Password encryption problem

Hi,

I am using this method to encrypt password.. When user enters special characters it gives an error..Please have a look at the method below and correct me

 private string base64Decode(string sData)
    {
         string result=null;
        try
        {
            System.Text.UTF8Encoding encoder = new System.Text.UTF8Encoding();

            System.Text.Decoder utf8Decode = encoder.GetDecoder();

            byte[] todecode_byte = Convert.FromBase64String(sData);

            int charCount = utf8Decode.GetCharCount(todecode_byte, 0, todecode_byte.Length);

            char[] decoded_char = new char[charCount];

            utf8Decode.GetChars(todecode_byte, 0, todecode_byte.Length, decoded_char, 0);

            result = new String(decoded_char);
        }
        catch
        { }

        return result;

    }