C.4. Encrypting Session Data

If the security of your database is in question, or if the data that you store in sessions is particularly sensitive, you might wish to encrypt all session data. I do not recommend this approach unless absolutely necessary, but if you feel that your situation warrants it, this section provides an example implementation.

The idea is pretty simple. In fact, in Chapter 8, you are shown how to implement your own session mechanism by calling session_set_save_handler( ). With a minor adjustment to the functions that store and retrieve data, you can encrypt data that you store in the database and decrypt the data that you retrieve:

    <?php

    function _read($id)
    {
      global $_sess_db;

      $algorithm = MCRYPT_BLOWFISH;
      $mode = MCRYPT_MODE_CBC;

      $id = mysql_real_escape_string($id);

      $sql = "SELECT data
              FROM   sessions
              WHERE  id = '$id'";

      if ($result = mysql_query($sql, $_sess_db))
      {
          $record = mysql_fetch_assoc($result);

          $data = base64_decode($record['data']);

          $iv_size = mcrypt_get_iv_size($algorithm, $mode);

          $ciphertext = substr($data, $iv_size);
          $iv = substr($data, 0, $iv_size);

          $crypt = new crypt();

          $crypt->iv = $iv;
          $crypt->ciphertext = $ciphertext;
          $crypt->decrypt();

          return $crypt->cleartext;
      }

      return '';
    }

    function _write($id, $data)
    {
      global $_sess_db;

      $access = time();

      $crypt = new crypt();

      $crypt->cleartext = $data;
      $crypt->generate_iv();
      $crypt->encrypt();

      $ciphertext = $crypt->ciphertext;
      $iv = $crypt->iv;

      $data = base64_encode($iv . $ciphertext);

      $id = mysql_real_escape_string($id);
      $access = mysql_real_escape_string($access);
      $data = mysql_real_escape_string($data);

      $sql = "REPLACE
              INTO    sessions
              VALUES  ('$id', '$access', '$data')";

      return mysql_query($sql, $_sess_db);
    }