courierust 1.0.0

//! AES block cipher (FIPS 197), encryption direction only.
//!
//! GCM mode (used by the TLS 1.3 AES suites) only ever invokes AES in
//! the encryption direction (the counter blocks are encrypted to produce
//! both keystream and the GHASH key), so no decryption path is needed.
//! The implementation is the standard S-box + xtime MixColumns form,
//! which is simple to audit; it operates on 16-byte blocks.

/// The AES S-box (FIPS 197 §5.1.1).
const 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,
];

/// Rcon (FIPS 197 §5.2).
const RCON: [u8; 10] = [0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36];

/// Multiply by 2 in GF(2^8) mod x^8+x^4+x^3+x+1.
#[inline]
fn xtime(b: u8) -> u8 {
    (b << 1) ^ (0x1b & (b >> 7).wrapping_neg())
}

/// An AES key schedule for either AES-128 or AES-256.
pub struct Aes {
    /// Round keys as 16-byte blocks.
    round_keys: alloc::vec::Vec<[u8; 16]>,
}

impl Aes {
    /// Expand `key` (16 or 32 bytes) into a round-key schedule.
    pub fn new(key: &[u8]) -> Option<Self> {
        let nk = match key.len() {
            16 => 4,
            32 => 8,
            _ => return None,
        };
        let nr = nk + 6; // 10 for 128, 14 for 256
        let mut w: alloc::vec::Vec<[u8; 4]> = alloc::vec::Vec::with_capacity(nk * (nr + 1));
        for i in 0..nk {
            let mut word = [0u8; 4];
            word.copy_from_slice(&key[i * 4..i * 4 + 4]);
            w.push(word);
        }
        for i in nk..nk * (nr + 1) {
            let mut temp = w[i - 1];
            if i % nk == 0 {
                temp = [
                    SBOX[temp[1] as usize],
                    SBOX[temp[2] as usize],
                    SBOX[temp[3] as usize],
                    SBOX[temp[0] as usize],
                ];
                temp[0] ^= RCON[(i / nk - 1) % RCON.len()];
            } else if nk > 6 && i % nk == 4 {
                temp = [
                    SBOX[temp[0] as usize],
                    SBOX[temp[1] as usize],
                    SBOX[temp[2] as usize],
                    SBOX[temp[3] as usize],
                ];
            }
            let prev = w[i - nk];
            w.push([
                prev[0] ^ temp[0],
                prev[1] ^ temp[1],
                prev[2] ^ temp[2],
                prev[3] ^ temp[3],
            ]);
        }
        let mut round_keys = alloc::vec::Vec::with_capacity(nr + 1);
        for r in 0..=nr {
            let mut block = [0u8; 16];
            for j in 0..4 {
                block[j * 4..j * 4 + 4].copy_from_slice(&w[r * 4 + j]);
            }
            round_keys.push(block);
        }
        Some(Self { round_keys })
    }

    /// Encrypt one 16-byte block in place.
    pub fn encrypt_block(&self, block: &mut [u8; 16]) {
        add_round_key(block, &self.round_keys[0]);
        for round in 1..self.round_keys.len() - 1 {
            for b in block.iter_mut() {
                *b = SBOX[*b as usize];
            }
            shift_rows(block);
            mix_columns(block);
            add_round_key(block, &self.round_keys[round]);
        }
        for b in block.iter_mut() {
            *b = SBOX[*b as usize];
        }
        shift_rows(block);
        add_round_key(block, &self.round_keys[self.round_keys.len() - 1]);
    }

    /// Number of rounds (10 for AES-128, 14 for AES-256).
    pub fn rounds(&self) -> usize {
        self.round_keys.len() - 1
    }
}

#[inline]
fn add_round_key(block: &mut [u8; 16], key: &[u8; 16]) {
    for i in 0..16 {
        block[i] ^= key[i];
    }
}

#[inline]
fn shift_rows(block: &mut [u8; 16]) {
    let s = *block;
    block[0] = s[0];
    block[1] = s[5];
    block[2] = s[10];
    block[3] = s[15];
    block[4] = s[4];
    block[5] = s[9];
    block[6] = s[14];
    block[7] = s[3];
    block[8] = s[8];
    block[9] = s[13];
    block[10] = s[2];
    block[11] = s[7];
    block[12] = s[12];
    block[13] = s[1];
    block[14] = s[6];
    block[15] = s[11];
}

