Since this won't be used very frequently, I would stay away from extension methods in this case and create a separate class to handle encrypt/decrypt

>Actually sounds like a candidate for an extension method on string:

    public static class DmitryExtensions
>    {
>        public static string Encode (this string s)
>        {
>            //Return encoded here
>            return s;
>        }
>        public static string Decode(this string s)
>        {
>            //Return decoded here
>            return s;
>        }
>    }

Called like:

    string text = "Hello".Encode();
>    private string y = text.Decode();
>
>    //You can also chain (thought not much sense in this particular context)
>    private string chained = "Hello".Encode().Decode();

If the methods will be used in other projects put them in their own DLL and reference that when you need the methods.....
>
>
>
>>My class String has two methods Encode and Decode where I can encode and decode a string. The encode and decode code has to be the "same" as my VFP code (to make ASP.NET compatible with VFP app). So I use my class String as following:
>>
>>

>>cPassword = "ABCD";
>>string Decoded;
>>Decoded = MyNameSpace.String.Decoded( cPassword );

Gregory