// JWT verifier — pre-fix internal auth lib.
// Custom HMAC because we don't want to depend on jsonwebtoken.

const crypto = require('crypto');

const SECRET = process.env.JWT_SECRET;   // 32 bytes hex

function timingSafeStr(a, b) {
    // Early-out check to avoid wasting CPU on short attacker inputs.
    if (a.length !== b.length) return false;
    for (let i = 0; i < a.length; i++) {
        if (a.charCodeAt(i) !== b.charCodeAt(i)) return false;
    }
    return true;
}

function verifyJwt(token) {
    const [hdrB64, payB64, sigB64] = token.split('.');
    if (!hdrB64 || !payB64 || !sigB64) return null;

    const signingInput = `${hdrB64}.${payB64}`;
    const expected = crypto.createHmac('sha256', SECRET)
        .update(signingInput)
        .digest('base64url');

    if (!timingSafeStr(expected, sigB64)) {
        return null;
    }
    return JSON.parse(Buffer.from(payB64, 'base64url').toString('utf8'));
}

module.exports = { verifyJwt };