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chacha20: simplify implementation #76

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tarcieri merged 1 commit into from
Jan 15, 2020
Merged

chacha20: simplify implementation #76

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70 changes: 16 additions & 54 deletions chacha20/src/block/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -45,66 +45,28 @@ impl Block {
],
};

block.rounds();
block.finish(key, iv, counter)
block.rounds(20)
}

/// Run the 20 rounds (i.e. 10 double rounds) of ChaCha20
#[inline]
fn rounds(&mut self) {
self.double_round();
self.double_round();
self.double_round();
self.double_round();
self.double_round();

self.double_round();
self.double_round();
self.double_round();
self.double_round();
self.double_round();
}

/// Double round function
#[inline]
fn double_round(&mut self) {
let state = &mut self.state;
quarter_round(0, 4, 8, 12, state);
quarter_round(1, 5, 9, 13, state);
quarter_round(2, 6, 10, 14, state);
quarter_round(3, 7, 11, 15, state);
quarter_round(0, 5, 10, 15, state);
quarter_round(1, 6, 11, 12, state);
quarter_round(2, 7, 8, 13, state);
quarter_round(3, 4, 9, 14, state);
}

/// Finish computing a block
#[inline]
fn finish(
self,
key: &[u32; KEY_WORDS],
iv: [u32; IV_WORDS],
counter: u64,
) -> [u32; STATE_WORDS] {
fn rounds(&mut self, count: usize) -> [u32; STATE_WORDS] {

@tarcieri tarcieri Jan 15, 2020

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Additionally this parameterizes the number of rounds enabling the implementation of ChaCha8 and ChaCha12. Again, this comes at no cost to performance (or constant-time, as the number of rounds is a constant).

let mut state = self.state;

state[0] = state[0].wrapping_add(CONSTANTS[0]);
state[1] = state[1].wrapping_add(CONSTANTS[1]);
state[2] = state[2].wrapping_add(CONSTANTS[2]);
state[3] = state[3].wrapping_add(CONSTANTS[3]);
state[4] = state[4].wrapping_add(key[0]);
state[5] = state[5].wrapping_add(key[1]);
state[6] = state[6].wrapping_add(key[2]);
state[7] = state[7].wrapping_add(key[3]);
state[8] = state[8].wrapping_add(key[4]);
state[9] = state[9].wrapping_add(key[5]);
state[10] = state[10].wrapping_add(key[6]);
state[11] = state[11].wrapping_add(key[7]);
state[12] = state[12].wrapping_add((counter & 0xffff_ffff) as u32);
state[13] = state[13].wrapping_add(((counter >> 32) & 0xffff_ffff) as u32);
state[14] = state[14].wrapping_add(iv[0]);
state[15] = state[15].wrapping_add(iv[1]);
for _ in 0..(count / 2) {
quarter_round(0, 4, 8, 12, &mut state);
quarter_round(1, 5, 9, 13, &mut state);
quarter_round(2, 6, 10, 14, &mut state);
quarter_round(3, 7, 11, 15, &mut state);
quarter_round(0, 5, 10, 15, &mut state);
quarter_round(1, 6, 11, 12, &mut state);
quarter_round(2, 7, 8, 13, &mut state);
quarter_round(3, 4, 9, 14, &mut state);
}

for i in 0..16 {
state[i] = state[i].wrapping_add(self.state[i]);
}

state
}
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