yana-rt 0.42.1

//! Pure-Rust SHA-256 + HMAC-SHA256 — no external crates.
//!
//! Why not the `sha2`/`hmac` crates (which Ruflo's proof.rs uses)? Because this
//! repo's build already broke once on a transitive edition2024 dep, and the
//! evidence guard must NEVER be the thing that fails a build — a safety layer
//! you can't compile is worse than none. SHA-256 is a fixed, well-specified
//! algorithm (FIPS 180-4); a self-contained ~80-line implementation is more
//! robust here than three more crates in the dependency tree.
//!
//! The HMAC construction follows RFC 2104. Test vectors below pin both against
//! the published standards so a refactor can't silently corrupt them.

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 digest of arbitrary bytes.
pub fn sha256(data: &[u8]) -> [u8; 32] {
    let mut h = H0;
    let mut msg = data.to_vec();
    let bit_len = (data.len() as u64).wrapping_mul(8);
    msg.push(0x80);
    while msg.len() % 64 != 56 {
        msg.push(0);
    }
    msg.extend_from_slice(&bit_len.to_be_bytes());

    for chunk in msg.chunks_exact(64) {
        let mut w = [0u32; 64];
        for (i, word) in chunk.chunks_exact(4).enumerate() {
            w[i] = u32::from_be_bytes([word[0], word[1], word[2], word[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 v = h;
        for i in 0..64 {
            let s1 = v[4].rotate_right(6) ^ v[4].rotate_right(11) ^ v[4].rotate_right(25);
            let ch = (v[4] & v[5]) ^ ((!v[4]) & v[6]);
            let t1 = v[7]
                .wrapping_add(s1)
                .wrapping_add(ch)
                .wrapping_add(K[i])
                .wrapping_add(w[i]);
            let s0 = v[0].rotate_right(2) ^ v[0].rotate_right(13) ^ v[0].rotate_right(22);
            let maj = (v[0] & v[1]) ^ (v[0] & v[2]) ^ (v[1] & v[2]);
            let t2 = s0.wrapping_add(maj);
            v[7] = v[6];
            v[6] = v[5];
            v[5] = v[4];
            v[4] = v[3].wrapping_add(t1);
            v[3] = v[2];
            v[2] = v[1];
            v[1] = v[0];
            v[0] = t1.wrapping_add(t2);
        }
        for i in 0..8 {
            h[i] = h[i].wrapping_add(v[i]);
        }
    }

    let mut out = [0u8; 32];
    for (i, word) in h.iter().enumerate() {
        out[i * 4..i * 4 + 4].copy_from_slice(&word.to_be_bytes());
    }
    out
}

/// HMAC-SHA256 (RFC 2104) over `msg` with `key`.
pub fn hmac_sha256(key: &[u8], msg: &[u8]) -> [u8; 32] {
    const BLOCK: usize = 64;
    let mut k = if key.len() > BLOCK {
        sha256(key).to_vec()
    } else {
        key.to_vec()
    };
    k.resize(BLOCK, 0);

    let mut ipad = [0x36u8; BLOCK];
    let mut opad = [0x5cu8; BLOCK];
    for i in 0..BLOCK {
        ipad[i] ^= k[i];
        opad[i] ^= k[i];
    }

    let mut inner = ipad.to_vec();
    inner.extend_from_slice(msg);
    let inner_hash = sha256(&inner);

    let mut outer = opad.to_vec();
    outer.extend_from_slice(&inner_hash);
    sha256(&outer)
}

/// Lowercase hex encoding (no external `hex` crate).
pub fn to_hex(bytes: &[u8]) -> String {
    let mut s = String::with_capacity(bytes.len() * 2);
    for b in bytes {
        s.push_str(&format!("{b:02x}"));
    }
    s
}

/// Constant-time comparison of two hex strings — avoids leaking, via timing,
/// how many leading characters of a forged signature were correct.
pub fn ct_eq(a: &str, b: &str) -> bool {
    let (a, b) = (a.as_bytes(), b.as_bytes());
    if a.len() != b.len() {
        return false;
    }
    let mut diff = 0u8;
    for i in 0..a.len() {
        diff |= a[i] ^ b[i];
    }
    diff == 0
}

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

    #[test]
    fn sha256_known_vectors() {
        // FIPS 180-4 / NIST published vectors
        assert_eq!(
            to_hex(&sha256(b"")),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
        assert_eq!(
            to_hex(&sha256(b"abc")),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
        assert_eq!(
            to_hex(&sha256(b"hello world")),
            "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
        );
    }

    #[test]
    fn hmac_known_vector() {
        // RFC 4231 test case 2: key="Jefe", msg="what do ya want for nothing?"
        let mac = hmac_sha256(b"Jefe", b"what do ya want for nothing?");
        assert_eq!(
            to_hex(&mac),
            "5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843"
        );
    }

    #[test]
    fn ct_eq_basic() {
        assert!(ct_eq("deadbeef", "deadbeef"));
        assert!(!ct_eq("deadbeef", "deadbeee"));
        assert!(!ct_eq("dead", "deadbeef"));
    }
}