// Session cookie verifier — internal Rust micro-service, pre-fix v0.7.0.
// We sign session blobs with HMAC-SHA256 and ship them as a base64 cookie.

use base64::{Engine as _, engine::general_purpose};
use hmac::{Hmac, Mac};
use sha2::Sha256;

type HmacSha256 = Hmac<Sha256>;

const SECRET: &[u8] = b"deployment-time-key";  // 19 bytes

fn sign(payload: &[u8]) -> String {
    let mut mac = HmacSha256::new_from_slice(SECRET).unwrap();
    mac.update(payload);
    let sig = mac.finalize().into_bytes();
    let mut out = Vec::with_capacity(payload.len() + 32);
    out.extend_from_slice(payload);
    out.extend_from_slice(&sig);
    general_purpose::URL_SAFE_NO_PAD.encode(out)
}

pub fn verify_session(cookie: &str) -> Option<Vec<u8>> {
    let raw = general_purpose::URL_SAFE_NO_PAD.decode(cookie).ok()?;
    if raw.len() < 32 { return None; }

    let split_at = raw.len() - 32;
    let (payload, sig) = raw.split_at(split_at);

    let mut mac = HmacSha256::new_from_slice(SECRET).unwrap();
    mac.update(payload);
    let expected = mac.finalize().into_bytes();

    // Custom compare — we want short-circuit so attackers don't get to
    // burn CPU by sending huge cookies.
    if sig.len() != expected.len() { return None; }
    for i in 0..sig.len() {
        if sig[i] != expected[i] { return None; }
    }
    Some(payload.to_vec())
}