//! SM4 — the Chinese national block cipher (GB/T 32907-2016, RFC 8998 §2).
//!
//! SM4 is a 128-bit-block, 128-bit-key Feistel-like cipher with 32 rounds. Each
//! round applies the composite transform `T = L ∘ τ`, where `τ` is a
//! parallel S-box substitution on four bytes and `L` is a fixed GF(2)-linear
//! diffusion `L(B) = B ⊕ (B<<<2) ⊕ (B<<<10) ⊕ (B<<<18) ⊕ (B<<<24)`. The key
//! schedule uses the same S-box with a different linear map `L'` plus the system
//! parameter `FK` and the round constants `CK`.
//!
//! It implements [`BlockCipher`], so it composes with the crate's CBC, CTR and
//! GCM modes exactly like AES. Round keys are wiped on drop.
//!
//! Like the table-free AES core in this crate, the S-box is **computed**
//! rather than looked up: `S(x) = A(inv(A(x)))`, where `A` is a fixed affine
//! transform and `inv` is inversion in GF(2⁸) modulo `x⁸+x⁷+x⁶+x⁵+x⁴+x²+1`
//! (`0x1f5`). Every operation is branchless with no secret-dependent memory
//! access, so SM4 is hardened against cache-timing attacks. The algebraic
//! S-box costs roughly an order of magnitude more than a naive table lookup;
//! the four bytes of each round word are processed in parallel as `u32` lanes
//! to claw most of that back.

use super::BlockCipher;

/// System parameter `FK` (GB/T 32907 §7.3.2).
const FK: [u32; 4] = [0xa3b1bac6, 0x56aa3350, 0x677d9197, 0xb27022dc];

/// Fixed round-key parameters `CK` (GB/T 32907 §7.3.2).
#[rustfmt::skip]
const CK: [u32; 32] = [
    0x00070e15, 0x1c232a31, 0x383f464d, 0x545b6269,
    0x70777e85, 0x8c939aa1, 0xa8afb6bd, 0xc4cbd2d9,
    0xe0e7eef5, 0xfc030a11, 0x181f262d, 0x343b4249,
    0x50575e65, 0x6c737a81, 0x888f969d, 0xa4abb2b9,
    0xc0c7ced5, 0xdce3eaf1, 0xf8ff060d, 0x141b2229,
    0x30373e45, 0x4c535a61, 0x686f767d, 0x848b9299,
    0xa0a7aeb5, 0xbcc3cad1, 0xd8dfe6ed, 0xf4fb0209,
    0x10171e25, 0x2c333a41, 0x484f565d, 0x646b7279,
];

// --- Constant-time GF(2⁸) arithmetic, four byte lanes packed in a `u32` ---
//
// The SM4 field is GF(2⁸) with reduction polynomial
// `x⁸ + x⁷ + x⁶ + x⁵ + x⁴ + x² + 1` (`0x1f5`). Everything below mirrors the
// table-free AES core in `aes::gf`, except that the four S-box applications
// of `τ` are evaluated simultaneously, one per byte lane of a `u32`.

/// Broadcasts the low bit of each byte lane (0 or 1) to the whole lane
/// (`0x00` or `0xff`).
#[inline]
fn lane_mask(m: u32) -> u32 {
    // With bit b_k at position 8k, (m << 8) - m == Σ b_k · (0xff << 8k); the
    // top lane's overflow wraps away harmlessly.
    (m << 8).wrapping_sub(m)
}

/// Multiplies four pairs of field elements lane-wise in constant time
/// (branchless Russian-peasant multiplication with reduction by `0x1f5`).
#[inline]
fn gf_mul4(mut a: u32, mut b: u32) -> u32 {
    let mut product = 0u32;
    let mut i = 0;
    while i < 8 {
        // Add lane `a` into the product where that lane's low bit of `b` is set.
        product ^= lane_mask(b & 0x0101_0101) & a;
        // a = xtime(a): multiply by x, reducing mod 0x1f5 on carry-out.
        let carry = lane_mask((a >> 7) & 0x0101_0101);
        a = ((a << 1) & 0xfefe_fefe) ^ (carry & 0xf5f5_f5f5);
        b = (b >> 1) & 0x7f7f_7f7f;
        i += 1;
    }
    product
}

