Struct rand_chacha::ChaCha20Rng

pub struct ChaCha20Rng { /* fields omitted */ }

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

ChaCha is a stream cipher designed by Daniel J. Bernstein¹, 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 eSTREAM².

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. We provide a SIMD implementation to support high throughput on a variety of common hardware platforms.

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 a 64-bit counter and 64-bit stream identifier as in Bernstein's implementation¹ 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   stream_id stream_id

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

---

1. D. J. Bernstein, ChaCha, a variant of Salsa20 ↩

2. eSTREAM: the ECRYPT Stream Cipher Project ↩

Methods

impl ChaCha20Rng

pub fn get_word_pos(&self) -> u128

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.

pub fn set_word_pos(&mut self, word_offset: u128)

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.

pub fn set_stream(&mut self, stream: u64)

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 ChaCha20Rng

fn clone(&self) -> ChaCha20Rng

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl From<ChaCha20Core> for ChaCha20Rng

fn from(core: ChaCha20Core) -> Self

Performs the conversion.

impl Debug for ChaCha20Rng

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl RngCore for ChaCha20Rng

fn next_u32(&mut self) -> u32

Return the next random u32.

fn next_u64(&mut self) -> u64

Return the next random u64.

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

Fill dest with random data.

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

Fill dest entirely with random data.

impl SeedableRng for ChaCha20Rng

type Seed = [u8; 32]

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

fn from_seed(seed: Self::Seed) -> Self

Create a new PRNG using the given seed.

fn seed_from_u64(state: u64) -> Self

Create a new PRNG using a u64 seed.

fn from_rng<R>(rng: R) -> Result<Self, Error> where
    R: RngCore, 

Create a new PRNG seeded from another Rng.

impl CryptoRng for ChaCha20Rng