1// Copyright 2019 Developers of the Rand project.
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9//! Interface to the random number generator of the operating system.
10
11use crate::{impls, CryptoRng, Error, RngCore};
12use getrandom::getrandom;
13
14/// A random number generator that retrieves randomness from the
15/// operating system.
16///
17/// This is a zero-sized struct. It can be freely constructed with `OsRng`.
18///
19/// The implementation is provided by the [getrandom] crate. Refer to
20/// [getrandom] documentation for details.
21///
22/// This struct is only available when specifying the crate feature `getrandom`
23/// or `std`. When using the `rand` lib, it is also available as `rand::rngs::OsRng`.
24///
25/// # Blocking and error handling
26///
27/// It is possible that when used during early boot the first call to `OsRng`
28/// will block until the system's RNG is initialised. It is also possible
29/// (though highly unlikely) for `OsRng` to fail on some platforms, most
30/// likely due to system mis-configuration.
31///
32/// After the first successful call, it is highly unlikely that failures or
33/// significant delays will occur (although performance should be expected to
34/// be much slower than a user-space PRNG).
35///
36/// # Usage example
37/// ```
38/// use rand_core::{RngCore, OsRng};
39///
40/// let mut key = [0u8; 16];
41/// OsRng.fill_bytes(&mut key);
42/// let random_u64 = OsRng.next_u64();
43/// ```
44///
45/// [getrandom]: https://crates.io/crates/getrandom
46#[cfg_attr(doc_cfg, doc(cfg(feature = "getrandom")))]
47#[derive(Clone, Copy, Debug, Default)]
48pub struct OsRng;
49
50impl CryptoRng for OsRng {}
51
52impl RngCore for OsRng {
53 fn next_u32(&mut self) -> u32 {
54 impls::next_u32_via_fill(self)
55 }
56
57 fn next_u64(&mut self) -> u64 {
58 impls::next_u64_via_fill(self)
59 }
60
61 fn fill_bytes(&mut self, dest: &mut [u8]) {
62 if let Err(e: Error) = self.try_fill_bytes(dest) {
63 panic!("Error: {}", e);
64 }
65 }
66
67 fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
68 getrandom(dest)?;
69 Ok(())
70 }
71}
72
73#[test]
74fn test_os_rng() {
75 let x = OsRng.next_u64();
76 let y = OsRng.next_u64();
77 assert!(x != 0);
78 assert!(x != y);
79}
80
81#[test]
82fn test_construction() {
83 let mut rng = OsRng::default();
84 assert!(rng.next_u64() != 0);
85}
86