algos/cs/security/

aes.rs

//! DISCLAIMER: This library is a toy example of the AES (Rijndael) block cipher in pure Rust.
//! It is *EXCLUSIVELY* for demonstration and educational purposes. Absolutely DO NOT use it
//! for real cryptographic or security-sensitive operations. It is not audited, not vetted,
//! and very likely insecure in practice. If you need AES or any cryptographic operations in
//! production, please use a vetted, well-reviewed cryptography library (e.g. RustCrypto).

use core::convert::TryInto;

/// AES block size in bytes (128 bits).
pub const AES_BLOCK_SIZE: usize = 16;

/// Represents key sizes for AES: 128, 192, or 256 bits.
#[derive(Debug, Clone, Copy)]
pub enum AesKeySize {
    Bits128,
    Bits192,
    Bits256,
}

/// An AES key schedule object, storing the round keys after expansion.
pub struct AesKey {
    pub round_keys: Vec<[u8; AES_BLOCK_SIZE]>,
    pub nr: usize, // number of rounds
}

impl AesKey {
    /// Create a new AES key from a given key material (raw bytes) and key size variant.
    ///
    /// # Panics
    /// Panics if the key material length doesn't match the indicated `AesKeySize`.
    ///
    /// DO NOT USE THIS FOR REAL SECURITY.
    pub fn new(key_data: &[u8], key_size: AesKeySize) -> Self {
        let (key_len, nr, nk) = match key_size {
            AesKeySize::Bits128 => (16, 10, 4),
            AesKeySize::Bits192 => (24, 12, 6),
            AesKeySize::Bits256 => (32, 14, 8),
        };
        assert_eq!(
            key_data.len(),
            key_len,
            "Key length mismatch for AES key size"
        );

        let expanded_len = AES_BLOCK_SIZE * (nr + 1);
        let mut round_keys = vec![0u8; expanded_len];
        // copy initial key
        round_keys[..key_len].copy_from_slice(key_data);

        key_expansion(&mut round_keys, nk, nr);

        // Convert round_keys to round-key blocks
        let mut round_blocks = Vec::with_capacity(nr + 1);
        for i in 0..(nr + 1) {
            let offset = i * AES_BLOCK_SIZE;
            let block: [u8; AES_BLOCK_SIZE] = round_keys[offset..offset + AES_BLOCK_SIZE]
                .try_into()
                .unwrap();
            round_blocks.push(block);
        }

        Self {
            round_keys: round_blocks,
            nr,
        }
    }
}

/// Encrypt a single 128-bit block `plaintext` in place using the provided AES key schedule.
/// *This is toy code. DO NOT use in production.*
///
/// # Panics
/// Panics if `plaintext.len() != 16`.
pub fn aes_encrypt_block(plaintext: &mut [u8; AES_BLOCK_SIZE], key: &AesKey) {
    add_round_key(plaintext, &key.round_keys[0]);

    for round in 1..key.nr {
        sub_bytes(plaintext);
        shift_rows(plaintext);
        mix_columns(plaintext);
        add_round_key(plaintext, &key.round_keys[round]);
    }

    // final round
    sub_bytes(plaintext);
    shift_rows(plaintext);
    add_round_key(plaintext, &key.round_keys[key.nr]);
}

/// Decrypt a single 128-bit block `ciphertext` in place using the provided AES key schedule.
/// *This is toy code. DO NOT use in production.*
///
/// # Panics
/// Panics if `ciphertext.len() != 16`.
pub fn aes_decrypt_block(ciphertext: &mut [u8; AES_BLOCK_SIZE], key: &AesKey) {
    add_round_key(ciphertext, &key.round_keys[key.nr]);
    inv_shift_rows(ciphertext);
    inv_sub_bytes(ciphertext);

    for round in (1..key.nr).rev() {
        add_round_key(ciphertext, &key.round_keys[round]);
        inv_mix_columns(ciphertext);
        inv_shift_rows(ciphertext);
        inv_sub_bytes(ciphertext);
    }

    // final
    add_round_key(ciphertext, &key.round_keys[0]);
}

// ---------------- Internal Implementation Details (toy) ---------------- //
// The following code includes S-Boxes, inverse S-Boxes, Rcon constants,
// and standard AES transformations. DO NOT rely on for real usage.

