<?php
/**
 * login_helpers.php — shared pieces of the 2-step login flow
 * (login_otp.php → login_confirm.php, with request_new_otp.php for resends).
 *
 * Pending-login session state (written by login_otp.php once the password has
 * been verified; the password itself is never kept in the session):
 *   login_data['username'] — normalised username/email the user typed
 *   login_user_id          — resolved user id
 *   password_verified_at   — Unix time the password was checked
 *   otp_attempts           — wrong OTP entries for the current code
 *   otp_resends            — OTP resends for this pending login
 */

// One answer for unknown username, wrong password and locked account, so the
// login form cannot be used to find out which accounts exist.
const LOGIN_GENERIC_FAILURE = 'Incorrect username or password, or the account is temporarily locked.';

// A verified password is good for this long before the user must type it again.
const LOGIN_PENDING_SECONDS = 600;

// Wrong OTP entries allowed per issued code; the next one ends the pending login.
const LOGIN_OTP_MAX_ATTEMPTS = 5;

// OTP resends allowed per pending login.
const LOGIN_OTP_MAX_RESENDS = 3;

/**
 * 6-digit TOTP (HMAC-SHA1, 3-minute step) keyed by the user's password hash, so
 * a password change invalidates it. Same algorithm db_auth.php re-derives on
 * every request.
 */
function login_generate_otp(string $secret_key, int $otp_time, int $time_step = 180, int $length = 6): string {
    $counter = floor($otp_time / $time_step);
    $data    = pack("NN", 0, $counter);
    $hash    = hash_hmac('sha1', $data, $secret_key, true);
    $offset  = ord(substr($hash, -1)) & 0x0F;
    $value   = unpack("N", substr($hash, $offset, 4));
    $otp     = ($value[1] & 0x7FFFFFFF) % pow(10, $length);

    return str_pad(strval($otp), $length, '0', STR_PAD_LEFT);
}

/**
 * Random 6-letter reference shown on the OTP screen and in the email. It used to
 * be derived from the OTP, which let anyone who saw the reference recover the
 * OTP offline by trying all 10^6 codes; a random value carries no information.
 */
function login_random_reference(): string {
    $ref = '';
    for ($i = 0; $i < 6; $i++) {
        $ref .= chr(65 + random_int(0, 25));
    }
    return $ref;
}

/** Whether the session holds a pending login whose password check is still fresh. */
function login_pending_valid(): bool {
    return !empty($_SESSION['login_user_id'])
        && !empty($_SESSION['password_verified_at'])
        && (time() - (int)$_SESSION['password_verified_at']) <= LOGIN_PENDING_SECONDS;
}

/** Answer with an HTTP status and a JSON message, then stop. */
function login_fail(int $status, string $message, array $extra = []): void {
    http_response_code($status);
    exit(json_encode(array_merge(['success' => 0, 'message' => $message], $extra)));
}