huskies/server/src/chat/transport/slack/verify.rs

//! Slack request signature verification.

use std::fmt::Write as FmtWrite;

/// Verify the Slack request signature using HMAC-SHA256.
///
/// Slack sends `X-Slack-Signature` and `X-Slack-Request-Timestamp` headers.
/// We compute `HMAC-SHA256(signing_secret, "v0:{timestamp}:{body}")` and
/// compare it to the provided signature.
///
/// This uses a constant-time comparison to prevent timing attacks.
pub(super) fn verify_slack_signature(
    signing_secret: &str,
    timestamp: &str,
    body: &[u8],
    signature: &str,
) -> bool {
    // Compute HMAC-SHA256 manually using the signing secret.
    // Slack signature format: v0={hex(HMAC-SHA256(secret, "v0:{ts}:{body}"))}
    let base_string = format!("v0:{timestamp}:");

    // Simple HMAC-SHA256 implementation using ring-style approach.
    // We use the hmac crate pattern with SHA-256.
    // Since we don't want to add a dependency, we'll use a manual approach:
    // HMAC(K, m) = H((K' ^ opad) || H((K' ^ ipad) || m))
    // where K' is the key padded/hashed to block size.

    let key = signing_secret.as_bytes();
    let block_size = 64; // SHA-256 block size

    // If key is longer than block size, hash it first.
    let key_block = if key.len() > block_size {
        let digest = sha256(key);
        let mut k = vec![0u8; block_size];
        k[..32].copy_from_slice(&digest);
        k
    } else {
        let mut k = vec![0u8; block_size];
        k[..key.len()].copy_from_slice(key);
        k
    };

    // Inner and outer padded keys.
    let mut ipad = vec![0x36u8; block_size];
    let mut opad = vec![0x5cu8; block_size];
    for i in 0..block_size {
        ipad[i] ^= key_block[i];
        opad[i] ^= key_block[i];
    }

    // Inner hash: H(ipad || message)
    let mut inner_data = ipad;
    inner_data.extend_from_slice(base_string.as_bytes());
    inner_data.extend_from_slice(body);
    let inner_hash = sha256(&inner_data);

    // Outer hash: H(opad || inner_hash)
    let mut outer_data = opad;
    outer_data.extend_from_slice(&inner_hash);
    let hmac_result = sha256(&outer_data);

    // Format as "v0={hex}"
    let mut expected = String::from("v0=");
    for byte in &hmac_result {
        write!(expected, "{byte:02x}").unwrap();
    }

    // Constant-time comparison.
    constant_time_eq(expected.as_bytes(), signature.as_bytes())
}

/// Minimal SHA-256 implementation (no external dependency).
///
/// This follows FIPS 180-4.  Only used for HMAC signature verification,
/// not for any security-critical path beyond webhook authentication.
fn sha256(data: &[u8]) -> [u8; 32] {
    let mut h: [u32; 8] = [
        0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab,
        0x5be0cd19,
    ];

    let 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,
    ];

    // Pre-processing: padding
    let bit_len = (data.len() as u64) * 8;
    let mut padded = data.to_vec();
    padded.push(0x80);
    while (padded.len() % 64) != 56 {
        padded.push(0);
    }
    padded.extend_from_slice(&bit_len.to_be_bytes());

    // Process each 512-bit block
    for chunk in padded.chunks_exact(64) {
        let mut w = [0u32; 64];
        for i in 0..16 {
            w[i] = u32::from_be_bytes([
                chunk[4 * i],
                chunk[4 * i + 1],
                chunk[4 * i + 2],
                chunk[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, mut b, mut c, mut d, mut e, mut f, mut g, mut hh] = h;

        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 = hh
                .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);

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

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

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

/// Constant-time byte comparison.
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
}

// ── Tests ───────────────────────────────────────────────────────────────

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

    // ── Signature verification ──────────────────────────────────────────

    #[test]
    fn verify_signature_with_known_values() {
        // Test with a known good signature.
        let secret = "8f742231b10e8888abcd99yez67291";
        let timestamp = "1531420618";
        let body = b"token=xyzz0WbapA4vBCDEFasx0q6G&team_id=T1DC2JH3J&team_domain=testteamnow&channel_id=G8PSS9T3V&channel_name=foobar&user_id=U2CERLKJA&user_name=roadrunner&command=%2Fwebhook-collect&text=&response_url=https%3A%2F%2Fhooks.slack.com%2Fcommands%2FT1DC2JH3J%2F397700885554%2F96rGlfmibIGlgcZRskXaIFfN&trigger_id=398738663015.47445629121.803a0bc887a14d10d2c659f";

        // Compute expected signature for this test case.
        let sig = compute_test_signature(secret, timestamp, body);

        assert!(verify_slack_signature(secret, timestamp, body, &sig));
    }

    #[test]
    fn verify_signature_rejects_bad_signature() {
        let secret = "test-secret";
        let timestamp = "1234567890";
        let body = b"test body";

        assert!(!verify_slack_signature(
            secret,
            timestamp,
            body,
            "v0=bad_signature_here"
        ));
    }

    #[test]
    fn verify_signature_rejects_wrong_secret() {
        let timestamp = "1234567890";
        let body = b"test body";

        let sig = compute_test_signature("correct-secret", timestamp, body);
        assert!(!verify_slack_signature(
            "wrong-secret",
            timestamp,
            body,
            &sig
        ));
    }

    /// Helper to compute a test signature using our sha256 + HMAC implementation.
    fn compute_test_signature(secret: &str, timestamp: &str, body: &[u8]) -> String {
        use std::fmt::Write;

        let key = secret.as_bytes();
        let block_size = 64;
        let key_block = if key.len() > block_size {
            let digest = sha256(key);
            let mut k = vec![0u8; block_size];
            k[..32].copy_from_slice(&digest);
            k
        } else {
            let mut k = vec![0u8; block_size];
            k[..key.len()].copy_from_slice(key);
            k
        };

        let mut ipad = vec![0x36u8; block_size];
        let mut opad = vec![0x5cu8; block_size];
        for i in 0..block_size {
            ipad[i] ^= key_block[i];
            opad[i] ^= key_block[i];
        }

        let base_string = format!("v0:{timestamp}:");
        let mut inner_data = ipad;
        inner_data.extend_from_slice(base_string.as_bytes());
        inner_data.extend_from_slice(body);
        let inner_hash = sha256(&inner_data);

        let mut outer_data = opad;
        outer_data.extend_from_slice(&inner_hash);
        let hmac_result = sha256(&outer_data);

        let mut expected = String::from("v0=");
        for byte in &hmac_result {
            write!(expected, "{byte:02x}").unwrap();
        }
        expected
    }

    // ── SHA-256 implementation ──────────────────────────────────────────

    #[test]
    fn sha256_empty_string() {
        let result = sha256(b"");
        let hex: String = result.iter().map(|b| format!("{b:02x}")).collect();
        assert_eq!(
            hex,
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
    }

    #[test]
    fn sha256_hello_world() {
        let result = sha256(b"hello world");
        let hex: String = result.iter().map(|b| format!("{b:02x}")).collect();
        assert_eq!(
            hex,
            "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
        );
    }

    // ── Constant-time comparison ────────────────────────────────────────

    #[test]
    fn constant_time_eq_same_values() {
        assert!(constant_time_eq(b"hello", b"hello"));
    }

    #[test]
    fn constant_time_eq_different_values() {
        assert!(!constant_time_eq(b"hello", b"world"));
    }

    #[test]
    fn constant_time_eq_different_lengths() {
        assert!(!constant_time_eq(b"hello", b"hi"));
    }
}