flux-platform 0.4.0

A local-first, AI-native developer automation platform: build, test, package, and deploy from a single .flux file, and make your repository legible to AI agents.
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//! A small, self-contained ChaCha20 stream cipher (RFC 8439).
//!
//! We implement it by hand rather than pulling a crypto crate, because the
//! usual choices drag in `getrandom`/`windows-sys` which this toolchain can't
//! link. Correctness is pinned by the RFC 8439 §2.4.2 test vector in the tests
//! below — this is the real cipher, not an ad-hoc scramble.

/// XOR `data` in place with the ChaCha20 keystream for `key`/`nonce`, starting
/// at block `counter`.
pub fn xor(key: &[u8; 32], nonce: &[u8; 12], counter: u32, data: &mut [u8]) {
    let mut block = [0u8; 64];
    let mut ctr = counter;
    for chunk in data.chunks_mut(64) {
        keystream_block(key, nonce, ctr, &mut block);
        for (b, k) in chunk.iter_mut().zip(block.iter()) {
            *b ^= *k;
        }
        ctr = ctr.wrapping_add(1);
    }
}

fn keystream_block(key: &[u8; 32], nonce: &[u8; 12], counter: u32, out: &mut [u8; 64]) {
    // Constants: "expand 32-byte k".
    let mut state = [0u32; 16];
    state[0] = 0x6170_7865;
    state[1] = 0x3320_646e;
    state[2] = 0x7962_2d32;
    state[3] = 0x6b20_6574;
    for i in 0..8 {
        state[4 + i] =
            u32::from_le_bytes([key[i * 4], key[i * 4 + 1], key[i * 4 + 2], key[i * 4 + 3]]);
    }
    state[12] = counter;
    for i in 0..3 {
        state[13 + i] = u32::from_le_bytes([
            nonce[i * 4],
            nonce[i * 4 + 1],
            nonce[i * 4 + 2],
            nonce[i * 4 + 3],
        ]);
    }

    let mut working = state;
    for _ in 0..10 {
        // Column rounds.
        quarter_round(&mut working, 0, 4, 8, 12);
        quarter_round(&mut working, 1, 5, 9, 13);
        quarter_round(&mut working, 2, 6, 10, 14);
        quarter_round(&mut working, 3, 7, 11, 15);
        // Diagonal rounds.
        quarter_round(&mut working, 0, 5, 10, 15);
        quarter_round(&mut working, 1, 6, 11, 12);
        quarter_round(&mut working, 2, 7, 8, 13);
        quarter_round(&mut working, 3, 4, 9, 14);
    }

    for i in 0..16 {
        let word = working[i].wrapping_add(state[i]);
        out[i * 4..i * 4 + 4].copy_from_slice(&word.to_le_bytes());
    }
}

fn quarter_round(s: &mut [u32; 16], a: usize, b: usize, c: usize, d: usize) {
    s[a] = s[a].wrapping_add(s[b]);
    s[d] = (s[d] ^ s[a]).rotate_left(16);
    s[c] = s[c].wrapping_add(s[d]);
    s[b] = (s[b] ^ s[c]).rotate_left(12);
    s[a] = s[a].wrapping_add(s[b]);
    s[d] = (s[d] ^ s[a]).rotate_left(8);
    s[c] = s[c].wrapping_add(s[d]);
    s[b] = (s[b] ^ s[c]).rotate_left(7);
}

#[cfg(test)]
mod tests {
    use super::*;

    /// RFC 8439 §2.4.2 encryption test vector.
    #[test]
    fn rfc8439_test_vector() {
        let key: [u8; 32] = std::array::from_fn(|i| i as u8);
        let nonce: [u8; 12] = [0, 0, 0, 0, 0, 0, 0, 0x4a, 0, 0, 0, 0];
        let plaintext = b"Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it.";

        let mut buf = plaintext.to_vec();
        xor(&key, &nonce, 1, &mut buf);

        // First 16 bytes of the expected ciphertext from the RFC.
        let expected_prefix = [
            0x6e, 0x2e, 0x35, 0x9a, 0x25, 0x68, 0xf9, 0x80, 0x41, 0xba, 0x07, 0x28, 0xdd, 0x0d,
            0x69, 0x81,
        ];
        assert_eq!(&buf[..16], &expected_prefix);

        // Round-trips back to plaintext.
        xor(&key, &nonce, 1, &mut buf);
        assert_eq!(&buf, plaintext);
    }
}