std::shuffle that may cause type::operator=(type&&) to be invoked on itself, which the library's debug mode detects and panics on. This is a known issue, see https://stackoverflow.com/questions/22915325/avoiding-self-assignment-in-stdshuffle

Variable Documentation

NUM_OS_RANDOM_BYTES

const int NUM_OS_RANDOM_BYTES = 32

static