I've just wanted to point some links about this there in general :

hxxp://www.woodmann.com/crackz/Tutorials/Anticrk.htm
hxxp://www.anticracking.sk/coding.html

And a must-have book by Pavol Cerven : Crackproof Your Software !

You can get it as online edition on google somewhere ...

Google-ing can give u good results on this subject ...

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Fr1c