//! For LICENSE check out https://github.com/0xPARC/plonky2-aes/blob/main/LICENSE
//!
//! Rust native implementation of [AES-GCM (Galois Counter
//! Mode)](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf)
//! having in mind the approach that will be done in-circuit; this means that it
//! is not written in the most rust idiomatic way, nor memory-efficient, but
//! simulating the behavior that we will do later inside the circuit.
//!

use crate::native_aes::{encrypt_block, flatten_state, key_expansion, State};

// supported tag length
const TAG_LEN: usize = 128;

/// Section 7.1 from https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
pub fn encrypt<const NK: usize, const NB: usize, const NR: usize>(
    key: &[u8; NK * NB],
    nonce: &[u8; 12],
    pt: &[u8], // plaintext
) -> (Vec<u8>, Vec<u8>)
where
    [(); 4 * (NR + 1)]:,
{
    let a: &[u8] = &[]; // additional authenticated data // TODO maybe as input
    let expanded_key: [[u8; 4]; 4 * (NR + 1)] = key_expansion::<NK, NB, NR>(&key);

    // 1. CIPH_K
    let h = encrypt_block::<NR>(&[0u8; 16], &expanded_key);
    let h = flatten_state(h);

    // 2. J_0
    let j0: [u8; 16] = if nonce.len() * 8 == 96 {
        // J_0 = IV || 0^31 || 1
        let mut out = [0u8; 16];
        out[..12].copy_from_slice(&nonce.as_slice());
        out[12..16].copy_from_slice(&[0x00, 0x00, 0x00, 0x01]);
        out
    } else {
        panic!("unsuported at initial version; nonce.len()=12 (96 bits)");
    };

    // 3. C=GCTR()
    let c = gctr(expanded_key, &inc32(j0), &pt);

    // 4. u, v
    // let u = (16 - ceil(c.len() % 16)) % 16 // TODO WIP
    let u: usize = 128 * ((c.len() * 8) as f64 / 128_f64).ceil() as usize - c.len() * 8;
    let v: usize = 128 * ((a.len() * 8) as f64 / 128_f64).ceil() as usize - a.len() * 8;

    // 5. S = GHASH()
    let a_len: [u8; 8] = (a.len() * 8).to_be_bytes();
    let c_len: [u8; 8] = (c.len() * 8).to_be_bytes();
    let ghash_input = &[
        a.to_vec(),
        vec![0u8; v],
        c.clone(),
        vec![0u8; u],
        a_len.to_vec(),
        c_len.to_vec(),
    ]
    .concat();
    let s = ghash(h, ghash_input);

    // 6. T=MSB(GCTR()))
    let t = msb_t(TAG_LEN, &gctr(expanded_key, &j0, &s));

    (c, t)
}

/// GCM-Ctr, Section 6.5, Algorithm 3
fn gctr<const NR: usize>(key: [[u8; 4]; 4 * (NR + 1)], icb: &[u8; 16], x: &[u8]) -> Vec<u8> {
    if x.is_empty() {
        return x.to_vec();
    }

    let n = ((x.len() * 8) as f64 / 128_f64).ceil() as usize;

    let mut y = vec![];
    let mut cb_i = icb.clone();
    for (i, x_i_raw) in x.chunks(16).enumerate() {
        // 5.
        if i > 1 {
            cb_i = inc32(cb_i);
        }

        let l = x_i_raw.len().min(16);
        let mut x_i = [0u8; 16];
        x_i[..l].copy_from_slice(x_i_raw);

        let ciph_cb_i = flatten_state(encrypt_block::<NR>(&cb_i, &key));

        let y_i = if i < n {
            xor_blocks(x_i, ciph_cb_i)
        } else {
            // last chunk, might be smaller than 16 bytes
            let msb_res = msb_t(x_i.len(), ciph_cb_i.as_slice());
            let l = msb_res.len().min(16);
            let mut m = [0u8; 16];
            m[..l].copy_from_slice(&msb_res);
            xor_blocks(x_i, m)
        };
        y.push(y_i);
    }
    y.concat()
}

/// Section 6.4, Algorithm 2
fn ghash(h: [u8; 16], x: &[u8]) -> [u8; 16] {
    assert!(x.len() % 16 == 0); // multiple of 128 bits
    let m = x.len() / 16;

    let mut y = [0u8; 16]; // (128 bits)
    for i in 0..m {
        let mut xi = [0u8; 16];
        xi.clone_from_slice(&x[i * 16..i * 16 + 16]);
        let y_xi = xor_blocks(y, xi);
        y = gf_2_128_mul(y_xi, h);
    }
    y
}
fn xor_blocks(b1: [u8; 16], b2: [u8; 16]) -> [u8; 16] {
    let mut r = [0u8; 16];
    for i in 0..16 {
        r[i] = b1[i] ^ b2[i];
    }
    r
}

/// multiplication of blocks (in GF(2^128)), Algorithm 1, Section 6.3
pub fn gf_2_128_mul(x: [u8; 16], y: [u8; 16]) -> [u8; 16] {
    // R = 11100001 || 0^120
    const R: [u8; 16] = [
        0xE1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00,
    ];

    let mut z = [0u8; 16];
    let mut v = y;
    for i in 0..128 {
        // xi: i-th bit of x
        let byte_index = i / 8;
        let bit_index = 7 - (i % 8);
        let xi = (x[byte_index] >> bit_index) & 1;

        if xi == 1 {
            z = xor_blocks(z, v);
        }
        let lsb = v[15] & 1;
        right_shift_one(&mut v);
        if lsb == 1 {
            v = xor_blocks(v, R);
        }
    }
    z
}
fn right_shift_one(block: &mut [u8; 16]) {
    let mut carry = 0u8;

    for byte in block.iter_mut() {
        let new_carry = *byte & 1;
        *byte = (*byte >> 1) | (carry << 7);
        carry = new_carry;
    }
}

