//! SHA-256 hash function (FIPS 180-4).
//!
//! 256-bit output, 512-bit (64-byte) blocks, Merkle-Damgård
//! construction with the standard `Ch` / `Maj` / `Σ₀` / `Σ₁` /
//! `σ₀` / `σ₁` round mixers.
//!
//! # Side-channel posture
//!
//! As an *unkeyed* hash, SHA-256 is itself **not SCA-sensitive**;
//! it has no secret input. The compression function is CT by
//! construction — fixed-iteration loop, constant rotations / shifts,
//! no table lookups indexed by secret-derived values.
//!
//! As a *keyed* primitive consumed by HMAC, RFC 6979 HMAC-DRBG,
//! or RSA-PSS / OAEP MGF1, the same compression function carries
//! a secret into its internal state and becomes vulnerable to
//! the carry-based DPA result of `belenky2023_cdpa_hmac_sha2`
//! (TCHES 2023/3) — any arithmetic-addition-based hash leaks the
//! key in 30 K – 275 K traces. Roadmap item `T2-D` (see
//! `arcana/doc/sca/countermeasures/hmac.rst`) ships a
//! `MaskedSha256` variant behind the `sca-protected` feature.

use crate::Hasher;

const H0: [u32; 8] = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
];

const K: [u32; 64] = [
    0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5, 0xd807aa98,
    0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786,
    0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152, 0xa831c66d, 0xb00327c8,
    0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
    0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819,
    0xd6990624, 0xf40e3585, 0x106aa070, 0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a,
    0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7,
    0xc67178f2,
];

/// SHA-256 hasher (FIPS 180-4). 256-bit output, 64-byte blocks.
///
/// The workhorse hash of TLS 1.2 / 1.3, JWS, X.509 SHA-2 signatures,
/// Bitcoin, and almost every other modern protocol that needs a
/// 128-bit-security hash function.
#[derive(Clone)]
pub struct Sha256 {
    state: [u32; 8],
    buf: [u8; 64],
    buf_len: usize,
    total_len: u64,
}

fn compress(state: &mut [u32; 8], block: &[u8]) {
    let mut w = [0u32; 64];
    for i in 0..16 {
        w[i] = u32::from_be_bytes([block[4 * i], block[4 * i + 1], block[4 * i + 2], block[4 * i + 3]]);
    }
    for i in 16..64 {
        let s0 = w[i - 15].rotate_right(7) ^ w[i - 15].rotate_right(18) ^ (w[i - 15] >> 3);
        let s1 = w[i - 2].rotate_right(17) ^ w[i - 2].rotate_right(19) ^ (w[i - 2] >> 10);
        w[i] = w[i - 16].wrapping_add(s0).wrapping_add(w[i - 7]).wrapping_add(s1);
    }

    let mut a = state[0];
    let mut b = state[1];
    let mut c = state[2];
    let mut d = state[3];
    let mut e = state[4];
    let mut f = state[5];
    let mut g = state[6];
    let mut h = state[7];

    for i in 0..64 {
        let s1 = e.rotate_right(6) ^ e.rotate_right(11) ^ e.rotate_right(25);
        let ch = (e & f) ^ ((!e) & g);
        let temp1 = h
            .wrapping_add(s1)
            .wrapping_add(ch)
            .wrapping_add(K[i])
            .wrapping_add(w[i]);
        let s0 = a.rotate_right(2) ^ a.rotate_right(13) ^ a.rotate_right(22);
        let maj = (a & b) ^ (a & c) ^ (b & c);
        let temp2 = s0.wrapping_add(maj);

        h = g;
        g = f;
        f = e;
        e = d.wrapping_add(temp1);
        d = c;
        c = b;
        b = a;
        a = temp1.wrapping_add(temp2);
    }

    state[0] = state[0].wrapping_add(a);
    state[1] = state[1].wrapping_add(b);
    state[2] = state[2].wrapping_add(c);
    state[3] = state[3].wrapping_add(d);
    state[4] = state[4].wrapping_add(e);
    state[5] = state[5].wrapping_add(f);
    state[6] = state[6].wrapping_add(g);
    state[7] = state[7].wrapping_add(h);
}

impl Sha256 {
    pub(crate) fn new_with_iv(iv: [u32; 8]) -> Self {
        Self {
            state: iv,
            buf: [0u8; 64],
            buf_len: 0,
            total_len: 0,
        }
    }
}

impl Hasher for Sha256 {
    const OUTPUT_LEN: usize = 32;
    const BLOCK_LEN: usize = 64;

    fn new() -> Self {
        Self {
            state: H0,
            buf: [0u8; 64],
            buf_len: 0,
            total_len: 0,
        }
    }

    fn update(&mut self, data: &[u8]) {
        let mut pos = 0;
        self.total_len += data.len() as u64;

        if self.buf_len > 0 {
            let need = 64 - self.buf_len;
            let take = need.min(data.len());
            self.buf[self.buf_len..self.buf_len + take].copy_from_slice(&data[..take]);
            self.buf_len += take;
            pos = take;
            if self.buf_len == 64 {
                let block = self.buf;
                compress(&mut self.state, &block);
                self.buf_len = 0;
            }
        }

        while pos + 64 <= data.len() {
            compress(&mut self.state, &data[pos..pos + 64]);
            pos += 64;
        }

        if pos < data.len() {
            let remaining = data.len() - pos;
            self.buf[..remaining].copy_from_slice(&data[pos..]);
            self.buf_len = remaining;
        }
    }

    fn finalize(self) -> Vec<u8> {
        let mut out = vec![0u8; 32];
        self.finalize_into(&mut out);
        out
    }

    fn finalize_into(mut self, out: &mut [u8]) {
        let bit_len = self.total_len * 8;
        let mut pad = [0u8; 72];
        pad[0] = 0x80;
        let pad_len = if self.buf_len < 56 {
            56 - self.buf_len
        } else {
            120 - self.buf_len
        };
        self.update(&pad[..pad_len]);
        self.update(&bit_len.to_be_bytes());

        for (i, word) in self.state.iter().enumerate() {
            let bytes = word.to_be_bytes();
            let start = i * 4;
            if start + 4 <= out.len() {
                out[start..start + 4].copy_from_slice(&bytes);
            } else if start < out.len() {
                let end = out.len() - start;
                out[start..].copy_from_slice(&bytes[..end]);
            }
        }
    }
}

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

    #[test]
    fn test_sha256_empty() {
        let digest = Sha256::hash(b"");
        let expected: [u8; 32] = [
            0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, 0x27, 0xae,
            0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55,
        ];
        assert_eq!(&digest[..], &expected[..]);
    }

    #[test]
    fn test_sha256_abc() {
        let digest = Sha256::hash(b"abc");
        let expected: [u8; 32] = [
            0xba, 0x78, 0x16, 0xbf, 0x8f, 0x01, 0xcf, 0xea, 0x41, 0x41, 0x40, 0xde, 0x5d, 0xae, 0x22, 0x23, 0xb0, 0x03,
            0x61, 0xa3, 0x96, 0x17, 0x7a, 0x9c, 0xb4, 0x10, 0xff, 0x61, 0xf2, 0x00, 0x15, 0xad,
        ];
        assert_eq!(&digest[..], &expected[..]);
    }
}