#[inline]
fn mix_columns(block: &mut [u8; 16]) {
    for col in 0..4 {
        let i = col * 4;
        let (a0, a1, a2, a3) = (block[i], block[i + 1], block[i + 2], block[i + 3]);
        block[i] = xtime(a0) ^ (xtime(a1) ^ a1) ^ a2 ^ a3;
        block[i + 1] = a0 ^ xtime(a1) ^ (xtime(a2) ^ a2) ^ a3;
        block[i + 2] = a0 ^ a1 ^ xtime(a2) ^ (xtime(a3) ^ a3);
        block[i + 3] = (xtime(a0) ^ a0) ^ a1 ^ a2 ^ xtime(a3);
    }
}

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

    #[test]
    fn aes128_fips197_vector() {
        // FIPS 197 Appendix C.1: AES-128 encrypt.
        let key = [
            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
            0x0e, 0x0f,
        ];
        let plaintext = [
            0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd,
            0xee, 0xff,
        ];
        let expected = [
            0x69, 0xc4, 0xe0, 0xd8, 0x6a, 0x7b, 0x04, 0x30, 0xd8, 0xcd, 0xb7, 0x80, 0x70, 0xb4,
            0xc5, 0x5a,
        ];
        let aes = Aes::new(&key).unwrap();
        let mut block = plaintext;
        aes.encrypt_block(&mut block);
        assert_eq!(&block[..], &expected[..]);
    }

    #[test]
    fn aes128_round_keys() {
        // FIPS 197 §A.1.
        let key = [
            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
            0x0e, 0x0f,
        ];
        let aes = Aes::new(&key).unwrap();
        assert_eq!(
            &aes.round_keys[1][..],
            &[
                0xd6, 0xaa, 0x74, 0xfd, 0xd2, 0xaf, 0x72, 0xfa, 0xda, 0xa6, 0x78, 0xf1, 0xd6, 0xab,
                0x76, 0xfe,
            ][..]
        );
        assert_eq!(
            &aes.round_keys[2][..],
            &[
                0xb6, 0x92, 0xcf, 0x0b, 0x64, 0x3d, 0xbd, 0xf1, 0xbe, 0x9b, 0xc5, 0x00, 0x68, 0x30,
                0xb3, 0xfe,
            ][..]
        );
        assert_eq!(
            &aes.round_keys[3][..],
            &[
                0xb6, 0xff, 0x74, 0x4e, 0xd2, 0xc2, 0xc9, 0xbf, 0x6c, 0x59, 0x0c, 0xbf, 0x04, 0x69,
                0xbf, 0x41,
            ][..]
        );
    }

    #[test]
    fn aes256_round_keys() {
        // FIPS 197 §A.3: for AES-256, the key is 32 bytes, so round key
        // 1 is the second half of the key, round key 2 is w[8..12].
        let key: Vec<u8> = (0u8..0x20).collect();
        let aes = Aes::new(&key).unwrap();
        assert_eq!(
            &aes.round_keys[1][..],
            &(0x10u8..0x20).collect::<Vec<u8>>()[..]
        );
        assert_eq!(
            &aes.round_keys[2][..],
            &[
                0xa5, 0x73, 0xc2, 0x9f, 0xa1, 0x76, 0xc4, 0x98, 0xa9, 0x7f, 0xce, 0x93, 0xa5, 0x72,
                0xc0, 0x9c,
            ][..]
        );
        assert_eq!(
            &aes.round_keys[3][..],
            &[
                0x16, 0x51, 0xa8, 0xcd, 0x02, 0x44, 0xbe, 0xda, 0x1a, 0x5d, 0xa4, 0xc1, 0x06, 0x40,
                0xba, 0xde,
            ][..]
        );
    }

    #[test]
    fn aes256_fips197_vector() {
        // FIPS 197 Appendix C.3: AES-256 encrypt.
        let key = [
            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
            0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b,
            0x1c, 0x1d, 0x1e, 0x1f,
        ];
        let plaintext = [
            0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd,
            0xee, 0xff,
        ];
        let expected = [
            0x8e, 0xa2, 0xb7, 0xca, 0x51, 0x67, 0x45, 0xbf, 0xea, 0xfc, 0x49, 0x90, 0x4b, 0x49,
            0x60, 0x89,
        ];
        let aes = Aes::new(&key).unwrap();
        let mut block = plaintext;
        aes.encrypt_block(&mut block);
        assert_eq!(&block[..], &expected[..]);
    }
}