tc_sm4 0.1.0

//! SM4 key schedule, round function, and block transformation.

use tc_zeroize::Zeroize;

use crate::{BLOCK_BYTES, KEY_BYTES};

/// Number of rounds, and so of round keys.
pub(crate) const ROUNDS: usize = 32;

/// The SM4 S-box.
#[rustfmt::skip]
const SBOX: [u8; 256] = [
    0xd6, 0x90, 0xe9, 0xfe, 0xcc, 0xe1, 0x3d, 0xb7, 0x16, 0xb6, 0x14, 0xc2, 0x28, 0xfb, 0x2c, 0x05,
    0x2b, 0x67, 0x9a, 0x76, 0x2a, 0xbe, 0x04, 0xc3, 0xaa, 0x44, 0x13, 0x26, 0x49, 0x86, 0x06, 0x99,
    0x9c, 0x42, 0x50, 0xf4, 0x91, 0xef, 0x98, 0x7a, 0x33, 0x54, 0x0b, 0x43, 0xed, 0xcf, 0xac, 0x62,
    0xe4, 0xb3, 0x1c, 0xa9, 0xc9, 0x08, 0xe8, 0x95, 0x80, 0xdf, 0x94, 0xfa, 0x75, 0x8f, 0x3f, 0xa6,
    0x47, 0x07, 0xa7, 0xfc, 0xf3, 0x73, 0x17, 0xba, 0x83, 0x59, 0x3c, 0x19, 0xe6, 0x85, 0x4f, 0xa8,
    0x68, 0x6b, 0x81, 0xb2, 0x71, 0x64, 0xda, 0x8b, 0xf8, 0xeb, 0x0f, 0x4b, 0x70, 0x56, 0x9d, 0x35,
    0x1e, 0x24, 0x0e, 0x5e, 0x63, 0x58, 0xd1, 0xa2, 0x25, 0x22, 0x7c, 0x3b, 0x01, 0x21, 0x78, 0x87,
    0xd4, 0x00, 0x46, 0x57, 0x9f, 0xd3, 0x27, 0x52, 0x4c, 0x36, 0x02, 0xe7, 0xa0, 0xc4, 0xc8, 0x9e,
    0xea, 0xbf, 0x8a, 0xd2, 0x40, 0xc7, 0x38, 0xb5, 0xa3, 0xf7, 0xf2, 0xce, 0xf9, 0x61, 0x15, 0xa1,
    0xe0, 0xae, 0x5d, 0xa4, 0x9b, 0x34, 0x1a, 0x55, 0xad, 0x93, 0x32, 0x30, 0xf5, 0x8c, 0xb1, 0xe3,
    0x1d, 0xf6, 0xe2, 0x2e, 0x82, 0x66, 0xca, 0x60, 0xc0, 0x29, 0x23, 0xab, 0x0d, 0x53, 0x4e, 0x6f,
    0xd5, 0xdb, 0x37, 0x45, 0xde, 0xfd, 0x8e, 0x2f, 0x03, 0xff, 0x6a, 0x72, 0x6d, 0x6c, 0x5b, 0x51,
    0x8d, 0x1b, 0xaf, 0x92, 0xbb, 0xdd, 0xbc, 0x7f, 0x11, 0xd9, 0x5c, 0x41, 0x1f, 0x10, 0x5a, 0xd8,
    0x0a, 0xc1, 0x31, 0x88, 0xa5, 0xcd, 0x7b, 0xbd, 0x2d, 0x74, 0xd0, 0x12, 0xb8, 0xe5, 0xb4, 0xb0,
    0x89, 0x69, 0x97, 0x4a, 0x0c, 0x96, 0x77, 0x7e, 0x65, 0xb9, 0xf1, 0x09, 0xc5, 0x6e, 0xc6, 0x84,
    0x18, 0xf0, 0x7d, 0xec, 0x3a, 0xdc, 0x4d, 0x20, 0x79, 0xee, 0x5f, 0x3e, 0xd7, 0xcb, 0x39, 0x48,
];

/// The fixed parameter constants `FK`.
const FK: [u32; 4] = [0xa3b1_bac6, 0x56aa_3350, 0x677d_9197, 0xb270_22dc];

