<?php
defined('BASEPATH') OR exit('No direct script access allowed');

/**
 * Two-Factor Authentication Library
 * Implements TOTP (Time-based One-Time Password) for Google Authenticator
 * Based on RFC 6238
 */
class TwoFactorAuth
{
    protected $ci;
    protected $codeLength = 6;
    protected $period = 30; // 30 seconds validity window

    public function __construct()
    {
        $this->ci =& get_instance();
    }

    /**
     * Generate a new secret key for TOTP
     * @return string Base32 encoded secret
     */
    public function generateSecret($length = 16)
    {
        $secret = '';
        $validChars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567'; // Base32 alphabet

        for ($i = 0; $i < $length; $i++) {
            $secret .= $validChars[random_int(0, 31)];
        }

        return $secret;
    }

    /**
     * Generate QR code URL for Google Authenticator
     * @param string $secret The user's TOTP secret
     * @param string $username The username
     * @param string $issuer The application name
     * @return string QR code URL
     */
    public function getQRCodeUrl($secret, $username, $issuer = 'DBOIS')
    {
        $encodedIssuer = rawurlencode($issuer);
        $encodedUsername = rawurlencode($username);

        $otpauthUrl = "otpauth://totp/{$encodedIssuer}:{$encodedUsername}?secret={$secret}&issuer={$encodedIssuer}";

        // Using Google Charts API for QR code generation
        return "https://chart.googleapis.com/chart?chs=200x200&chld=M|0&cht=qr&chl=" . urlencode($otpauthUrl);
    }

    /**
     * Verify a TOTP code
     * @param string $secret The user's secret
     * @param string $code The code to verify
     * @param int $discrepancy Time window (default 1 = +/- 30 seconds)
     * @return bool True if code is valid
     */
    public function verifyCode($secret, $code, $discrepancy = 1)
    {
        $currentTimeSlice = floor(time() / $this->period);

        for ($i = -$discrepancy; $i <= $discrepancy; $i++) {
            $calculatedCode = $this->getCode($secret, $currentTimeSlice + $i);
            if ($this->timingSafeEquals($calculatedCode, $code)) {
                return true;
            }
        }

        return false;
    }

    /**
     * Generate TOTP code for a given time slice
     * @param string $secret The secret key
     * @param int $timeSlice The time slice (defaults to current)
     * @return string The generated code
     */
    public function getCode($secret, $timeSlice = null)
    {
        if ($timeSlice === null) {
            $timeSlice = floor(time() / $this->period);
        }

        $secretKey = $this->base32Decode($secret);

        // Pack time into binary string
        $time = chr(0).chr(0).chr(0).chr(0).pack('N*', $timeSlice);

        // Hash with secret
        $hash = hash_hmac('sha1', $time, $secretKey, true);

        // Use last 4 bits as offset
        $offset = ord(substr($hash, -1)) & 0x0F;

        // Get 4 bytes from offset
        $hashPart = substr($hash, $offset, 4);

        // Unpack binary value
        $value = unpack('N', $hashPart);
        $value = $value[1];

        // Only 32 bits
        $value = $value & 0x7FFFFFFF;

        // Get the required digits
        $modulo = pow(10, $this->codeLength);

        return str_pad($value % $modulo, $this->codeLength, '0', STR_PAD_LEFT);
    }

    /**
     * Decode Base32 string
     * @param string $secret Base32 encoded string
     * @return string Binary string
     */
    protected function base32Decode($secret)
    {
        if (empty($secret)) {
            return '';
        }

        $base32chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';
        $base32charsFlipped = array_flip(str_split($base32chars));

        $paddingCharCount = substr_count($secret, '=');
        $allowedValues = array(6, 4, 3, 1, 0);

        if (!in_array($paddingCharCount, $allowedValues)) {
            return false;
        }

        for ($i = 0; $i < 4; $i++) {
            if ($paddingCharCount == $allowedValues[$i] &&
                substr($secret, -($allowedValues[$i])) != str_repeat('=', $allowedValues[$i])) {
                return false;
            }
        }

        $secret = str_replace('=', '', $secret);
        $secret = str_split($secret);
        $binaryString = '';

        for ($i = 0; $i < count($secret); $i = $i + 8) {
            $x = '';
            if (!in_array($secret[$i], $base32charsFlipped)) {
                return false;
            }

            for ($j = 0; $j < 8; $j++) {
                $x .= str_pad(base_convert(@$base32charsFlipped[@$secret[$i + $j]], 10, 2), 5, '0', STR_PAD_LEFT);
            }

            $eightBits = str_split($x, 8);

            for ($z = 0; $z < count($eightBits); $z++) {
                $binaryString .= (($y = chr(base_convert($eightBits[$z], 2, 10))) || ord($y) == 48) ? $y : '';
            }
        }

        return $binaryString;
    }

    /**
     * Timing-safe string comparison
     * @param string $safeString Known string
     * @param string $userString User provided string
     * @return bool True if strings match
     */
    protected function timingSafeEquals($safeString, $userString)
    {
        if (function_exists('hash_equals')) {
            return hash_equals($safeString, $userString);
        }

        $safeLen = strlen($safeString);
        $userLen = strlen($userString);

        if ($userLen != $safeLen) {
            return false;
        }

        $result = 0;

        for ($i = 0; $i < $userLen; $i++) {
            $result |= (ord($safeString[$i]) ^ ord($userString[$i]));
        }

        return $result === 0;
    }
}