Struct `ChaCha8Rng`

struct ChaCha8Rng { ... }

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.

D. J. Bernstein, ChaCha, a variant of Salsa20 ↩ ↩²
eSTREAM: the ECRYPT Stream Cipher Project ↩

Implementations

`impl ChaCha8Rng`

fn get_word_pos(self: &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.

fn set_word_pos(self: &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.

fn set_stream(self: &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.

fn get_stream(self: &Self) -> u64

Get the stream number.

fn get_seed(self: &Self) -> [u8; 32]

Get the seed.

`impl Clone for ChaCha8Rng`

fn clone(self: &Self) -> ChaCha8Rng

`impl CryptoRng for ChaCha8Rng`

`impl Debug for ChaCha8Rng`

fn fmt(self: &Self, f: &mut Formatter<'_>) -> Result

`impl Eq for ChaCha8Rng`

`impl Freeze for ChaCha8Rng`

`impl From for ChaCha8Rng`

fn from(core: ChaCha8Core) -> Self

`impl PartialEq for ChaCha8Rng`

fn eq(self: &Self, rhs: &ChaCha8Rng) -> bool

`impl RefUnwindSafe for ChaCha8Rng`

`impl RngCore for ChaCha8Rng`

fn next_u32(self: &mut Self) -> u32
fn next_u64(self: &mut Self) -> u64
fn fill_bytes(self: &mut Self, bytes: &mut [u8])

`impl SeedableRng for ChaCha8Rng`

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

`impl Send for ChaCha8Rng`

`impl Serialize for ChaCha8Rng`

fn serialize<S>(self: &Self, s: S) -> Result<<S as >::Ok, <S as >::Error> where S: Serializer

`impl Sync for ChaCha8Rng`

`impl Unpin for ChaCha8Rng`

`impl UnsafeUnpin for ChaCha8Rng`

`impl UnwindSafe for ChaCha8Rng`

`impl<'de> Deserialize for ChaCha8Rng`

fn deserialize<D>(d: D) -> Result<Self, <D as >::Error> where D: Deserializer<'de>

`impl<R> TryCryptoRng for ChaCha8Rng`

`impl<R> TryRngCore for ChaCha8Rng`

fn try_next_u32(self: &mut Self) -> Result<u32, <R as TryRngCore>::Error>
fn try_next_u64(self: &mut Self) -> Result<u64, <R as TryRngCore>::Error>
fn try_fill_bytes(self: &mut Self, dst: &mut [u8]) -> Result<(), <R as TryRngCore>::Error>

`impl<T> Any for ChaCha8Rng`

fn type_id(self: &Self) -> TypeId

`impl<T> Borrow for ChaCha8Rng`

fn borrow(self: &Self) -> &T

`impl<T> BorrowMut for ChaCha8Rng`

fn borrow_mut(self: &mut Self) -> &mut T

`impl<T> CloneToUninit for ChaCha8Rng`

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

`impl<T> DeserializeOwned for ChaCha8Rng`

`impl<T> From for ChaCha8Rng`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T> ToOwned for ChaCha8Rng`

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

`impl<T, U> Into for ChaCha8Rng`

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

`impl<T, U> TryFrom for ChaCha8Rng`

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

`impl<T, U> TryInto for ChaCha8Rng`

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>

`impl<V, T> VZip for ChaCha8Rng`

fn vzip(self: Self) -> V