cryptograph 0.1.12

All you need for encrypthing your messages.
Documentation 
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/// DES Substitution Boxes (S-Boxes).
///
/// Each S-Box maps a 6-bit input to a 4-bit output.
/// There are 8 S-Boxes used in DES, each with:
///
/// - 4 rows
/// - 16 columns
///
/// Structure:
///
/// SBOXES[box][row][column]
///
/// # Notes
///
/// - Input: 48-bit block (8 × 6 bits)
/// - Output: 32-bit block (8 × 4 bits)
/// - Core nonlinear component of DES
pub const SBOXES: [[[u8; 16]; 4]; 8] = [
    // S1
    [
        [14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7],
        [0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8],
        [4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0],
        [15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13],
    ],
    // S2
    [
        [15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10],
        [3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5],
        [0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15],
        [13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9],
    ],
    // S3
    [
        [10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8],
        [13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1],
        [13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7],
        [1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12],
    ],
    // S4
    [
        [7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15],
        [13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9],
        [10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4],
        [3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14],
    ],
    // S5
    [
        [2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9],
        [14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6],
        [4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14],
        [11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3],
    ],
    // S6
    [
        [12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11],
        [10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8],
        [9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6],
        [4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13],
    ],
    // S7
    [
        [4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1],
        [13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6],
        [1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2],
        [6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12],
    ],
    // S8
    [
        [13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7],
        [1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2],
        [7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8],
        [2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11],
    ],
];

/// Applies DES S-Box substitution.
///
/// Splits a 48-bit input into eight 6-bit blocks,
/// then applies the corresponding S-Box to each block.
///
/// # Arguments
///
/// * `right_x` - 48-bit expanded right half
///
/// # Returns
///
/// 32-bit substituted value
///
/// # Example
///
/// ```rust
/// use cryptograph::cryptography::des::s_box::s_box;
///
/// let input: u64 = 0x123456789ABC;
/// let result = s_box(input);
///
/// assert!(result <= u32::MAX);
/// ```
///
/// # Algorithm
///
/// For each 6-bit chunk:
///
/// - Row = first and last bit
/// - Column = middle four bits
/// - Lookup in SBOXES
///
/// # Notes
///
/// - This is the nonlinear step in DES
/// - Critical for cryptographic security
pub fn s_box(right_x: u64) -> u32 {
    let mut sbox: u32 = 0;

    for (table, i) in (0..(48 / 6)).rev().enumerate() {
        let bits: u8 = ((right_x >> (i * 6)) & 0b111111) as u8;

        let bit = bits & 1;
        let bit2 = bits >> 5 & 1;

        let column = (bits >> 1 & 0b1111) as usize;
        let row = (bit << 1 | bit2) as usize;

        let b: u32 = SBOXES[table][row][column] as u32;

        sbox <<= 4;
        sbox |= b;
    }

    sbox
}