Struct rand_chacha::ChaCha8Rng[][src]

pub struct ChaCha8Rng { /* fields omitted */ }
Expand description

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.

Implementations

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.

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.

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.

Get the stream number.

Get the seed.

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Performs the conversion.

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

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

Seed type, which is restricted to types mutably-dereferenceable 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 using a u64 seed. Read more

Create a new PRNG seeded from another Rng. Read more

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

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Upcast to an RngCore trait object.

Performs the conversion.

Performs the conversion.

The resulting type after obtaining ownership.

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

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

recently added

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

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.