Struct rand::prng::chacha::ChaChaRng[][src]

pub struct ChaChaRng(_);

A cryptographically secure random number generator that uses the ChaCha algorithm.

ChaCha is a stream cipher designed by Daniel J. Bernstein 1, that we use as an RNG. It is an improved variant of the Salsa20 cipher family, which was selected as one of the "stream ciphers suitable for widespread adoption" by eSTREAM 2.

ChaCha uses add-rotate-xor (ARX) operations as its basis. These are safe against timing attacks, although that is mostly a concern for ciphers and not for RNGs. Also it is very suitable for SIMD implementation. Here we do not provide a SIMD implementation yet, except for what is provided by auto-vectorisation.

With the ChaCha algorithm it is possible to choose the number of rounds the core algorithm should run. The number of rounds is a tradeoff between performance and security, where 8 rounds is the minimum potentially secure configuration, and 20 rounds is widely used as a conservative choice. We use 20 rounds in this implementation, but hope to allow type-level configuration in the future.

We use a 64-bit counter and 64-bit stream identifier as in Benstein's implementation 1 except that we use a stream identifier in place of a nonce. A 64-bit counter over 64-byte (16 word) blocks allows 1 ZiB of output before cycling, and the stream identifier allows 264 unique streams of output per seed. Both counter and stream are initialized to zero but may be set via set_word_pos and set_stream.

The word layout is:

constant constant constant constant
seed     seed     seed     seed
seed     seed     seed     seed
counter  counter  nonce    nonce

This implementation uses an output buffer of sixteen u32 words, and uses BlockRng to implement the RngCore methods.

Methods

impl ChaChaRng
[src]

Deprecated since 0.5.0

: use the FromEntropy or SeedableRng trait

Create an ChaCha random number generator using the default fixed key of 8 zero words.

Examples

use rand::{RngCore, ChaChaRng};

let mut ra = ChaChaRng::new_unseeded();
println!("{:?}", ra.next_u32());
println!("{:?}", ra.next_u32());

Since this equivalent to a RNG with a fixed seed, repeated executions of an unseeded RNG will produce the same result. This code sample will consistently produce:

  • 2917185654
  • 2419978656

Get the offset from the start of the stream, in 32-bit words.

Since the generated blocks are 16 words (24) long and the counter is 64-bits, the offset is a 68-bit number. Sub-word offsets are not supported, hence the result can simply be multiplied by 4 to get a byte-offset.

Note: this function is currently only available when the i128_support feature is enabled. In the future this will be enabled by default.

Set the offset from the start of the stream, in 32-bit words.

As with get_word_pos, we use a 68-bit number. Since the generator simply cycles at the end of its period (1 ZiB), we ignore the upper 60 bits.

Note: this function is currently only available when the i128_support feature is enabled. In the future this will be enabled by default.

Set the stream number.

This is initialized to zero; 264 unique streams of output are available per seed/key.

Note that in order to reproduce ChaCha output with a specific 64-bit nonce, one can convert that nonce to a u64 in little-endian fashion and pass to this function. In theory a 96-bit nonce can be used by passing the last 64-bits to this function and using the first 32-bits as the most significant half of the 64-bit counter (which may be set indirectly via set_word_pos), but this is not directly supported.

Trait Implementations

impl Clone for ChaChaRng
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Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl Debug for ChaChaRng
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Formats the value using the given formatter. Read more

impl RngCore for ChaChaRng
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Return the next random u32. Read more

Return the next random u64. Read more

Fill dest with random data. Read more

Fill dest entirely with random data. Read more

impl SeedableRng for ChaChaRng
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Seed type, which is restricted to types mutably-dereferencable as u8 arrays (we recommend [u8; N] for some N). Read more

Create a new PRNG using the given seed. Read more

Create a new PRNG seeded from another Rng. Read more

Create a new PRNG using a u64 seed. Read more

impl CryptoRng for ChaChaRng
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impl From<ChaChaCore> for ChaChaRng
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Performs the conversion.

Auto Trait Implementations

impl Send for ChaChaRng

impl Sync for ChaChaRng