Expand description
Interface to the random number generator of the operating system.
Platform sources
| OS | interface |
|---|---|
| Linux, Android | getrandom system call if available, otherwise /dev/urandom after successfully polling /dev/random |
| Windows | RtlGenRandom |
| macOS | getentropy() if available, otherwise /dev/random (identical to /dev/urandom) |
| iOS | SecRandomCopyBytes |
| FreeBSD | getrandom() if available, otherwise kern.arandom |
| OpenBSD | getentropy |
| NetBSD | kern.arandom |
| Dragonfly BSD | /dev/random |
| Solaris, illumos | getrandom system call if available, otherwise /dev/random |
| Fuchsia OS | cprng_draw |
| Redox | rand: |
| CloudABI | cloudabi_sys_random_get |
| Haiku | /dev/random (identical to /dev/urandom) |
| L4RE, SGX, UEFI | RDRAND |
| Hermit | RDRAND as sys_rand is currently broken. |
| VxWorks | randABytes after checking entropy pool initialization with randSecure |
| Web browsers | Crypto.getRandomValues (see Support for WebAssembly and asm.js) |
| Node.js | crypto.randomBytes (see Support for WebAssembly and asm.js) |
| WASI | __wasi_random_get |
Getrandom 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.
Unsupported targets
By default, compiling getrandom for an unsupported target will result in
a compilation error. If you want to build an application which uses getrandom
for such target, you can either:
- Use
[replace]or[patch]section in yourCargo.tomlto switch to a custom implementation with a support of your target. - Enable the
dummyfeature to have getrandom use an implementation that always fails at run-time on unsupported targets.
Support for WebAssembly and asm.js
Getrandom supports all of Rust’s current wasm32 targets, and it works with
both Node.js and web browsers. The three Emscripten targets
asmjs-unknown-emscripten, wasm32-unknown-emscripten, and
wasm32-experimental-emscripten use Emscripten’s /dev/random emulation.
The WASI target wasm32-wasi uses the __wasi_random_get function
defined by the WASI standard.
Getrandom also supports wasm32-unknown-unknown by directly calling
JavaScript methods. Rust currently has two ways to do this: bindgen and
stdweb. Getrandom supports using either one by enabling the
wasm-bindgen or stdweb crate features. Note that if both features are
enabled, wasm-bindgen will be used. If neither feature is enabled, calls
to getrandom will always fail at runtime.
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; in these cases we always choose to block.
On Linux (when the getrandom system call is not available) and on NetBSD
reading from /dev/urandom never blocks, even when the OS hasn’t collected
enough entropy yet. To avoid returning low-entropy bytes, we first read from
/dev/random and only switch to /dev/urandom once this has succeeded.
Error handling
We always choose failure over returning insecure “random” bytes. In general,
on supported platforms, failure is highly unlikely, though not impossible.
If an error does occur, then it is likely that it will occur on every call to
getrandom, hence after the first successful call one can be reasonably
confident that no errors will occur.
On unsupported platforms, getrandom always fails. See the Error type
for more information on what data is returned on failure.
Structs
A small and no_std compatible error type.
Functions
Fill dest with random bytes from the system’s preferred random number
source.