Keygenning Checksum based algos

Hi,
Sorry for the late reply. Suppose I get a crackme program. (I did get one :-) ). It takes the name and finds the checksum using a function whcih uses some ROL's etc. Then it takes the serial and find its checksum using ANOTHER function. Then the serial is valid only if bothe the cksums are equal. How do I solve this?

Thomas Antony

Let's name function that takes the name and finds the checksum as F1(), and name another function (for serial) as F2().
You need build function F2'() which is inverse of F2() i.e. F2(F2'(x)) == F2'(F2(x)) == x.

After that to get valid serial for any name just calculate F2'(F1(Name)).