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

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

/// Round constant for key expansion
static RCON: [u8; 255] = {
    let mut rcon = [0u8; 255];
    rcon[0] = 0x00;
    rcon[1] = 0x01;
    rcon[2] = 0x02;
    rcon[3] = 0x04;
    rcon[4] = 0x08;
    rcon[5] = 0x10;
    rcon[6] = 0x20;
    rcon[7] = 0x40;
    rcon[8] = 0x80;
    rcon[9] = 0x1B;
    rcon[10] = 0x36;
    // Only first 11 values are needed for AES-128
    rcon
};

fn sub_bytes(state: &mut [u8; AES_BLOCK_SIZE]) {
    for b in state.iter_mut() {
        *b = SBOX[*b as usize];
    }
}

fn inv_sub_bytes(state: &mut [u8; AES_BLOCK_SIZE]) {
    for b in state.iter_mut() {
        *b = INV_SBOX[*b as usize];
    }
}

fn shift_rows(state: &mut [u8; AES_BLOCK_SIZE]) {
    // row 1 shift by 1
    let row1 = [state[1], state[5], state[9], state[13]];
    state[1] = row1[1];
    state[5] = row1[2];
    state[9] = row1[3];
    state[13] = row1[0];

    // row 2 shift by 2
    let row2 = [state[2], state[6], state[10], state[14]];
    state[2] = row2[2];
    state[6] = row2[3];
    state[10] = row2[0];
    state[14] = row2[1];

    // row 3 shift by 3
    let row3 = [state[3], state[7], state[11], state[15]];
    state[3] = row3[3];
    state[7] = row3[0];
    state[11] = row3[1];
    state[15] = row3[2];
}

fn inv_shift_rows(state: &mut [u8; AES_BLOCK_SIZE]) {
    // row 1 shift right by 1
    let row1 = [state[1], state[5], state[9], state[13]];
    state[1] = row1[3];
    state[5] = row1[0];
    state[9] = row1[1];
    state[13] = row1[2];

    // row 2 shift right by 2
    let row2 = [state[2], state[6], state[10], state[14]];
    state[2] = row2[2];
    state[6] = row2[3];
    state[10] = row2[0];
    state[14] = row2[1];

    // row 3 shift right by 3
    let row3 = [state[3], state[7], state[11], state[15]];
    state[3] = row3[1];
    state[7] = row3[2];
    state[11] = row3[3];
    state[15] = row3[0];
}

fn xtime(x: u8) -> u8 {
    if (x & 0x80) != 0 {
        (x << 1) ^ 0x1B
    } else {
        x << 1
    }
}

fn mix_columns(state: &mut [u8; AES_BLOCK_SIZE]) {
    for col in 0..4 {
        let base = col * 4;
        let t = state[base] ^ state[base + 1] ^ state[base + 2] ^ state[base + 3];
        let temp0 = state[base];
        let temp1 = state[base + 1];
        let temp2 = state[base + 2];
        let temp3 = state[base + 3];

        state[base] ^= t ^ xtime(temp0 ^ temp1);
        state[base + 1] ^= t ^ xtime(temp1 ^ temp2);
        state[base + 2] ^= t ^ xtime(temp2 ^ temp3);
        state[base + 3] ^= t ^ xtime(temp3 ^ temp0);
    }
}

fn inv_mix_columns(state: &mut [u8; AES_BLOCK_SIZE]) {
    // The standard approach is to multiply the state columns by the inverse of the MDS matrix
    // We'll do it in the typical inline approach:
    for col in 0..4 {
        let base = col * 4;
        let a0 = state[base];
        let a1 = state[base + 1];
        let a2 = state[base + 2];
        let a3 = state[base + 3];

