random.h File Reference

#include <crypto/chacha20.h>
#include <crypto/common.h>
#include <span.h>
#include <uint256.h>
#include <util/check.h>
#include <bit>
#include <cassert>
#include <chrono>
#include <concepts>
#include <cstdint>
#include <limits>
#include <type_traits>
#include <vector>

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Classes
class	RandomMixin< T >
	===================== RANDOM NUMBER GENERATION CLASSES ===================== More...
class	FastRandomContext
	Fast randomness source. More...
class	InsecureRandomContext
	xoroshiro128++ PRNG. More...

Concepts
concept	RandomNumberGenerator
	A concept for RandomMixin-based random number generators.
concept	StdChronoDuration
	A concept for C++ std::chrono durations.

Functions
void	RandomInit ()
	Overall design of the RNG and entropy sources. More...
void	RandAddPeriodic () noexcept
	Gather entropy from various expensive sources, and feed them to the PRNG state. More...
void	RandAddEvent (const uint32_t event_info) noexcept
	Gathers entropy from the low bits of the time at which events occur. More...
void	GetRandBytes (Span< uint8_t > bytes) noexcept
	================== BASE RANDOMNESS GENERATION FUNCTIONS ==================== More...
void	GetStrongRandBytes (Span< uint8_t > bytes) noexcept
	Gather entropy from various sources, feed it into the internal PRNG, and generate random data using it. More...
double	MakeExponentiallyDistributed (uint64_t uniform) noexcept
	Given a uniformly random uint64_t, return an exponentially distributed double with mean 1. More...
uint256	GetRandHash () noexcept
	========== CONVENIENCE FUNCTIONS FOR COMMONLY USED RANDOMNESS ========== More...
template<typename I , RandomNumberGenerator R>
void	Shuffle (I first, I last, R &&rng)
	More efficient than using std::shuffle on a FastRandomContext. More...
bool	Random_SanityCheck ()
	=============== MISCELLANEOUS TEST-ONLY FUNCTIONS ====================== More...

Function Documentation

GetRandBytes()

void GetRandBytes ( Span< uint8_t >  bytes )

noexcept

================== BASE RANDOMNESS GENERATION FUNCTIONS ====================

All produced randomness is eventually generated by one of these functions. 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.

In test mode (see SeedRandomForTest in src/test/util/random.h), the normal PRNG state is bypassed, and a deterministic, seeded, PRNG is used instead.

Thread-safe.

Definition at line 690 of file random.cpp.

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GetRandHash()

uint256 GetRandHash ( )

inlinenoexcept

========== CONVENIENCE FUNCTIONS FOR COMMONLY USED RANDOMNESS ==========

Generate a random uint256.

Definition at line 494 of file random.h.

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GetStrongRandBytes()

void GetStrongRandBytes ( Span< uint8_t >  bytes )

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.

The normal PRNG is never bypassed here, even in test mode.

Thread-safe.

Definition at line 695 of file random.cpp.

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MakeExponentiallyDistributed()

double MakeExponentiallyDistributed ( uint64_t  uniform )

noexcept

Given a uniformly random uint64_t, return an exponentially distributed double with mean 1.

Definition at line 803 of file random.cpp.

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RandAddEvent()

void RandAddEvent ( const uint32_t  event_info )

noexcept

Gathers entropy from the low bits of the time at which events occur.

Should be called with a uint32_t describing the event at the time an event occurs.

Thread-safe.

Definition at line 704 of file random.cpp.

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RandAddPeriodic()

void RandAddPeriodic ( )

noexcept

Gather entropy from various expensive sources, and feed them to the PRNG state.

Thread-safe.

Definition at line 700 of file random.cpp.

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Random_SanityCheck()

bool Random_SanityCheck

(

)

=============== MISCELLANEOUS TEST-ONLY FUNCTIONS ======================

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

This does not measure the quality of randomness, but it does test that GetOSRand() overwrites all 32 bytes of the output given a maximum number of tries.

Loop until all bytes have been overwritten at least once, or max number tries reached.

Definition at line 730 of file random.cpp.

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RandomInit()

void RandomInit

(

)

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:

• GetRandBytes, GetRandHash, GetRandDur, 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 GetStrongRandBytes() function performs 'slow' seeding, including everything that fast seeding includes, but additionally:
  • OS entropy (/dev/urandom, getrandom(), ...). The application will terminate if this entropy source fails.
  • 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.
• RandAddPeriodic() seeds everything that fast seeding includes, but additionally:
  • A high-precision timestamp
  • Dynamic environment data (performance monitoring, ...)
  • Strengthen the entropy for 10 ms using repeated SHA512. This is run once every minute.
• 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.
  • Dynamic environment data (performance monitoring, ...)
  • Static environment data
  • 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.

During tests, the RNG can be put into a special deterministic mode, in which the output of all RNG functions, with the exception of GetStrongRandBytes(), is replaced with the output of a deterministic RNG. This deterministic RNG does not gather entropy, and is unaffected by RandAddPeriodic() or RandAddEvent(). It produces pseudorandom data that only depends on the seed it was initialized with, possibly until it is reinitialized. ==================== INITIALIZATION AND ADDING ENTROPY ================= 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.

Definition at line 796 of file random.cpp.

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Shuffle()

template<typename I , RandomNumberGenerator 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

Definition at line 512 of file random.h.

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