Php加密，解密不带特殊字符，数字长度应为6-8

在php中，我想加密和解密数据。我不想使用特殊字符。我只有 6-8 位数字与字母和数字。对于加密和解密，我已经制作了此功能，但有时它无法解密加密的数字，也无法解密其长度与特殊字符。

function random_password($length = 8) {
    $password = "";
    $possible = "2346789bcdfghjkmnpqrtvwxyzBCDFGHJKLMNPQRTVWXYZ";
    $maxlength = strlen($possible);
    if ($length > $maxlength) {
        $length = $maxlength;
    }
    $i = 0;
    while ($i < $length) {
        $char = substr($possible, mt_rand(0, $maxlength-1), 1);
        if (!strstr($password, $char)) {
            $password .= $char;
            $i++;
        }
    }
    return $password;
}
function encrypt($plaintext, $salt) {
    $td = mcrypt_module_open('cast-256', '', 'ecb', '');
    $iv = mcrypt_create_iv (mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
    mcrypt_generic_init($td, $salt, $iv);
    $encrypted_data = mcrypt_generic($td, $plaintext);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    $encoded_64 = base64_encode($encrypted_data);
    return trim($encoded_64);
}
function decrypt($crypttext, $salt) {
    $decoded_64=base64_decode($crypttext);
    $td = mcrypt_module_open('cast-256', '', 'ecb', '');
    $iv = mcrypt_create_iv (mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
    mcrypt_generic_init($td, $salt, $iv);
    $decrypted_data = mdecrypt_generic($td, $decoded_64);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    return trim($decrypted_data);
}