寻求一种可逆加密算法，加密数据库自增主键，要求密文越短越好。

<code><?php $key='123';
$orign='qaz123...';
$pwd=do_mencrypt($orign, $key);
echo '明文:'.$orign."<br>";
echo '加密结果:'.$pwd."<br>";
echo '解密结果:'.do_mdecrypt($pwd, $key)."<br>";


function do_mencrypt($input, $key)
{
    $input = str_replace("\n", "", $input);
    $input = str_replace("\t", "", $input);
    $input = str_replace("\r", "", $input);
    $key = substr(md5($key), 0, 24);
    $td = mcrypt_module_open('tripledes', '', 'ecb', '');
    $iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
    mcrypt_generic_init($td, $key, $iv);
    $encrypted_data = mcrypt_generic($td, $input);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    return trim(chop(base64_encode($encrypted_data)));
}


function do_mdecrypt($input, $key)
{
    $input = str_replace("\n", "", $input);
    $input = str_replace("\t", "", $input);
    $input = str_replace("\r", "", $input);
    $input = trim(chop(base64_decode($input)));
    $td = mcrypt_module_open('tripledes', '', 'ecb', '');
    $key = substr(md5($key), 0, 24);
    $iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
    mcrypt_generic_init($td, $key, $iv);
    $decrypted_data = mdecrypt_generic($td, $input);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    return trim(chop($decrypted_data));
}</code>