Unpredictable random numbers
12345678 | #include <random>
int main() {
std::random_device r;
std::seed_seq seed_seq{r(), r(), r(), r(), r(), r()};
std::mt19937 engine{seed_seq};
} |
This pattern is licensed under the CC0 Public Domain Dedication.
Intent
Seed a random number engine with greater unpredictability.
Description
A pseudo-random number engine is initialised with a seed, which determines the initial state of the engine. When seeding with a single 32 bit value (as is typically provided by std::random_device), the engine can be in one of only 232 states. By providing an an engine with a greater amount of random seed data, we increase the number of possible states and therefore the unpredicitability of the engine.
On line 4, we create a std::random_device as a source of
non-deterministic random numbers (if such a source is available).
On line 5, we initialise a std::seed_seq
with several initial seeds produced by the std::random_device.
On line 7, we seed a random number engine with this std::seed_seq,
which provides the engine with a sequence of evenly
distributed seeds with low bias (although not
bias-free).
Providing more initial seeds will increase the unpredictability of
the engine by increasing the number of possible seed sequences that
might be generated.
While a std::seed_seq can be used to initialize multiple random
number engines, bear in mind that a particular std::seed_seq will
produce the same sequence of seeds each time it is used.
Note: An unpredictable random number engine is not necessarily cryptographically secure.
