CryptoLicensing uses the latest military strength, state-of-the-art cryptographic technology to generate secure and unbreakable license codes to ensure that your software and intellectual property is protected. Cryptographic licenses are unbreakable even when using brute force computing power. Furthermore, since the licenses can only be generated using the private key (which only you possess), this means that it is impossible for a hacker to develop a ‘keygen’ (key generator) for your software. This is an excellent first line of defense. It is upto you to provide the next line of defense. The following are some useful tips and guidelines for designing effective licenses using CryptoLicensing and writing effective license validation code using the CryptoLicensing validation API. The philosophy is simple: to make it as difficult as possible for the hacker to ‘crack’ your software and cause the hacker to lose interest in your software or not make it worthwhile for him/her.

Check license repeatedly

Instead of checking the license at software startup only, check the license at multiple points of time during your software execution and from multiple places in your code. Do not to use a single license validation routine in your software. Check for a license from different places in your software, but don’t call a single license validation routine to do this. This ensures that a hacker cannot bypass your licensing scheme by simply hacking a single piece of code in your software and forces the hacker to locate every piece of license validation code in your program.

Use multiple licensing checking layers

In addition to the normal license validation done every time your software is executed, add another layer which checks for a license when a certain random criteria is met. For example, you can check for a license on the 2nd Monday of each month, or every 63 days, or on the February 29th of a leap year. If a hacker bypasses the normal license checking code, the additional ‘guerilla’ layer is still present which will check for a license and cause the check to fail.

Use encrypted dlls

Move some important part of your software in a dll and encrypt the dll using a key. When generating licenses using CryptoLicensing, embed this key in your license codes (use the custom user-data feature of CryptoLicensing for this) and decrypt the dll using this key before loading it. This ensures that even if a hacker bypasses licensing checking code, your software will not function correctly since it will not be able to load the encrypted dll file.

Check for the hash of your exe/dll to detect tampering

You can include the hash/checksum of your exe/dll in your license codes generated using CryptoLicensing (use the custom user-data feature of CryptoLicensing for this). When your exe/dll is loaded, you can recalculate the hash of the exe/dll, and compare it with that included in the license code. If different, this means that your exe has been tampered or modified. Similar to license checking, perform this check at multiple points of time and place in your software.

Do not display failures immediately

If a license check fails, note this but do not immediately display to the user that the check has failed. Instead wait for some other part of your software to run and notify the user there. This makes it more difficult for the hacker to locate and bypass your license checking.

Don’t use explicit error messages

When notifying the user of a failed license check, don’t use explicit and obvious messages such as ‘License not valid’. Instead, display a generic failure message and ask the user to contact your support department. He may not know that the cause of the error is a failed license check, and when contacted, you may try to make a sale and convert the illegal user into a licensed user or even catch a hacker if he contacts you.

Crash or fail or output incorrect results when license check fails

In combination with above two, when a license check fails or when you determine that your software has been hacked or your license scheme has been compromised, note this and when your software perform some processing at a later point of time, you can crash your software or output incorrect results. Sometimes, a hacker may also lose interest in your ‘buggy’ program which does not even display informative error messages!

Embed user information in generated licenses

When generating licenses, embed some data about the user in the license such as the user’s name, email or company name (use the custom user-data feature of CryptoLicensing for this). Display this information prominently at multiple places in your software. This is a very effective way to discourage the user from sharing his/her license code with others.

Original article : http://www.ssware.com/articles/writing-effective-license-checking-code-and-designing-effective-licenses-with-cryptolicensing.htm