Struct ChaCha 

pub struct ChaCha<const ROUNDS: usize, const IS_IETF: bool> { /* private fields */ }

ChaCha stream cipher.

IS_IETF selects the nonce/counter layout:

• false (DJB original): 64-bit counter at words 12–13, 64-bit nonce at words 14–15.
• true (IETF / RFC 8439): 32-bit counter at word 12, 96-bit nonce at words 13–15.

The IETF layout can only encrypt up to CHACHA20_IETF_MAX_LEN bytes per (key, nonce) pair; xor_keystream panics instead of wrapping the counter.

Implementations

impl<const ROUNDS: usize> ChaCha<ROUNDS, false>

pub fn new(key: &[u8; 32], nonce: &[u8; 8]) -> ChaCha<ROUNDS, false>

Create a new ChaCha instance with the DJB nonce layout (8-byte nonce, 64-bit counter).

pub fn set_counter(&mut self, counter: u64)

Set the ChaCha counter (words 12 and 13). It can be used to move forward and backward in the keystream.

impl<const ROUNDS: usize> ChaCha<ROUNDS, true>

pub fn new(key: &[u8; 32], nonce: &[u8; 12]) -> ChaCha<ROUNDS, true>

Create a new ChaCha instance with the IETF (RFC 8439) nonce layout (12-byte nonce, 32-bit counter).

pub fn set_counter(&mut self, counter: u32)

Set the ChaCha counter (word 12). The counter is a u32. It can be used to move forward and backward in the keystream.

The (key, nonce, counter) triple must never be reused across distinct messages. Setting the counter near u32::MAX leaves little room before the counter space is exhausted; streaming past it will panic rather than reuse keystream (see CHACHA20_IETF_MAX_LEN).

Trait Implementations

impl<const ROUNDS: usize, const IS_IETF: bool> Drop for ChaCha<ROUNDS, IS_IETF>

fn drop(&mut self)

Executes the destructor for this type.

fn pin_drop(self: Pin<&mut Self>)

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

Execute the destructor for this type, but different to Drop::drop, it requires self to be pinned.

impl<const ROUNDS: usize, const IS_IETF: bool> StreamCipher for ChaCha<ROUNDS, IS_IETF>

fn xor_keystream(&mut self, in_out: &mut [u8])

XOR plaintext with the ChaCha keystream.

Panics

For the IETF layout, panics if this call would advance the 32-bit block counter past u32::MAX and silently reuse keystream. This happens when encrypting more than CHACHA20_IETF_MAX_LEN bytes under one key/nonce, or after set_counter jumps near the end of the counter space.

impl<const ROUNDS: usize, const IS_IETF: bool> Zeroize for ChaCha<ROUNDS, IS_IETF>

fn zeroize(&mut self)

Zero out this object from memory using Rust intrinsics which ensure the zeroization operation is not “optimized away” by the compiler.