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Since this crate was written, the arbitrary crate has vastly improved. It uses a better approach to translating raw, fuzzer-provided bytes into property-testing style tests, and is where I am investing my time now. As such, this crate is archived, and you should use that one instead.


BufRng is a "random" number generator that simply yields pre-determined values from a buffer, and yields 0s once the buffer is exhausted.


This RNG is not suitable for anything other than testing and fuzzing! It is not suitable for cryptography! It is not suitable for generating pseudo-random numbers!



BufRng is useful for reinterpreting raw input bytes from libFuzzer or AFL as an RNG that is used with structure-aware test case generators (e.g. quickcheck::Arbitrary). This combines the power of coverage-guided fuzzing with structure-aware fuzzing.


Let's say we are developing a crate to convert back and forth between RGB and HSL color representations.

First, we can implement quickcheck::Arbitrary for our color types to get structure-aware test case generators. Then, we can use these with quickcheck's own test runner infrastructure to assert various properties about our code (such as it never panics, or that RGB -> HSL -> RGB is the identity function) and quickcheck will generate random instances of Rgb and Hsl to check this property against.

/// A color represented with RGB.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Rgb {
    pub r: u8,
    pub g: u8,
    pub b: u8,

impl Rgb {
    pub fn to_hsl(&self) -> Hsl {
        // ...

/// A color represented with HSL.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct Hsl {
    pub h: f64,
    pub s: f64,
    pub l: f64,

impl Hsl {
    pub fn to_rgb(&self) -> Rgb {
        // ...

// Implementations of `quickcheck::Arbitrary` to create structure-aware test
// case generators for `Rgb` and `Hsl`.

use rand::prelude::*;
use quickcheck::{Arbitrary, Gen};

impl Arbitrary for Rgb {
    fn arbitrary<G: Gen>(g: &mut G) -> Self {
        Rgb {
            r: g.gen(),
            g: g.gen(),
            b: g.gen(),

impl Arbitrary for Hsl {
    fn arbitrary<G: Gen>(g: &mut G) -> Self {
        Hsl {
            h: g.gen_range(0.0, 360.0),
            s: g.gen_range(0.0, 1.0),
            l: g.gen_range(0.0, 1.0),

// Properties that we can have `quickcheck` assert for us.

pub fn rgb_to_hsl_doesnt_panic(rgb: Rgb) {
    let _ = rgb.to_hsl();

pub fn rgb_to_hsl_to_rgb_is_identity(rgb: Rgb) {
    assert_eq!(rgb, rgb.to_hsl().to_rgb());

mod tests {
    quickcheck::quickcheck! {
        fn rgb_to_hsl_doesnt_panic(rgb: Rgb) -> bool {

    quickcheck::quickcheck! {
        fn rgb_to_hsl_to_rgb_is_identity(rgb: Rgb) -> bool {

Finally, we can reuse our existing structure-aware test case generators (the Arbitrary impls) with libFuzzer of AFL inputs with BufRng. Thus we can leverage coverage-guided fuzzing — where the fuzzer is observing code coverage while tests are running, and trying to maximize the paths the inputs cover — with our existing structure-aware generators.

The following snippet is with cargo fuzz and libFuzzer, but the concepts would apply equally well to AFL, for example.

// my-rgb-to-hsl-crate/fuzz/fuzz_targets/


extern crate libfuzzer_sys;

use bufrng::BufRng;
use my_rgb_to_hsl_crate::{rgb_to_hsl_doesnt_panic, rgb_to_hsl_to_rgb_is_identity, Rgb};
use quickcheck::Arbitrary;

fuzz_target!(|data: &[u8]| {
    // Create a `BufRng` from the raw data given to us by the fuzzer.
    let mut rng = BufRng::new(data);

    // Generate an `Rgb` instance with it.
    let rgb = Rgb::arbitrary(&mut rng);

    // Assert our properties!


An RNG that generates "random" numbers copied from a given buffer






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