/// The fixed key-schedule constants `CK`.
#[rustfmt::skip]
const CK: [u32; 32] = [
    0x0007_0e15, 0x1c23_2a31, 0x383f_464d, 0x545b_6269,
    0x7077_7e85, 0x8c93_9aa1, 0xa8af_b6bd, 0xc4cb_d2d9,
    0xe0e7_eef5, 0xfc03_0a11, 0x181f_262d, 0x343b_4249,
    0x5057_5e65, 0x6c73_7a81, 0x888f_969d, 0xa4ab_b2b9,
    0xc0c7_ced5, 0xdce3_eaf1, 0xf8ff_060d, 0x141b_2229,
    0x3037_3e45, 0x4c53_5a61, 0x686f_767d, 0x848b_9299,
    0xa0a7_aeb5, 0xbcc3_cad1, 0xd8df_e6ed, 0xf4fb_0209,
    0x1017_1e25, 0x2c33_3a41, 0x484f_565d, 0x646b_7279,
];

/// Expands the key into the round keys for the requested direction.
/// Variable time: secret-dependent S-box lookups during key expansion.
pub(crate) fn expand_key(
    for_encryption: bool,
    key: &[u8; KEY_BYTES],
    round_keys: &mut [u32; ROUNDS],
) {
    // Mix the first four words with FK, then advance a sliding window.
    let mut window = [0_u32; 4];
    for ((word, chunk), fixed) in window.iter_mut().zip(key.chunks_exact(4)).zip(FK) {
        *word = u32::from_be_bytes(chunk.try_into().unwrap()) ^ fixed;
    }

    for (round_key, constant) in round_keys.iter_mut().zip(CK) {
        *round_key = window[0] ^ t_prime(window[1] ^ window[2] ^ window[3] ^ constant);
        window = [window[1], window[2], window[3], *round_key];
    }

    // Decryption traverses the same round keys in reverse.
    if !for_encryption {
        round_keys.reverse();
    }
    window.zeroize();
}

/// Runs the round loop; the direction is already encoded in `round_keys`.
/// Variable time: secret-dependent S-box lookups during every round.
pub(crate) fn process_block(
    round_keys: &[u32; ROUNDS],
    input: &[u8; BLOCK_BYTES],
    output: &mut [u8; BLOCK_BYTES],
) {
    let mut x = [0_u32; 4];
    for (word, chunk) in x.iter_mut().zip(input.chunks_exact(4)) {
        *word = u32::from_be_bytes(chunk.try_into().unwrap());
    }

    for round in round_keys.chunks_exact(4) {
        x[0] ^= t(x[1] ^ x[2] ^ x[3] ^ round[0]);
        x[1] ^= t(x[2] ^ x[3] ^ x[0] ^ round[1]);
        x[2] ^= t(x[3] ^ x[0] ^ x[1] ^ round[2]);
        x[3] ^= t(x[0] ^ x[1] ^ x[2] ^ round[3]);
    }

    // Final reversal: output X35, X34, X33, X32.
    for (word, chunk) in x.iter().rev().zip(output.chunks_exact_mut(4)) {
        chunk.copy_from_slice(&word.to_be_bytes());
    }
    x.zeroize();
}

/// The nonlinear substitution `tau`: the S-box applied to each byte.
fn tau(a: u32) -> u32 {
    let mut bytes = a.to_be_bytes();
    for byte in &mut bytes {
        *byte = SBOX[usize::from(*byte)];
    }
    let result = u32::from_be_bytes(bytes);
    bytes.zeroize();
    result
}

/// The round mixer-substitution `T = L . tau`.
fn t(z: u32) -> u32 {
    let b = tau(z);
    b ^ b.rotate_left(2) ^ b.rotate_left(10) ^ b.rotate_left(18) ^ b.rotate_left(24)
}

/// The key-schedule mixer-substitution `T' = L' . tau`, which differs from
/// [`t`] only in the rotations of its linear part.
fn t_prime(z: u32) -> u32 {
    let mut b = tau(z);
    let result = b ^ b.rotate_left(13) ^ b.rotate_left(23);
    b.zeroize();
    result
}

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

    #[test]
    fn the_s_box_is_a_permutation() {
        let mut seen = [false; 256];
        for &value in &SBOX {
            assert!(!seen[usize::from(value)], "duplicate entry {value:#04x}");
            seen[usize::from(value)] = true;
        }
    }

    #[test]
    fn the_decryption_schedule_is_the_encryption_schedule_reversed() {
        let key = [0xa5_u8; KEY_BYTES];
        let mut encryption = [0; ROUNDS];
        let mut decryption = [0; ROUNDS];
        expand_key(true, &key, &mut encryption);
        expand_key(false, &key, &mut decryption);
        encryption.reverse();
        assert_eq!(encryption, decryption);
    }
}