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random.h File Reference
#include <crypto/chacha20.h>
#include <crypto/common.h>
#include <uint256.h>
#include <chrono>
#include <cstdint>
#include <limits>
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Classes

class  FastRandomContext
 Fast randomness source. More...
 

Functions

void GetRandBytes (uint8_t *buf, int num) noexcept
 Overall design of the RNG and entropy sources. More...
 
uint64_t GetRand (uint64_t nMax) noexcept
 
std::chrono::microseconds GetRandMicros (std::chrono::microseconds duration_max) noexcept
 
int GetRandInt (int nMax) noexcept
 
uint256 GetRandHash () noexcept
 
void GetStrongRandBytes (uint8_t *buf, int num) noexcept
 Gather entropy from various sources, feed it into the internal PRNG, and generate random data using it. 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...
 
template<typename I , typename R >
void Shuffle (I first, I last, R &&rng)
 More efficient than using std::shuffle on a FastRandomContext. More...
 
void GetOSRand (uint8_t *ent32)
 Get 32 bytes of system entropy. More...
 
bool Random_SanityCheck ()
 Check that OS randomness is available and returning the requested number of bytes. More...
 
void RandomInit ()
 Initialize global RNG state and log any CPU features that are used. More...
 

Variables

static const int NUM_OS_RANDOM_BYTES = 32
 Number of random bytes returned by GetOSRand. More...
 

Function Documentation

◆ GetOSRand()

void GetOSRand ( uint8_t *  ent32)

Get 32 bytes of system entropy.

Do not use this in application code: use GetStrongRandBytes instead.

Fall back to /dev/urandom if there is no specific method implemented to get system entropy for this OS.

Definition at line 315 of file random.cpp.

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◆ GetRand()

uint64_t GetRand ( uint64_t  nMax)
noexcept

Definition at line 641 of file random.cpp.

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◆ GetRandBytes()

void GetRandBytes ( uint8_t *  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.
    • 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. 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.

Definition at line 625 of file random.cpp.

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◆ GetRandHash()

uint256 GetRandHash ( )
noexcept

Definition at line 654 of file random.cpp.

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◆ GetRandInt()

int GetRandInt ( int  nMax)
noexcept

Definition at line 650 of file random.cpp.

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◆ GetRandMicros()

std::chrono::microseconds GetRandMicros ( std::chrono::microseconds  duration_max)
noexcept

Definition at line 646 of file random.cpp.

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◆ GetStrongRandBytes()

void GetStrongRandBytes ( uint8_t *  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.

Definition at line 628 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 635 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 631 of file random.cpp.

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◆ Random_SanityCheck()

bool Random_SanityCheck ( )

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 701 of file random.cpp.

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◆ 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.

Definition at line 781 of file random.cpp.

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◆ Shuffle()

template<typename I , typename R >
void Shuffle ( first,
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 230 of file random.h.

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Variable Documentation

◆ NUM_OS_RANDOM_BYTES

const int NUM_OS_RANDOM_BYTES = 32
static

Number of random bytes returned by GetOSRand.

When changing this constant make sure to change all call sites, and make sure that the underlying OS APIs for all platforms support the number. (many cap out at 256 bytes).

Definition at line 247 of file random.h.