chacha20 0.9.0

The ChaCha20 stream cipher (RFC 8439) implemented in pure Rust using traits from the RustCrypto `cipher` crate, with optional architecture-specific hardware acceleration (AVX2, SSE2). Additionally provides the ChaCha8, ChaCha12, XChaCha20, XChaCha12 and XChaCha8 stream ciphers, and also optional rand_core-compatible RNGs based on those ciphers.
Documentation 
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//! Portable implementation which does not rely on architecture-specific
//! intrinsics.

use crate::{Block, ChaChaCore, Unsigned, STATE_WORDS};
use cipher::{
    consts::{U1, U64},
    BlockSizeUser, ParBlocksSizeUser, StreamBackend,
};

pub(crate) struct Backend<'a, R: Unsigned>(pub(crate) &'a mut ChaChaCore<R>);

impl<'a, R: Unsigned> BlockSizeUser for Backend<'a, R> {
    type BlockSize = U64;
}

impl<'a, R: Unsigned> ParBlocksSizeUser for Backend<'a, R> {
    type ParBlocksSize = U1;
}

impl<'a, R: Unsigned> StreamBackend for Backend<'a, R> {
    #[inline(always)]
    fn gen_ks_block(&mut self, block: &mut Block) {
        let res = run_rounds::<R>(&self.0.state);
        self.0.state[12] = self.0.state[12].wrapping_add(1);

        for (chunk, val) in block.chunks_exact_mut(4).zip(res.iter()) {
            chunk.copy_from_slice(&val.to_le_bytes());
        }
    }
}

#[inline(always)]
fn run_rounds<R: Unsigned>(state: &[u32; STATE_WORDS]) -> [u32; STATE_WORDS] {
    let mut res = *state;

    for _ in 0..R::USIZE {
        // column rounds
        quarter_round(0, 4, 8, 12, &mut res);
        quarter_round(1, 5, 9, 13, &mut res);
        quarter_round(2, 6, 10, 14, &mut res);
        quarter_round(3, 7, 11, 15, &mut res);

        // diagonal rounds
        quarter_round(0, 5, 10, 15, &mut res);
        quarter_round(1, 6, 11, 12, &mut res);
        quarter_round(2, 7, 8, 13, &mut res);
        quarter_round(3, 4, 9, 14, &mut res);
    }

    for (s1, s0) in res.iter_mut().zip(state.iter()) {
        *s1 = s1.wrapping_add(*s0);
    }
    res
}

/// The ChaCha20 quarter round function
fn quarter_round(a: usize, b: usize, c: usize, d: usize, state: &mut [u32; STATE_WORDS]) {
    state[a] = state[a].wrapping_add(state[b]);
    state[d] ^= state[a];
    state[d] = state[d].rotate_left(16);

    state[c] = state[c].wrapping_add(state[d]);
    state[b] ^= state[c];
    state[b] = state[b].rotate_left(12);

    state[a] = state[a].wrapping_add(state[b]);
    state[d] ^= state[a];
    state[d] = state[d].rotate_left(8);

    state[c] = state[c].wrapping_add(state[d]);
    state[b] ^= state[c];
    state[b] = state[b].rotate_left(7);
}