logo
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136

//! The Salsa20 core function.

use crate::{rounds::Rounds, Key, Nonce, BLOCK_SIZE, CONSTANTS, STATE_WORDS};
use core::{convert::TryInto, marker::PhantomData, mem};

/// The Salsa20 core function.
// TODO(tarcieri): zeroize support
pub struct Core<R: Rounds> {
    /// Internal state of the core function
    state: [u32; STATE_WORDS],

    /// Number of rounds to perform
    rounds: PhantomData<R>,
}

impl<R: Rounds> Core<R> {
    /// Initialize core function with the given key and IV
    pub fn new(key: &Key, iv: &Nonce) -> Self {
        #[allow(unsafe_code)]
        let mut state: [u32; STATE_WORDS] = unsafe { mem::zeroed() };
        state[0] = CONSTANTS[0];

        for (i, chunk) in key[..16].chunks(4).enumerate() {
            state[1 + i] = u32::from_le_bytes(chunk.try_into().unwrap());
        }

        state[5] = CONSTANTS[1];

        for (i, chunk) in iv.chunks(4).enumerate() {
            state[6 + i] = u32::from_le_bytes(chunk.try_into().unwrap());
        }

        state[8] = 0;
        state[9] = 0;
        state[10] = CONSTANTS[2];

        for (i, chunk) in key[16..].chunks(4).enumerate() {
            state[11 + i] = u32::from_le_bytes(chunk.try_into().unwrap());
        }

        state[15] = CONSTANTS[3];

        Self {
            state,
            rounds: PhantomData,
        }
    }

    /// Generate output, overwriting data already in the buffer
    pub fn generate(&mut self, output: &mut [u8]) {
        debug_assert_eq!(output.len(), BLOCK_SIZE);

        let mut state = self.state;
        self.rounds(&mut state);

        for (i, chunk) in output.chunks_mut(4).enumerate() {
            chunk.copy_from_slice(&state[i].to_le_bytes());
        }
    }

    /// Apply generated keystream to the output buffer
    pub fn apply_keystream(&mut self, counter: u64, output: &mut [u8]) {
        debug_assert_eq!(output.len(), BLOCK_SIZE);
        self.counter_setup(counter);

        let mut state = self.state;
        self.rounds(&mut state);

        for (i, chunk) in output.chunks_mut(4).enumerate() {
            for (a, b) in chunk.iter_mut().zip(&state[i].to_le_bytes()) {
                *a ^= *b;
            }
        }
    }

    #[inline]
    pub(crate) fn counter_setup(&mut self, counter: u64) {
        self.state[8] = (counter & 0xffff_ffff) as u32;
        self.state[9] = ((counter >> 32) & 0xffff_ffff) as u32;
    }

    /// Run the 20 rounds (i.e. 10 double rounds) of Salsa20
    #[inline]
    fn rounds(&mut self, state: &mut [u32; STATE_WORDS]) {
        for _ in 0..(R::COUNT / 2) {
            // column rounds
            quarter_round(0, 4, 8, 12, state);
            quarter_round(5, 9, 13, 1, state);
            quarter_round(10, 14, 2, 6, state);
            quarter_round(15, 3, 7, 11, state);

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

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

impl<R: Rounds> From<[u32; STATE_WORDS]> for Core<R> {
    fn from(state: [u32; STATE_WORDS]) -> Core<R> {
        Self {
            state,
            rounds: PhantomData,
        }
    }
}

#[inline]
#[allow(clippy::many_single_char_names)]
pub(crate) fn quarter_round(
    a: usize,
    b: usize,
    c: usize,
    d: usize,
    state: &mut [u32; STATE_WORDS],
) {
    let mut t: u32;

    t = state[a].wrapping_add(state[d]);
    state[b] ^= t.rotate_left(7) as u32;

    t = state[b].wrapping_add(state[a]);
    state[c] ^= t.rotate_left(9) as u32;

    t = state[c].wrapping_add(state[b]);
    state[d] ^= t.rotate_left(13) as u32;

    t = state[d].wrapping_add(state[c]);
    state[a] ^= t.rotate_left(18) as u32;
}