getrandom: system's random number generator

getrandom is a Rust library for retrieving random data from (operating) system sources.

It is assumed that the system always provides high-quality, cryptographically secure random data, ideally backed by hardware entropy sources. This crate derives its name from the Linux getrandom syscall but is cross-platform, roughly supporting the same set of platforms as Rust's std library.

This is a low-level API. Most users should prefer using a higher-level random-number library like rand.

Usage

Add the getrandom dependency to your Cargo.toml file:

[dependencies]
getrandom = "0.3"

Then invoke the fill function on a byte buffer to fill it with random data:

fn get_random_u128() -> Result<u128, getrandom::Error> {
    let mut buf = [0u8; 16];
    getrandom::fill(&mut buf)?;
    Ok(u128::from_ne_bytes(buf))
}

Supported targets

Pull Requests that add support for new targets to getrandom are always welcome.

Opt-in backends

getrandom also provides optional (opt-in) backends, which allow users to customize the source of randomness based on their specific needs:

Opt-in backends can be enabled using the getrandom_backend configuration flag. The flag can be set either by specifying the rustflags field in .cargo/config.toml (note that it can be done on a per-target basis), or by using the RUSTFLAGS environment variable:

RUSTFLAGS='--cfg getrandom_backend="linux_getrandom"' cargo build

Enabling an opt-in backend will replace the backend used by default. Doing this for an incorrect target (e.g. using linux_getrandom while compiling for a Windows target) will result in a compilation error. Be extremely careful while using opt-in backends, as incorrect configuration may result in vulnerable applications or applications that always panic.

Note that using an opt-in backend in a library (e.g. for tests or benchmarks) WILL NOT have any effect on its downstream users.

WebAssembly support

This crate fully supports the WASI and Emscripten targets. However, the wasm32-unknown-unknown target (i.e. the target used by wasm-pack) is not automatically supported since, from the target name alone, we cannot deduce which JavaScript interface should be used (or if JavaScript is available at all).

Instead, if the wasm_js backend is enabled, this crate will assume that you are building for an environment containing JavaScript, and will call the appropriate methods. Both web browser (main window and Web Workers) and Node.js environments are supported, invoking the methods described above using the wasm-bindgen toolchain.

To enable the wasm_js backend, you can add the following lines to your project's .cargo/config.toml file:

[target.wasm32-unknown-unknown]
rustflags = ['--cfg', 'getrandom_backend="wasm_js"']

Node.js ES module support

Node.js supports both CommonJS modules and ES modules. Due to limitations in wasm-bindgen's module support, we cannot directly support ES Modules running on Node.js. However, on Node v15 and later, the module author can add a simple shim to support the Web Cryptography API:

import { webcrypto } from 'node:crypto'
globalThis.crypto = webcrypto

This crate will then use the provided webcrypto implementation.

Custom backend

If this crate does not support your target out of the box or you have to use a non-default entropy source, then you can provide a custom implementation. You need to enable the custom backend as described in the configuration flags section. Next, you need to define an extern function with the following signature:

use getrandom::Error;

#[no_mangle]
unsafe extern "Rust" fn __getrandom_v03_custom(
    dest: *mut u8,
    len: usize,
) -> Result<(), Error> {
    todo!()
}

This function should, ideally, be defined in the root crate of your project, e.g. in your main.rs. This function MUST be defined only once for your project, i.e. upstream library crates SHOULD NOT define it outside of tests and benchmarks. Improper configuration of this backend may result in linking errors.

The function accepts a pointer to a buffer that should be filled with random data and its length in bytes. Note that the buffer MAY be uninitialized. On success, the function should return Ok(()) and fully fill the input buffer; otherwise, it should return an error value.

If you are confident that getrandom is not used in your project, but it gets pulled nevertheless by one of your dependencies, then you can use the following custom backend, which always returns the "unsupported" error:

use getrandom::Error;

#[no_mangle]
unsafe extern "Rust" fn __getrandom_v03_custom(
    dest: *mut u8,
    len: usize,
) -> Result<(), Error> {
    Err(Error::UNSUPPORTED)
}

Platform Support

This crate generally supports the same operating system and platform versions that the Rust standard library does. Additional targets may be supported using the opt-in custom backend.

This means that as Rust drops support for old versions of operating systems (such as old Linux kernel versions, Android API levels, etc.) in stable releases, getrandom may create new patch releases that remove support for outdated platform versions.

/dev/urandom fallback on Linux and Android

On Linux targets, the /dev/urandom fallback is present only if either target_env is musl, or target_arch is one of the following: aarch64, arm, powerpc, powerpc64, s390x, x86, x86_64. Other supported targets require kernel versions that support the getrandom system call, so the fallback is not needed.

On Android targets the fallback is present only for the following target_arches: aarch64, arm, x86, x86_64. Other target_arches (e.g. RISC-V) require sufficiently high API levels.

The fallback can be disabled by enabling the linux_getrandom opt-in backend. Note that doing so will bump minimum supported Linux kernel version to 3.17 and Android API level to 23 (Marshmallow).

Early boot

Sometimes, early in the boot process, the OS has not collected enough entropy to securely seed its RNG. This is especially common on virtual machines, where standard "random" events are hard to come by.

Some operating system interfaces always block until the RNG is securely seeded. This can take anywhere from a few seconds to more than a minute. 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), reading from /dev/urandom never blocks, even when the OS hasn't collected enough entropy yet. To avoid returning low-entropy bytes, we first poll /dev/random and only switch to /dev/urandom once this has succeeded.

On OpenBSD, this kind of entropy accounting isn't available, and on NetBSD, blocking on it is discouraged. On these platforms, nonblocking interfaces are used, even when reliable entropy may not be available. On the platforms where it is used, the reliability of entropy accounting itself isn't free from controversy. This library provides randomness sourced according to the platform's best practices, but each platform has its own limits on the grade of randomness it can promise in environments with few sources of entropy.

Error handling

We always prioritize failure over returning known insecure "random" bytes. Generally, on supported platforms, failure is highly unlikely, though not impossible. If an error does occur, it is likely that it will occur on every call to getrandom. Therefore, after the first successful call, one can be reasonably confident that no errors will occur.

Panic handling

We strive to eliminate all potential panics from our backend implementations. In other words, when compiled with optimizations enabled, the generated binary code for getrandom functions should not contain any panic branches. Even if the platform misbehaves and returns an unexpected result, our code should correctly handle it and return an error, e.g. Error::UNEXPECTED.

Sanitizer support

If your code uses fill_uninit and you enable memory sanitization (i.e. -Zsanitizer=memory), you need to pass the getrandom_sanitize configuration flag to enable unpoisoning of the destination buffer filled by fill_uninit.

For example, it can be done as follows (requires a Nightly compiler):

RUSTFLAGS="-Zsanitizer=memory --cfg getrandom_sanitize" \
    cargo test -Zbuild-std --target=x86_64-unknown-linux-gnu

Minimum Supported Rust Version

This crate requires Rust 1.63 or later.

License

The getrandom library is distributed under either of

• Apache License, Version 2.0
• MIT license

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.