/// increment the right-most 32 bits of the given block (128 bits). Section 2.
pub fn inc32(b: [u8; 16]) -> [u8; 16] {
    let mut r = b.clone();
    // counter = last 32bits
    let counter = u32::from_be_bytes([r[12], r[13], r[14], r[15]]);
    let incremented = counter.wrapping_add(1);
    let bytes = incremented.to_be_bytes();
    r[12..16].copy_from_slice(&bytes);
    r
}

/// returns the t left-most (most-significant in BE) bits of the block
pub fn msb_t(t: usize, block: &[u8]) -> Vec<u8> {
    assert!(t <= 128, "MSB_t err: max 128 bits");

    let mut out = Vec::with_capacity((t + 7) / 8); // ceiling bytes
    let mut bits_left = t;
    for byte in block.iter() {
        if bits_left == 0 {
            break;
        }
        if bits_left >= 8 {
            out.push(*byte);
            bits_left -= 8;
        } else {
            // take the top `bits_left` bits of the byte
            let mask = 0xFFu8 << (8 - bits_left);
            out.push(byte & mask);
            break;
        }
    }
    out
}

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

    use aes_gcm::{
        aead::{Aead, AeadCore, Key, KeyInit, OsRng},
        Aes128Gcm, Nonce,
    };

    #[test]
    fn test_aes_gcm_encrypt_nist_vector() {
        // https://csrc.nist.gov/Projects/Cryptographic-Algorithm-Validation-Program/CAVP-TESTING-BLOCK-CIPHER-MODES

        // test vector at line 7
        let key: [u8; 16] = [
            0xcf, 0x06, 0x3a, 0x34, 0xd4, 0xa9, 0xa7, 0x6c, 0x2c, 0x86, 0x78, 0x7d, 0x3f, 0x96,
            0xdb, 0x71,
        ];
        let nonce: [u8; 12] = [
            0x11, 0x3b, 0x97, 0x85, 0x97, 0x18, 0x64, 0xc8, 0x3b, 0x01, 0xc7, 0x87,
        ];
        let plaintext = [];
        let _aad: &[u8] = &[];
        let expected_ciphertext: &[u8] = &[];
        let expected_tag: &[u8] = &[
            0x72, 0xac, 0x84, 0x93, 0xe3, 0xa5, 0x22, 0x8b, 0x5d, 0x13, 0x0a, 0x69, 0xd2, 0x51,
            0x0e, 0x42,
        ];
        let (c, tag) = encrypt::<4, 4, 10>(&key, &nonce, &plaintext);
        assert_eq!(c, expected_ciphertext);
        assert_eq!(tag, expected_tag);

        // test vector at line 4417
        let key: [u8; 16] = [
            0xe9, 0x8b, 0x72, 0xa9, 0x88, 0x1a, 0x84, 0xca, 0x6b, 0x76, 0xe0, 0xf4, 0x3e, 0x68,
            0x64, 0x7a,
        ];
        let nonce: [u8; 12] = [
            0x8b, 0x23, 0x29, 0x9f, 0xde, 0x17, 0x40, 0x53, 0xf3, 0xd6, 0x52, 0xba,
        ];
        let plaintext = [
            0x28, 0x28, 0x6a, 0x32, 0x12, 0x93, 0x25, 0x3c, 0x3e, 0x0a, 0xa2, 0x70, 0x4a, 0x27,
            0x80, 0x32,
        ];
        let _aad: &[u8] = &[];
        let expected_ciphertext: &[u8] = &[
            0x5a, 0x3c, 0x1c, 0xf1, 0x98, 0x5d, 0xbb, 0x8b, 0xed, 0x81, 0x80, 0x36, 0xfd, 0xd5,
            0xab, 0x42,
        ];
        let expected_tag: &[u8] = &[
            0x23, 0xc7, 0xab, 0x0f, 0x95, 0x2b, 0x70, 0x91, 0xcd, 0x32, 0x48, 0x35, 0x04, 0x3b,
            0x5e, 0xb5,
        ];
        let (c, tag) = encrypt::<4, 4, 10>(&key, &nonce, &plaintext);
        assert_eq!(c, expected_ciphertext);
        assert_eq!(tag, expected_tag);
    }

    /// test checking against external aes-gcm lib
    #[test]
    fn test_with_external_lib() -> anyhow::Result<()> {
        let key: &[u8; 16] = &[42; 16];
        let nonce: &[u8; 12] = &[111; 12];
        let pt: &[u8] = &[42u8; 16];

        let key_ext: &Key<Aes128Gcm> = key.into();
        let nonce_ext = Nonce::from_slice(nonce);
        let cipher = Aes128Gcm::new(&key_ext);

        let ciphertext = cipher.encrypt(&nonce_ext, pt.as_ref()).unwrap();

        let plaintext = cipher.decrypt(&nonce_ext, ciphertext.as_ref()).unwrap();
        assert_eq!(plaintext, pt);

        let (c, t) = encrypt::<4, 4, 10>(key, nonce, &pt);
        assert_eq!([c, t].concat(), ciphertext);
        Ok(())
    }
}