Trait RngCore

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pub trait RngCore {
    // Required methods
    fn next_u32(&mut self) -> u32;
    fn next_u64(&mut self) -> u64;
    fn fill_bytes(&mut self, dest: &mut [u8]);
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error>;
}

Expand description

The core of a random number generator.

This trait encapsulates the low-level functionality common to all generators, and is the “back end”, to be implemented by generators. End users should normally use the Rng trait from the rand crate, which is automatically implemented for every type implementing RngCore.

Three different methods for generating random data are provided since the optimal implementation of each is dependent on the type of generator. There is no required relationship between the output of each; e.g. many implementations of fill_bytes consume a whole number of u32 or u64 values and drop any remaining unused bytes. The same can happen with the next_u32 and next_u64 methods, implementations may discard some random bits for efficiency.

The try_fill_bytes method is a variant of fill_bytes allowing error handling; it is not deemed sufficiently useful to add equivalents for next_u32 or next_u64 since the latter methods are almost always used with algorithmic generators (PRNGs), which are normally infallible.

Implementers should produce bits uniformly. Pathological RNGs (e.g. always returning the same value, or never setting certain bits) can break rejection sampling used by random distributions, and also break other RNGs when seeding them via SeedableRng::from_rng.

Algorithmic generators implementing SeedableRng should normally have portable, reproducible output, i.e. fix Endianness when converting values to avoid platform differences, and avoid making any changes which affect output (except by communicating that the release has breaking changes).

Typically an RNG will implement only one of the methods available in this trait directly, then use the helper functions from the impls module to implement the other methods.

It is recommended that implementations also implement:

Debug with a custom implementation which does not print any internal state (at least, CryptoRngs should not risk leaking state through Debug).
Serialize and Deserialize (from Serde), preferably making Serde support optional at the crate level in PRNG libs.
Clone, if possible.
never implement Copy (accidental copies may cause repeated values).
do not implement Default for pseudorandom generators, but instead implement SeedableRng, to guide users towards proper seeding. External / hardware RNGs can choose to implement Default.
Eq and PartialEq could be implemented, but are probably not useful.

§Example

A simple example, obviously not generating very random output:

#![allow(dead_code)]
use rand_core::{RngCore, Error, impls};

struct CountingRng(u64);

impl RngCore for CountingRng {
    fn next_u32(&mut self) -> u32 {
        self.next_u64() as u32
    }

    fn next_u64(&mut self) -> u64 {
        self.0 += 1;
        self.0
    }

    fn fill_bytes(&mut self, dest: &mut [u8]) {
        impls::fill_bytes_via_next(self, dest)
    }

    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        Ok(self.fill_bytes(dest))
    }
}

Required Methods§

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fn next_u32(&mut self) -> u32

Return the next random u32.

RNGs must implement at least one method from this trait directly. In the case this method is not implemented directly, it can be implemented using self.next_u64() as u32 or via impls::next_u32_via_fill.

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fn next_u64(&mut self) -> u64

Return the next random u64.

RNGs must implement at least one method from this trait directly. In the case this method is not implemented directly, it can be implemented via impls::next_u64_via_u32 or via impls::next_u64_via_fill.

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fn fill_bytes(&mut self, dest: &mut [u8])

Fill dest with random data.

RNGs must implement at least one method from this trait directly. In the case this method is not implemented directly, it can be implemented via impls::fill_bytes_via_next or via RngCore::try_fill_bytes; if this generator can fail the implementation must choose how best to handle errors here (e.g. panic with a descriptive message or log a warning and retry a few times).

This method should guarantee that dest is entirely filled with new data, and may panic if this is impossible (e.g. reading past the end of a file that is being used as the source of randomness).

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fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error>

Fill dest entirely with random data.

This is the only method which allows an RNG to report errors while generating random data thus making this the primary method implemented by external (true) RNGs (e.g. OsRng) which can fail. It may be used directly to generate keys and to seed (infallible) PRNGs.

Other than error handling, this method is identical to RngCore::fill_bytes; thus this may be implemented using Ok(self.fill_bytes(dest)) or fill_bytes may be implemented with self.try_fill_bytes(dest).unwrap() or more specific error handling.