<?php

class Duo {
  const DUO_PREFIX = "TX";
  const APP_PREFIX = "APP";
  const AUTH_PREFIX = "AUTH";
  
  const DUO_EXPIRE = 300;
  const APP_EXPIRE = 3600;
  
  const IKEY_LEN = 20;
  const SKEY_LEN = 40;
  const AKEY_LEN = 40; // if this changes you have to change ERR_AKEY
  
  const ERR_USER = 'ERR|The username passed to sign_request() is invalid.';
  const ERR_IKEY = 'ERR|The Duo integration key passed to sign_request() is invalid.';
  const ERR_SKEY = 'ERR|The Duo secret key passed to sign_request() is invalid.';
  const ERR_AKEY = "ERR|The application secret key passed to sign_request() must be at least 40 characters.";
  
  private static function sign_vals($key, $vals, $prefix, $expire) {
    $exp = time() + $expire;
    
    $val = $vals . '|' . $exp;
    $b64 = base64_encode($val);
    $cookie = $prefix . '|' . $b64;
    
    $sig = hash_hmac("sha1", $cookie, $key);
    return $cookie . '|' . $sig;
  }
  
  private static function parse_vals($key, $val, $prefix) {
    $ts = time();
    list($u_prefix, $u_b64, $u_sig) = explode('|', $val);
    
    $sig = hash_hmac("sha1", $u_prefix . '|' . $u_b64, $key);
    if (hash_hmac("sha1", $sig, $key) != hash_hmac("sha1", $u_sig, $key)) {
      return null;
    }
  
    if ($u_prefix != $prefix) {
      return null;
    }
    
    list($user, $ikey, $exp) = explode('|', base64_decode($u_b64));
  
    if ($ts >= intval($exp)) {
      return null;
    }
  
    return $user;
  }
  
  public static function signRequest($ikey, $skey, $akey, $username) {
    if (!isset($username) || strlen($username) == 0){
      return self::ERR_USER;
    }
    if (!isset($ikey) || strlen($ikey) != self::IKEY_LEN) {
      return self::ERR_IKEY;
    }
    if (!isset($skey) || strlen($skey) != self::SKEY_LEN) {
      return self::ERR_SKEY;
    }
    if (!isset($akey) || strlen($akey) < self::AKEY_LEN) {
      return self::ERR_AKEY;
    }
    
    $vals = $username . '|' . $ikey;
    
    $duo_sig = self::sign_vals($skey, $vals, self::DUO_PREFIX, self::DUO_EXPIRE);
    $app_sig = self::sign_vals($akey, $vals, self::APP_PREFIX, self::APP_EXPIRE);
    
    return $duo_sig . ':' . $app_sig;
  }
  
  public static function verifyResponse($ikey, $skey, $akey, $sig_response) {
    list($auth_sig, $app_sig) = explode(':', $sig_response);
    
    $auth_user = self::parse_vals($skey, $auth_sig, self::AUTH_PREFIX);
    $app_user = self::parse_vals($akey, $app_sig, self::APP_PREFIX);
    
    if ($auth_user != $app_user) {
      return null;
    }
    
    return $auth_user;
  }
}
