smallrand

Random number generation with absolutely minimal dependencies and no unsafe code.

This crate provides a lightweight alternative to rand. It implements the same two algorithms as rand's SmallRng and StdRng (Xoshiro256++ and ChaCha12), using the same aliases, and provides all basic functions you expect including uniformly distributed integers and floats in a user-specified range. Those who are sometimes frustrated by rand's API might prefer smallrand's API.

The crate is intended to be easy to audit. It is small and uses no unsafe code. Its only dependency is getrandom, and that is only used on non-Linux/Unix platforms.

It can also be built as no-std, in which case you'll have to provide your own seeds.

Quick start

use smallrand::StdRng;
let mut rng = StdRng::new();
let coin_flip : bool = rng.random();
let some_int = rng.random::<u32>();
let uniformly_distributed : u32 = rng.range(0..=42);
let a_float : f64 = rng.range(0.0..42.0);

FAQ

• Where does the seed come from?
  • By default, the seed is read from /dev/urandom on Linux-like platforms, and comes from the getrandom crate for others. You can also implement your own EntropySource and use that to provide the seed.
• Why don't you get the seeds from hash_map::RandomState like fastrand does and remove the dependency on getrandom?
  • RandomState reads 128 bits of entropy from the system's entropy source at startup. It then uses a non-secure algorithm to derive more seeds from that. This provides limited entropy and is not good enough as a default for everyone. However, you can opt out of depending on getrandom by building without the allow-getrandom feature flag, in which case RandomState will be used. Note that /dev/urandom is always used on Unix-like platforms, regardless.
• Why would I choose this over rand?
  • rand is large and difficult to audit. Its dependencies (as of version 0.9) include zerocopy, which contains a huge amount of unsafe code.
  • Its API encourages you to use thread local RNG instances. This creates unnecessary (thread) global state, which is almost always a bad idea. Since it is thread local, you also get one RNG per thread in the thread pool if your code is async.
  • Unlike rand, smallrand crate does not require you to import any traits or anything else beyond the RNG you're using.
  • This crate has minimal dependencies and does not intend to change much, so you won't have to update it very often.
  • This crate compiles faster than rand due to its smaller size and minimal dependencies.
• Why would I choose this over fastrand?
  • If you think the algorithms used are preferable to Wyrand.
  • fastrand gets its entropy from std::collections::hash_map::RandomState. This provides somewhat limited entropy (see above), although perhaps enough to initialize Wyrand given its smaller state.
  • Just like rand its API encourages you to use thread local RNG instances.
• How fast is this compared to rand?
  • smallrand seems to be slightly faster overall on a Apple M1 (see Speed below).
• Is the StdRng cryptographically secure?
  • Just as with StdRng in rand it might be (depending on how you define the term), but this not in any way guaranteed. See also the next section.
• Can this be used "no-std"?
  • Yes, please see the crate documentation for an example.

Security

StdRng uses the ChaCha crypto algorithm with 12 rounds. Current thinking seems to be that 8 rounds is sufficient (Too Much Crypto), but 12 is currently used for extra security margin. This algorithm is well respected and is currently unbroken, and is as such not predictable. It can likely be used to implement random generators that are cryptographically secure in practice, but please note that no guarantees of any kind are made that this particular implementation is cryptographically secure.

Also note that for a random generator implementation to be certifiable as cryptographically secure, it needs to be implemented according to NIST SP 800-90A. ChaCha is not one of the approved algorithms allowed by NIST SP 800-90A.

SmallRng uses Xoshiro256++ which is a predictable RNG. An attacker that is able to observe its output will be able to calculate its internal state and predict its output, which means that it is not cryptographically secure. It has this in common with other algorithms of similar size and complexity, like PCG and Wyrand.

smallrand makes a modest effort to detect fatal failures of the entropy source when creating an StdRng with new(), including the Health Tests of NIST SP 800-90B.

Speed

smallrand has been benchmarked against the v.0.9 of the rand crate using criterion on a MacBook Air M1:

Algorithm	Operation	rand	smallrand
SmallRng (Xoshiro256++)	generate u64	1.145ns	1.141ns
SmallRng (Xoshiro256++)	fill 256 bytes	38.66ns	35.99ns
SmallRng (Xoshiro256++)	range (u64)	3.84ns	1.46ns
SmallRng (Xoshiro256++)	range (f64)	1.17ns	1.24ns
StdRng (Chacha 12)	fill 256 bytes	254.8ns	233.1ns
StdRng (Chacha 12)	generate u64	8.64ns	7.32ns

In these benchmarks, smallrand is a little faster overall than rand on this platform, although rand is a little faster at generating uniformly distributed f64 in a specified range. This could be because smallrand uses a different algorithm that uses the full dynamic range of the mantissa even for very small values (rand does not). On the other hand, smallrand is 2.6 times faster then rand at generating uniformly distributed u64 in a specified range (using SmallRng).