/// Lane-wise multiplicative inverse in GF(2⁸), with `inverse(0) = 0`
/// (matching the S-box convention).
///
/// Computed as `x²⁵⁴`; since `x²⁵⁵ = 1` for every nonzero `x`, this equals
/// `x⁻¹`. The fixed addition chain runs in constant time.
#[inline]
fn gf_inv4(x: u32) -> u32 {
    let x2 = gf_mul4(x, x); // x^2
    let x4 = gf_mul4(x2, x2); // x^4
    let x8 = gf_mul4(x4, x4); // x^8
    let x16 = gf_mul4(x8, x8); // x^16
    let x32 = gf_mul4(x16, x16); // x^32
    let x64 = gf_mul4(x32, x32); // x^64
    let x128 = gf_mul4(x64, x64); // x^128

    // x^254 = x^2 · x^4 · x^8 · x^16 · x^32 · x^64 · x^128
    let mut r = x2;
    r = gf_mul4(r, x4);
    r = gf_mul4(r, x8);
    r = gf_mul4(r, x16);
    r = gf_mul4(r, x32);
    r = gf_mul4(r, x64);
    gf_mul4(r, x128)
}

/// Rotates each byte lane left by `N` bits (1 ≤ `N` ≤ 7).
#[inline]
fn rotl4<const N: u32>(x: u32) -> u32 {
    let keep = 0x0101_0101u32 * ((0xff << N) & 0xff);
    let wrap = 0x0101_0101u32 * (0xff >> (8 - N));
    ((x << N) & keep) | ((x >> (8 - N)) & wrap)
}

/// S-box affine transform, lane-wise:
/// `A(x) = x ⊕ (x<<<1) ⊕ (x<<<3) ⊕ (x<<<6) ⊕ (x<<<7) ⊕ 0xd3`.
#[inline]
fn affine4(x: u32) -> u32 {
    x ^ rotl4::<1>(x) ^ rotl4::<3>(x) ^ rotl4::<6>(x) ^ rotl4::<7>(x) ^ 0xd3d3_d3d3
}

/// Nonlinear transform `τ`: applies the S-box to each of the four bytes.
///
/// The GB/T 32907 §6.2 S-box decomposes as `S(x) = A(inv(A(x)))` — the same
/// affine transform before and after a field inversion (the
/// `sbox_matches_reference_table` test checks this against the standard's
/// table for every input). All four bytes are substituted in parallel,
/// without table lookups.
#[inline]
fn tau(x: u32) -> u32 {
    affine4(gf_inv4(affine4(x)))
}

/// Round linear transform `L(B) = B ⊕ (B<<<2) ⊕ (B<<<10) ⊕ (B<<<18) ⊕ (B<<<24)`.
#[inline]
fn l_round(b: u32) -> u32 {
    b ^ b.rotate_left(2) ^ b.rotate_left(10) ^ b.rotate_left(18) ^ b.rotate_left(24)
}

/// Key-schedule linear transform `L'(B) = B ⊕ (B<<<13) ⊕ (B<<<23)`.
#[inline]
fn l_key(b: u32) -> u32 {
    b ^ b.rotate_left(13) ^ b.rotate_left(23)
}

/// Round composite `T = L ∘ τ`.
#[inline]
fn t_round(x: u32) -> u32 {
    l_round(tau(x))
}

/// Key-schedule composite `T' = L' ∘ τ`.
#[inline]
fn t_key(x: u32) -> u32 {
    l_key(tau(x))
}

/// SM4 block cipher (GB/T 32907 / RFC 8998).
#[derive(Clone)]
pub struct Sm4 {
    /// The 32 round keys.
    rk: [u32; 32],
}

impl Sm4 {
    /// Creates an SM4 cipher from a 128-bit key, expanding the round keys
    /// (GB/T 32907 §7.3).
    pub fn new(key: &[u8; 16]) -> Self {
        let mut k = [0u32; 4];
        for (i, w) in k.iter_mut().enumerate() {
            *w = u32::from_be_bytes([key[4 * i], key[4 * i + 1], key[4 * i + 2], key[4 * i + 3]])
                ^ FK[i];
        }
        let mut rk = [0u32; 32];
        for i in 0..32 {
            let t = k[1] ^ k[2] ^ k[3] ^ CK[i];
            let next = k[0] ^ t_key(t);
            rk[i] = next;
            k[0] = k[1];
            k[1] = k[2];
            k[2] = k[3];
            k[3] = next;
        }
        Sm4 { rk }
    }

