Crate rand_os

source ·
Expand description

Interface to the random number generator of the operating system.

OsRng is the preferred external source of entropy for most applications. Commonly it is used to initialize a user-space RNG, which can then be used to generate random values with much less overhead than OsRng.

You may prefer to use EntropyRng instead of OsRng. It is unlikely, but not entirely theoretical, for OsRng to fail. In such cases EntropyRng falls back on a good alternative entropy source.

OsRng::new() is guaranteed to be very cheap (after the first successful call), and will never consume more than one file handle per process.

§Usage example

use rand_os::OsRng;
use rand_os::rand_core::RngCore;

let mut os_rng = OsRng::new().unwrap();
let mut key = [0u8; 16];
os_rng.fill_bytes(&mut key);
let random_u64 = os_rng.next_u64();

§Platform sources

Linux, Androidgetrandom system call if available, otherwise /dev/urandom after reading from /dev/random once
macOS, iOSSecRandomCopyBytes
OpenBSD, Bitriggetentropy
NetBSD/dev/urandom after reading from /dev/random once
Dragonfly BSD/dev/random
Solaris, illumosgetrandom system call if available, otherwise /dev/random
Fuchsia OScprng_draw
Haiku/dev/random (identical to /dev/urandom)
Web browsersCrypto.getRandomValues (see Support for WebAssembly and ams.js)
Node.jscrypto.randomBytes (see Support for WebAssembly and ams.js)

Rand doesn’t have a blanket implementation for all Unix-like operating systems that reads from /dev/urandom. This ensures all supported operating systems are using the recommended interface and respect maximum buffer sizes.

§Support for WebAssembly and ams.js

The three Emscripten targets asmjs-unknown-emscripten, wasm32-unknown-emscripten and wasm32-experimental-emscripten use Emscripten’s emulation of /dev/random on web browsers and Node.js.

The bare WASM target wasm32-unknown-unknown tries to call the javascript methods directly, using either stdweb or wasm-bindgen depending on what features are activated for this crate. Note that if both features are enabled wasm-bindgen will be used.

§Early boot

It is possible that early in the boot process the OS hasn’t had enough time yet to collect entropy to securely seed its RNG, especially on virtual machines.

Some operating systems always block the thread until the RNG is securely seeded. This can take anywhere from a few seconds to more than a minute. Others make a best effort to use a seed from before the shutdown and don’t document much.

A few, Linux, NetBSD and Solaris, offer a choice between blocking, and getting an error. With try_fill_bytes we choose to get the error (ErrorKind::NotReady), while the other methods use a blocking interface.

On Linux (when the genrandom system call is not available) and on NetBSD reading from /dev/urandom never blocks, even when the OS hasn’t collected enough entropy yet. As a countermeasure we try to do a single read from /dev/random until we know the OS RNG is initialized (and store this in a global static).

§Panics and error handling

We cannot guarantee that OsRng will fail, but if it does, it will likely be either when OsRng::new() is first called or when data is first read. If you wish to catch errors early, then test reading of at least one byte from OsRng via try_fill_bytes. If this succeeds, it is extremely unlikely that any further errors will occur.

Only try_fill_bytes is able to report the cause of an error; the other RngCore methods may (depending on the error kind) retry several times, but must eventually panic if the error persists.



  • A random number generator that retrieves randomness straight from the operating system.