Random number generators and adapters
Computers cannot produce random numbers from nowhere. We classify random number generators as follows:
- “True” random number generators (TRNGs) use hard-to-predict data sources (e.g. the high-resolution parts of event timings and sensor jitter) to harvest random bit-sequences, apply algorithms to remove bias and estimate available entropy, then combine these bits into a byte-sequence or an entropy pool. This job is usually done by the operating system or a hardware generator (HRNG).
- “Pseudo”-random number generators (PRNGs) use algorithms to transform a seed into a sequence of pseudo-random numbers. These generators can be fast and produce well-distributed unpredictable random numbers (or not). They are usually deterministic: given algorithm and seed, the output sequence can be reproduced. They have finite period and eventually loop; with many algorithms this period is fixed and can be proven sufficiently long, while others are chaotic and the period depends on the seed.
- “Cryptographically secure” pseudo-random number generators (CSPRNGs) are the sub-set of PRNGs which are secure. Security of the generator relies both on hiding the internal state and using a strong algorithm.
All PRNGs require a seed to produce their random number sequence. The
SeedableRng trait provides three ways of constructing PRNGs:
from_seedaccepts a type specific to the PRNG
from_rngallows a PRNG to be seeded from any other RNG
seed_from_u64allows any PRNG to be seeded from a
from_entropysecurely seeds a PRNG from fresh entropy
rand_core crate when implementing your own RNGs.
This crate provides several random number generators:
OsRngis an interface to the operating system’s random number source. Typically the operating system uses a CSPRNG with entropy provided by a TRNG and some type of on-going re-seeding.
ThreadRng, provided by the
thread_rngfunction, is a handle to a thread-local CSPRNG with periodic seeding from
OsRng. Because this is local, it is typically much faster than
OsRng. It should be secure, though the paranoid may prefer
StdRngis a CSPRNG chosen for good performance and trust of security (based on reviews, maturity and usage). The current algorithm is ChaCha12, which is well established and rigorously analysed.
StdRngprovides the algorithm used by
ThreadRngbut without periodic reseeding.
SmallRng] is an insecure PRNG designed to be fast, simple, require little memory, and have good output quality.
The algorithms selected for
StdRng and [
SmallRng] may change in any
release and may be platform-dependent, therefore they should be considered
rdrand crate provides an interface to the RDRAND and
RDSEED instructions available in modern Intel and AMD CPUs.
rand_jitter crate provides a user-space implementation of
entropy harvesting from CPU timer jitter, but is very slow and has
PRNGs: Several companion crates are available, providing individual or
families of PRNG algorithms. These provide the implementations behind
StdRng and [
SmallRng] but can also be used directly, indeed should
be used directly when reproducibility matters.
Some suggestions are:
A full list can be found by searching for crates with the
A random number generator that retrieves randomness from the operating system.
The standard RNG. The PRNG algorithm in
StdRng is chosen to be efficient
on the current platform, to be statistically strong and unpredictable
(meaning a cryptographically secure PRNG).
A reference to the thread-local generator