cargoless-cas 0.2.0

//! Minimal, dependency-free SHA-256 (FIPS 180-4).
//!
//! Rationale for hand-rolling instead of pulling `sha2`/`blake3`: the whole
//! workspace builds CI-only with **no committed `Cargo.lock`** and a
//! deliberately minimal dependency tree — the same reasoning that makes
//! `cargoless-proto` refuse `serde` and daemon-core hand-roll its debounce/ignore
//! logic (cold-build time is what AC#1/#2 are measured against). SHA-256 is a
//! fixed, ~120-line, `#![forbid(unsafe_code)]`-clean algorithm whose output is
//! pinned by the FIPS 180-4 test vectors exercised in this module's tests. It
//! is a *stable* hash with zero supply-chain or cold-build cost, which is
//! exactly what the CAS key contract needs.
//!
//! The contract (`cargoless-proto`) deliberately leaves the hash algorithm
//! unspecified and owned here; this module is that choice.

use core::fmt::Write as _;

const H0: [u32; 8] = [
    0x6a09_e667,
    0xbb67_ae85,
    0x3c6e_f372,
    0xa54f_f53a,
    0x510e_527f,
    0x9b05_688c,
    0x1f83_d9ab,
    0x5be0_cd19,
];

#[rustfmt::skip]
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 `data` as a lowercase 64-char hex string.
///
/// This is the single hashing primitive the CAS is built on: it produces both
/// the per-component [`cargoless_proto::ContentHash`](cargoless_proto::ContentHash) values
/// and the derived [`cargoless_proto::InputHash`](cargoless_proto::InputHash) CAS key.
#[must_use]
pub fn sha256_hex(data: &[u8]) -> String {
    let digest = sha256(data);
    let mut s = String::with_capacity(64);
    for b in digest {
        // Infallible: writing to a String never errors.
        let _ = write!(s, "{b:02x}");
    }
    s
}

fn sha256(data: &[u8]) -> [u8; 32] {
    let mut h = H0;
    let bit_len = (data.len() as u64).wrapping_mul(8);

    // Pad: 0x80, then zeros until len % 64 == 56, then 8-byte big-endian length.
    let mut msg = data.to_vec();
    msg.push(0x80);
    while msg.len() % 64 != 56 {
        msg.push(0);
    }
    msg.extend_from_slice(&bit_len.to_be_bytes());

    for block in msg.chunks_exact(64) {
        let mut w = [0u32; 64];
        for (word, src) in w.iter_mut().take(16).zip(block.chunks_exact(4)) {
            *word = u32::from_be_bytes([src[0], src[1], src[2], src[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, mut b, mut c, mut d, mut e, mut f, mut g, mut hh] = h;
        for (ki, wi) in K.iter().zip(w.iter()) {
            let s1 = e.rotate_right(6) ^ e.rotate_right(11) ^ e.rotate_right(25);
            let ch = (e & f) ^ ((!e) & g);
            let t1 = hh
                .wrapping_add(s1)
                .wrapping_add(ch)
                .wrapping_add(*ki)
                .wrapping_add(*wi);
            let s0 = a.rotate_right(2) ^ a.rotate_right(13) ^ a.rotate_right(22);
            let maj = (a & b) ^ (a & c) ^ (b & c);
            let t2 = s0.wrapping_add(maj);
            hh = g;
            g = f;
            f = e;
            e = d.wrapping_add(t1);
            d = c;
            c = b;
            b = a;
            a = t1.wrapping_add(t2);
        }

        for (slot, v) in h.iter_mut().zip([a, b, c, d, e, f, g, hh]) {
            *slot = slot.wrapping_add(v);
        }
    }

    let mut out = [0u8; 32];
    for (chunk, word) in out.chunks_exact_mut(4).zip(h) {
        chunk.copy_from_slice(&word.to_be_bytes());
    }
    out
}

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

    #[test]
    fn fips_180_4_known_vectors() {
        // The canonical SHA-256 test vectors. If these ever change, the CAS
        // key space has silently moved and every cached artifact is orphaned —
        // so this test is the contract that the hash is *stable forever*.
        assert_eq!(
            sha256_hex(b""),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
        assert_eq!(
            sha256_hex(b"abc"),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
        assert_eq!(
            sha256_hex(b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"),
            "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1"
        );
    }

    #[test]
    fn long_input_crosses_block_boundary() {
        // 1,000,000 'a' — exercises multi-block + length padding.
        let a = vec![b'a'; 1_000_000];
        assert_eq!(
            sha256_hex(&a),
            "cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0"
        );
    }

    #[test]
    fn avalanche_and_stability() {
        // Different inputs ⇒ different digests; identical inputs ⇒ identical.
        assert_ne!(sha256_hex(b"input-a"), sha256_hex(b"input-b"));
        assert_ne!(sha256_hex(b"a"), sha256_hex(b"A"));
        assert_eq!(sha256_hex(b"stable"), sha256_hex(b"stable"));
        assert_eq!(sha256_hex(b"abc").len(), 64);
    }
}