src/lib/zircon/rust/src/cprng.rs

// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! Type-safe bindings for the Zircon kernel's CPRNG.
//!
use crate::{ok, Status};
use fuchsia_zircon_sys as sys;

/// Draw random bytes from the kernel's CPRNG to fill `buffer`. This function
/// always fills the buffer.
///
/// Wraps the
/// [zx_cprng_draw](https://fuchsia.dev/fuchsia-src/reference/syscalls/cprng_draw.md)
/// syscall.
pub fn cprng_draw(buffer: &mut [u8]) {
    unsafe { sys::zx_cprng_draw(buffer.as_mut_ptr(), buffer.len()) };
}

/// Mix the given entropy into the kernel CPRNG.
///
/// The buffer must have length less than `ZX_CPRNG_ADD_ENTROPY_MAX_LEN`.
///
/// Wraps the
/// [zx_cprng_add_entropy](https://fuchsia.dev/fuchsia-src/reference/syscalls/cprng_add_entropy.md)
/// syscall.
pub fn cprng_add_entropy(buffer: &[u8]) -> Result<(), Status> {
    let status = unsafe { sys::zx_cprng_add_entropy(buffer.as_ptr(), buffer.len()) };
    ok(status)
}

#[cfg(test)]
mod tests {
    use super::*;

    fn check_buffer(buffer: &mut [u8]) {
        let mut first_zero = 0;
        let mut last_zero = 0;
        for _ in 0..30 {
            cprng_draw(buffer);
            if buffer[0] == 0 {
                first_zero += 1;
            }
            if buffer.len() > 1 && buffer[buffer.len() - 1] == 0 {
                last_zero += 1;
            }
        }
        assert_ne!(first_zero, 30);
        assert_ne!(last_zero, 30);
    }

    #[test]
    fn cprng() {
        let mut buffer = [0; 20];
        check_buffer(&mut buffer);
    }

    #[test]
    fn cprng_large() {
        const SIZE: usize = sys::ZX_CPRNG_DRAW_MAX_LEN + 1;
        let mut buffer = [0; SIZE];
        check_buffer(&mut buffer);

        for mut s in buffer.chunks_mut(SIZE / 3) {
            check_buffer(&mut s);
        }
    }

    #[test]
    fn cprng_add() {
        let buffer = [0, 1, 2];
        assert_eq!(cprng_add_entropy(&buffer), Ok(()));
    }
}