Protecting "Trial Versions" of assemblies

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Recently I saw a post on one of the MS newsgroups looking for a place to store a "File" so that you would be able to read so you'd know if the user of your Trial software was using it beyond the specified trial expiration date.

There were a number of answers, but none of them used the approach that seemed so obvious to me: Why do you need to worry about a file, when you can embed the expiration date directly into the assembly!

Now this is by no means secure, and is certainly not immune from decompilation - not by a long shot. However, it should give those who have any kind of imagination some food for thought. Bear in mind, I'm keeping it very simple, and I haven't even encrypted the expiration date:


using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Reflection ;

[assembly: AssemblyConfiguration("06/10/2006")] // this can be encrypted.
[assembly: AssemblyVersion("1.0.0.0")]
namespace AssemblyConfigurationTest
{
public class WebForm1 : System.Web.UI.Page
{
DateTime ExpirationDate;
private void Page_Load(object sender, System.EventArgs e)
{
//Page.Trace.IsEnabled =true; // you can uncomment this and other lines to see the values without having to be "Debug".
Assembly asm = Assembly.GetExecutingAssembly();
string fullHashString= asm.FullName +asm.GetName().Version.ToString();
Trace.Write(fullHashString);
int asmHash = fullHashString.GetHashCode();
object[] objArray=asm.GetCustomAttributes(false) ;
int hash ;
foreach (object obj in objArray)
{
AssemblyConfigurationAttribute conf =
obj as AssemblyConfigurationAttribute;
if (conf != null)
{
hash = conf.Configuration.GetHashCode();
//Trace.Write(hash + ": " +asmHash);
if(hash !=-579392602 || asmHash!=-1991291623)
throw new InvalidOperationException("Assembly Has been Altered! Bad, Bad!");
this.ExpirationDate=Convert.ToDateTime(conf.Configuration) ;
}
}
Response.Write("This Trial Version Expires: " +this.ExpirationDate.ToString());
}
Peter has been active on the C# and other MS newsgroups, has had samples at gotdotnet.com (now codegallery.com) that have been downloaded over 56,000 times, and has written numerous articles for eggheadcafe.com, for which he is a co-founder, and MSDNAA sites. He has also been a technical reviewer on books from Prentice Hall, Manning and other publishers. Peter was a Microsoft MVP for 10 years. Peter was born and raised in New York City, and after a stint as a jazz musician playing contrabass with the Robert Hunt Trio, migrated to sunny Florida, where he was hired as a rookie Financial Consultant at Merrill Lynch in Orlando, specializing in Corporate Services. It was at Merrill that Peter realized computers and technology were his forte. He spent his last few years at Merrill developing real – time computerized trading and and stock charting program using advanced technical analysis indicators, some of which are his own inventions. In 1997, he moved on to architect and serve as lead developer on a number of innovative web-based and middleware solutions for the healthcare and financial services industry.

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