<?php
declare(strict_types=1);

/*
 * Two-factor authentication for admins: TOTP (RFC 6238, SHA-1, 6 digits, 30 s),
 * compatible with Google/Microsoft Authenticator. The shared secret is stored
 * encrypted (AES-256-GCM) with security.app_key, which lives outside the
 * document root. Recovery codes are stored as HMACs keyed with the same key.
 */

const TOTP_PERIOD = 30;
const TOTP_DIGITS = 6;
const TOTP_WINDOW = 1;             // accept one step before/after for clock drift
const OTP_MAX_FAILS = 5;           // wrong codes within LOGIN_WINDOW_MIN before the login is dropped
const RECOVERY_CODE_COUNT = 10;
const BASE32_ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';

function app_key(): string
{
    $key = base64_decode((string)(security_config()['app_key'] ?? ''), true);
    if ($key === false || strlen($key) !== 32) {
        throw new RuntimeException('security.app_key belum diisi (32 byte base64); dibutuhkan untuk OTP.');
    }
    return $key;
}

function base32_encode(string $bin): string
{
    $bits = '';
    foreach (str_split($bin) as $ch) {
        $bits .= str_pad(decbin(ord($ch)), 8, '0', STR_PAD_LEFT);
    }
    $out = '';
    foreach (str_split($bits, 5) as $chunk) {
        $out .= BASE32_ALPHABET[bindec(str_pad($chunk, 5, '0'))];
    }
    return $out;
}

function base32_decode(string $b32): string
{
    $bits = '';
    foreach (str_split(strtoupper(preg_replace('/[^A-Za-z2-7]/', '', $b32))) as $ch) {
        $bits .= str_pad(decbin(strpos(BASE32_ALPHABET, $ch)), 5, '0', STR_PAD_LEFT);
    }
    $out = '';
    foreach (str_split($bits, 8) as $byte) {
        if (strlen($byte) === 8) {
            $out .= chr(bindec($byte));
        }
    }
    return $out;
}

function totp_new_secret(): string
{
    return base32_encode(random_bytes(20));   // 160-bit, as recommended by RFC 4226
}

function totp_code(string $secret, int $step): string
{
    $hash = hash_hmac('sha1', pack('J', $step), base32_decode($secret), true);
    $offset = ord($hash[19]) & 0x0f;
    $bin = ((ord($hash[$offset]) & 0x7f) << 24) | (ord($hash[$offset + 1]) << 16)
         | (ord($hash[$offset + 2]) << 8) | ord($hash[$offset + 3]);
    return str_pad((string)($bin % (10 ** TOTP_DIGITS)), TOTP_DIGITS, '0', STR_PAD_LEFT);
}

/** The matching time step, or null. Steps at or before $lastStep are refused (no replay). */
function totp_verify(string $secret, string $code, ?int $lastStep): ?int
{
    $code = preg_replace('/\D/', '', $code);
    if (strlen($code) !== TOTP_DIGITS) {
        return null;
    }
    $now = intdiv(time(), TOTP_PERIOD);
    for ($s = $now - TOTP_WINDOW; $s <= $now + TOTP_WINDOW; $s++) {
        if (($lastStep === null || $s > $lastStep) && hash_equals(totp_code($secret, $s), $code)) {
            return $s;
        }
    }
    return null;
}

function totp_uri(string $secret, string $username): string
{
    $label = rawurlencode('SERUBI Bank Papua:' . $username);
    return "otpauth://totp/$label?secret=$secret&issuer=" . rawurlencode('SERUBI Bank Papua') . '&period=' . TOTP_PERIOD . '&digits=' . TOTP_DIGITS;
}

function secret_encrypt(string $plain): string
{
    $iv = random_bytes(12);
    $tag = '';
    $ct = openssl_encrypt($plain, 'aes-256-gcm', app_key(), OPENSSL_RAW_DATA, $iv, $tag);
    return base64_encode($iv . $tag . $ct);
}

function secret_decrypt(string $stored): string
{
    $raw = (string)base64_decode($stored, true);
    $plain = openssl_decrypt(substr($raw, 28), 'aes-256-gcm', app_key(), OPENSSL_RAW_DATA, substr($raw, 0, 12), substr($raw, 12, 16));
    if ($plain === false) {
        throw new RuntimeException('OTP secret tidak dapat dibuka (app_key berubah?).');
    }
    return $plain;
}

function recovery_hash(string $code): string
{
    return hash_hmac('sha256', strtoupper(preg_replace('/[^A-Za-z0-9]/', '', $code)), app_key());
}

/** Fresh one-time codes like "K7QD-M2XP": [plain codes to show once, hashes to store]. */
function recovery_codes_new(): array
{
    $plain = [];
    for ($i = 0; $i < RECOVERY_CODE_COUNT; $i++) {
        $c = substr(base32_encode(random_bytes(5)), 0, 8);
        $plain[] = substr($c, 0, 4) . '-' . substr($c, 4);
    }
    return [$plain, array_map('recovery_hash', $plain)];
}