        state[base] = mul(a0, 0x0e) ^ mul(a1, 0x0b) ^ mul(a2, 0x0d) ^ mul(a3, 0x09);
        state[base + 1] = mul(a0, 0x09) ^ mul(a1, 0x0e) ^ mul(a2, 0x0b) ^ mul(a3, 0x0d);
        state[base + 2] = mul(a0, 0x0d) ^ mul(a1, 0x09) ^ mul(a2, 0x0e) ^ mul(a3, 0x0b);
        state[base + 3] = mul(a0, 0x0b) ^ mul(a1, 0x0d) ^ mul(a2, 0x09) ^ mul(a3, 0x0e);
    }
}

fn mul(x: u8, y: u8) -> u8 {
    // Galois Field (2^8) multiplication
    let mut r = 0;
    let mut a = x;
    let mut b = y;
    for _ in 0..8 {
        if (b & 1) == 1 {
            r ^= a;
        }
        let hi_bit_set = (a & 0x80) != 0;
        a <<= 1;
        if hi_bit_set {
            a ^= 0x1b;
        }
        b >>= 1;
    }
    r
}

fn add_round_key(state: &mut [u8; AES_BLOCK_SIZE], round_key: &[u8; AES_BLOCK_SIZE]) {
    for (s, k) in state.iter_mut().zip(round_key) {
        *s ^= *k;
    }
}

// Key Expansion routines
fn key_expansion(expanded: &mut [u8], nk: usize, nr: usize) {
    let total_words = (nr + 1) * 4; // number of 32-bit words
    let mut i = nk;
    while i < total_words {
        let mut temp = [
            expanded[(i - 1) * 4],
            expanded[(i - 1) * 4 + 1],
            expanded[(i - 1) * 4 + 2],
            expanded[(i - 1) * 4 + 3],
        ];

        if i % nk == 0 {
            // rotate
            temp = [temp[1], temp[2], temp[3], temp[0]];
            // sub
            for t in temp.iter_mut() {
                *t = SBOX[*t as usize];
            }
            // rcon
            temp[0] ^= RCON[i / nk];
        } else if nk > 6 && i % nk == 4 {
            for t in temp.iter_mut() {
                *t = SBOX[*t as usize];
            }
        }

        let wprev = (i - nk) * 4;
        for (j, tj) in temp.iter().enumerate() {
            expanded[i * 4 + j] = expanded[wprev + j] ^ tj;
        }
        i += 1;
    }
}

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

    // Example test using NIST known test vectors for AES-128 single block
    // "Fips-197" example: key=2b7e151628aed2a6abf7158809cf4f3c, plaintext=6bc1bee22e409f96e93d7e117393172a
    // ciphertext=3ad77bb40d7a3660a89ecaf32466ef97
    #[test]
    fn test_aes128_encrypt_block() {
        let key_data = hex_to_bytes("2b7e151628aed2a6abf7158809cf4f3c");
        let mut block = hex_to_array("6bc1bee22e409f96e93d7e117393172a");

        let aes_key = AesKey::new(&key_data, AesKeySize::Bits128);

        aes_encrypt_block(&mut block, &aes_key);

        let expected = hex_to_array("3ad77bb40d7a3660a89ecaf32466ef97");
        assert_eq!(block, expected);
    }

    #[test]
    fn test_aes128_decrypt_block() {
        let key_data = hex_to_bytes("2b7e151628aed2a6abf7158809cf4f3c");
        let mut block = hex_to_array("3ad77bb40d7a3660a89ecaf32466ef97");

        let aes_key = AesKey::new(&key_data, AesKeySize::Bits128);

        aes_decrypt_block(&mut block, &aes_key);

        let expected = hex_to_array("6bc1bee22e409f96e93d7e117393172a");
        assert_eq!(block, expected);
    }

    // Helpers
    fn hex_to_bytes(s: &str) -> Vec<u8> {
        (0..s.len())
            .step_by(2)
            .map(|i| u8::from_str_radix(&s[i..i + 2], 16).unwrap())
            .collect()
    }

    fn hex_to_array(s: &str) -> [u8; 16] {
        let bytes = hex_to_bytes(s);
        bytes.try_into().unwrap()
    }
}