# [−] [src] 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 the `set_word_pos` and `set_stream` methods.

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` [src]

#### `pub fn get_word_pos(&self) -> u128` [src]

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)` [src]

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)` [src]

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 From<ChaCha20Core> for ChaCha20Rng` [src]

#### `fn from(core: ChaCha20Core) -> Self` [src]

Performs the conversion.

### `impl Clone for ChaCha20Rng` [src]

#### `fn clone(&self) -> ChaCha20Rng` [src]

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`1.0.0 [src]

Performs copy-assignment from `source`. Read more

### `impl Debug for ChaCha20Rng` [src]

#### `fn fmt(&self, f: &mut Formatter) -> Result` [src]

Formats the value using the given formatter. Read more

### `impl RngCore for ChaCha20Rng` [src]

#### `fn next_u32(&mut self) -> u32` [src]

Return the next random `u32`. Read more

#### `fn next_u64(&mut self) -> u64` [src]

Return the next random `u64`. Read more

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

Fill `dest` with random data. Read more

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

Fill `dest` entirely with random data. Read more

### `impl CryptoRng for ChaCha20Rng` [src]

### `impl SeedableRng for ChaCha20Rng` [src]

#### `type Seed = [u8; 32]`

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

#### `fn from_seed(seed: Self::Seed) -> Self` [src]

Create a new PRNG using the given seed. Read more

#### `fn seed_from_u64(state: u64) -> Self` [src]

Create a new PRNG using a `u64` seed. Read more

#### `fn from_rng<R>(rng: R) -> Result<Self, Error> where
R: RngCore,` [src]

Create a new PRNG seeded from another `Rng`. Read more

#### `fn from_entropy() -> Self` [src]

Creates a new instance of the RNG seeded via [`getrandom`]. Read more

## Auto Trait Implementations

### `impl Send for ChaCha20Rng`

### `impl Sync for ChaCha20Rng`

## Blanket Implementations

### `impl<T> From<T> for T` [src]

#### `fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `fn to_owned(&self) -> T` [src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&self, target: &mut T)` [src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into`)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `fn into(self) -> U` [src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `fn borrow_mut(&mut self) -> &mut T` [src]

Mutably borrows from an owned value. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `fn borrow(&self) -> &T` [src]

Immutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<V, T> VZip<V> for T where
V: MultiLane<T>,` [src]

#### `fn vzip(self) -> V` [src]

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