arcly-stream 0.1.4

//! A small, dependency-free **SHA-256 / HMAC-SHA-256** implementation.
//!
//! Used by [`TokenAuthenticator`](crate::auth::TokenAuthenticator) so the crate
//! can ship production token verification without pulling a crypto stack into
//! the dependency tree. Both algorithms are exact, well-specified standards
//! (FIPS 180-4 and RFC 2104) and are checked against the published test vectors
//! in this module's tests — this is *not* a novel scheme.
//!
//! Internal: nothing here is part of the public API. Safe Rust only
//! (`#![forbid(unsafe_code)]` applies crate-wide).

// FIPS 180-4 §4.2.2 round constants.
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,
];

// FIPS 180-4 §5.3.3 initial hash value.
const H0: [u32; 8] = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
];

const BLOCK: usize = 64;

/// One-shot SHA-256 over `msg`, returning the 32-byte digest.
pub(crate) fn sha256(msg: &[u8]) -> [u8; 32] {
    let mut h = H0;

    // Pad: message || 0x80 || 0x00* || u64-be bit length, to a multiple of 64.
    let bit_len = (msg.len() as u64).wrapping_mul(8);
    let mut padded = Vec::with_capacity(msg.len() + 9 + 63);
    padded.extend_from_slice(msg);
    padded.push(0x80);
    while padded.len() % BLOCK != 56 {
        padded.push(0);
    }
    padded.extend_from_slice(&bit_len.to_be_bytes());

    let mut w = [0u32; 64];
    for chunk in padded.chunks_exact(BLOCK) {
        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 (hv, vv) in h.iter_mut().zip(v.iter()) {
            *hv = hv.wrapping_add(*vv);
        }
    }

    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-SHA-256 (RFC 2104) of `msg` under `key`, returning the 32-byte tag.
pub(crate) fn hmac_sha256(key: &[u8], msg: &[u8]) -> [u8; 32] {
    // Normalize the key to one block.
    let mut block = [0u8; BLOCK];
    if key.len() > BLOCK {
        block[..32].copy_from_slice(&sha256(key));
    } else {
        block[..key.len()].copy_from_slice(key);
    }

    let mut ipad = [0u8; BLOCK];
    let mut opad = [0u8; BLOCK];
    for i in 0..BLOCK {
        ipad[i] = block[i] ^ 0x36;
        opad[i] = block[i] ^ 0x5c;
    }

    let mut inner = Vec::with_capacity(BLOCK + msg.len());
    inner.extend_from_slice(&ipad);
    inner.extend_from_slice(msg);
    let inner_hash = sha256(&inner);

    let mut outer = Vec::with_capacity(BLOCK + 32);
    outer.extend_from_slice(&opad);
    outer.extend_from_slice(&inner_hash);
    sha256(&outer)
}

/// Lowercase hex-encode bytes.
pub(crate) fn to_hex(bytes: &[u8]) -> String {
    const HEX: &[u8; 16] = b"0123456789abcdef";
    let mut s = String::with_capacity(bytes.len() * 2);
    for &b in bytes {
        s.push(HEX[(b >> 4) as usize] as char);
        s.push(HEX[(b & 0x0f) as usize] as char);
    }
    s
}

/// Constant-time equality for two byte slices of equal length. Returns `false`
/// for differing lengths. Avoids leaking match position via early return.
pub(crate) fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
    if a.len() != b.len() {
        return false;
    }
    let mut diff = 0u8;
    for (x, y) in a.iter().zip(b.iter()) {
        diff |= x ^ y;
    }
    diff == 0
}

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

    #[test]
    fn sha256_known_vectors() {
        // FIPS 180-4 / NIST examples.
        assert_eq!(
            to_hex(&sha256(b"")),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
        assert_eq!(
            to_hex(&sha256(b"abc")),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
        assert_eq!(
            to_hex(&sha256(
                b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"
            )),
            "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1"
        );
    }

    #[test]
    fn hmac_sha256_rfc4231_vectors() {
        // RFC 4231 Test Case 2.
        assert_eq!(
            to_hex(&hmac_sha256(b"Jefe", b"what do ya want for nothing?")),
            "5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843"
        );
        // RFC 4231 Test Case 1.
        assert_eq!(
            to_hex(&hmac_sha256(&[0x0b; 20], b"Hi There")),
            "b0344c61d8db38535ca8afceaf0bf12b881dc200c9833da726e9376c2e32cff7"
        );
    }

    #[test]
    fn hmac_handles_keys_longer_than_a_block() {
        // 80-byte key (> 64) exercises the key-hashing path; RFC 4231 Test Case 4
        // uses a long key — here we just check it runs and is deterministic.
        let tag = hmac_sha256(&[0xaa; 80], b"Test Using Larger Than Block-Size Key");
        assert_eq!(
            tag,
            hmac_sha256(&[0xaa; 80], b"Test Using Larger Than Block-Size Key")
        );
        assert_ne!(
            tag,
            hmac_sha256(&[0xab; 80], b"Test Using Larger Than Block-Size Key")
        );
    }

    #[test]
    fn constant_time_eq_matches_semantics() {
        assert!(constant_time_eq(b"abc", b"abc"));
        assert!(!constant_time_eq(b"abc", b"abd"));
        assert!(!constant_time_eq(b"abc", b"ab"));
    }
}