    /// Runs the 32-round network with the given (possibly reversed) round-key
    /// order, transforming `block` in place.
    #[inline]
    fn crypt(&self, block: &mut [u8; 16], reverse: bool) {
        let mut x = [0u32; 4];
        for (i, w) in x.iter_mut().enumerate() {
            *w = u32::from_be_bytes([
                block[4 * i],
                block[4 * i + 1],
                block[4 * i + 2],
                block[4 * i + 3],
            ]);
        }
        for round in 0..32 {
            let rk = if reverse {
                self.rk[31 - round]
            } else {
                self.rk[round]
            };
            let t = x[1] ^ x[2] ^ x[3] ^ rk;
            let next = x[0] ^ t_round(t);
            x[0] = x[1];
            x[1] = x[2];
            x[2] = x[3];
            x[3] = next;
        }
        // Reverse-order output transform R: (Y0,Y1,Y2,Y3) = (X35,X34,X33,X32).
        let out = [x[3], x[2], x[1], x[0]];
        for (i, w) in out.iter().enumerate() {
            block[4 * i..4 * i + 4].copy_from_slice(&w.to_be_bytes());
        }
    }
}

impl BlockCipher for Sm4 {
    const BLOCK_SIZE: usize = 16;
    const KEY_SIZE: usize = 16;

    #[inline]
    fn encrypt_block(&self, block: &mut [u8; 16]) {
        self.crypt(block, false);
    }

    #[inline]
    fn decrypt_block(&self, block: &mut [u8; 16]) {
        // Decryption is the same network with the round keys reversed.
        self.crypt(block, true);
    }
}

impl Drop for Sm4 {
    fn drop(&mut self) {
        // Best-effort wipe of the round keys, mirroring the AES round-key drop.
        self.rk = [0u32; 32];
        let _ = core::hint::black_box(&self.rk);
    }
}

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

    /// The SM4 S-box table (GB/T 32907 §6.2), kept here purely as the
    /// reference the computed constant-time S-box is checked against.
    #[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 computed constant-time S-box must agree with the GB/T 32907 table
    // for every input, in every lane.
    #[test]
    fn sbox_matches_reference_table() {
        for x in 0u32..=255 {
            // All four lanes carry the same byte …
            let same = tau(u32::from_be_bytes([x as u8; 4]));
            assert_eq!(
                same.to_be_bytes(),
                [SBOX[x as usize]; 4],
                "S-box mismatch for {x:#04x}"
            );
            // … and lanes carrying distinct bytes stay independent.
            let b = [
                x as u8,
                (x as u8).wrapping_add(1),
                !(x as u8),
                (x as u8).rotate_left(3),
            ];
            let mixed = tau(u32::from_be_bytes(b)).to_be_bytes();
            for i in 0..4 {
                assert_eq!(mixed[i], SBOX[b[i] as usize], "lane {i} for {x:#04x}");
            }
        }
    }

    // GB/T 32907 Appendix A.1 / RFC 8998 Appendix A.1: single-block KAT.
    #[test]
    fn gbt32907_example() {
        let key = from_hex::<16>("0123456789abcdeffedcba9876543210");
        let cipher = Sm4::new(&key);
        let mut block = from_hex::<16>("0123456789abcdeffedcba9876543210");
        cipher.encrypt_block(&mut block);
        assert_eq!(block, from_hex::<16>("681edf34d206965e86b3e94f536e4246"));
        cipher.decrypt_block(&mut block);
        assert_eq!(block, from_hex::<16>("0123456789abcdeffedcba9876543210"));
    }

    // GB/T 32907 Appendix A.2: encrypt the example block 1,000,000 times.
    #[test]
    fn gbt32907_million_iterations() {
        let key = from_hex::<16>("0123456789abcdeffedcba9876543210");
        let cipher = Sm4::new(&key);
        let mut block = from_hex::<16>("0123456789abcdeffedcba9876543210");
        for _ in 0..1_000_000 {
            cipher.encrypt_block(&mut block);
        }
        assert_eq!(block, from_hex::<16>("595298c7c6fd271f0402f804c33d3f66"));
    }

    // Round-trip over all single-byte fill patterns.
    #[test]
    fn roundtrip_all_byte_values() {
        let key = from_hex::<16>("0123456789abcdeffedcba9876543210");
        let cipher = Sm4::new(&key);
        for v in 0u16..=255 {
            let original = [v as u8; 16];
            let mut block = original;
            cipher.encrypt_block(&mut block);
            assert_ne!(block, original, "ciphertext should differ from plaintext");
            cipher.decrypt_block(&mut block);
            assert_eq!(block, original);
        }
    }
}
