How to Right Valid lic file

[dellos]

var validationFailures = license.Validate()  
                                .ExpirationDate(systemDateTime: DateTime.Now)  
                                .When(lic => lic.Type == LicenseType.Trial)  
                                .And()  
                                .Signature(publicKey)  
                                .AssertValidLicense();

if somehow cracker knows I was using this lib then he watches the real created .lic file to know what fields my app was checking.
then he creates a .lic file using the same lib with his private key and passphrase and the same field structure. then I thought this fake .lic file would be accept in my app cause the above code would validate the fake one too!!!

Is there some misunderstood or something I need to change to make it right using the Lib?