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// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2019 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or
#include <crypto/chacha20.h>
#include <crypto/common.h>
#include <uint256.h>
#include <chrono> // For std::chrono::microseconds
#include <cstdint>
#include <limits>
* 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.
void GetRandBytes(unsigned char* buf, int num) noexcept;
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;
* 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.
void GetStrongRandBytes(unsigned char* buf, int num) noexcept;
* Gather entropy from various expensive sources, and feed them to the PRNG state.
* Thread-safe.
void RandAddPeriodic() 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.
void RandAddEvent(const uint32_t event_info) noexcept;
* Fast randomness source. This is seeded once with secure random data, but
* is completely deterministic and does not gather more entropy after that.
* This class is not thread-safe.
class FastRandomContext {
bool requires_seed;
ChaCha20 rng;
unsigned char bytebuf[64];
int bytebuf_size;
uint64_t bitbuf;
int bitbuf_size;
void RandomSeed();
void FillByteBuffer()
if (requires_seed) {
rng.Keystream(bytebuf, sizeof(bytebuf));
bytebuf_size = sizeof(bytebuf);
void FillBitBuffer()
bitbuf = rand64();
bitbuf_size = 64;
explicit FastRandomContext(bool fDeterministic = false) noexcept;
/** Initialize with explicit seed (only for testing) */
explicit FastRandomContext(const uint256& seed) noexcept;
// Do not permit copying a FastRandomContext (move it, or create a new one to get reseeded).
FastRandomContext(const FastRandomContext&) = delete;
FastRandomContext(FastRandomContext&&) = delete;
FastRandomContext& operator=(const FastRandomContext&) = delete;
/** Move a FastRandomContext. If the original one is used again, it will be reseeded. */
FastRandomContext& operator=(FastRandomContext&& from) noexcept;
/** Generate a random 64-bit integer. */
uint64_t rand64() noexcept
if (bytebuf_size < 8) FillByteBuffer();
uint64_t ret = ReadLE64(bytebuf + 64 - bytebuf_size);
bytebuf_size -= 8;
return ret;
/** Generate a random (bits)-bit integer. */
uint64_t randbits(int bits) noexcept {
if (bits == 0) {
return 0;
} else if (bits > 32) {
return rand64() >> (64 - bits);
} else {
if (bitbuf_size < bits) FillBitBuffer();
uint64_t ret = bitbuf & (~(uint64_t)0 >> (64 - bits));
bitbuf >>= bits;
bitbuf_size -= bits;
return ret;
/** Generate a random integer in the range [0..range). */
uint64_t randrange(uint64_t range) noexcept
int bits = CountBits(range);
while (true) {
uint64_t ret = randbits(bits);
if (ret <= range) return ret;
/** Generate random bytes. */
std::vector<unsigned char> randbytes(size_t len);
/** Generate a random 32-bit integer. */
uint32_t rand32() noexcept { return randbits(32); }
/** generate a random uint256. */
uint256 rand256() noexcept;
/** Generate a random boolean. */
bool randbool() noexcept { return randbits(1); }
// Compatibility with the C++11 UniformRandomBitGenerator concept
typedef uint64_t result_type;
static constexpr uint64_t min() { return 0; }
static constexpr uint64_t max() { return std::numeric_limits<uint64_t>::max(); }
inline uint64_t operator()() noexcept { return rand64(); }
/** 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
template<typename I, typename R>
void Shuffle(I first, I last, R&& rng)
while (first != last) {
size_t j = rng.randrange(last - first);
if (j) {
using std::swap;
swap(*first, *(first + j));
/* 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).
static const int NUM_OS_RANDOM_BYTES = 32;
/** Get 32 bytes of system entropy. Do not use this in application code: use
* GetStrongRandBytes instead.
void GetOSRand(unsigned char *ent32);
/** Check that OS randomness is available and returning the requested number
* of bytes.
bool Random_SanityCheck();
* 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.
void RandomInit();
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