Crackme N.3 by COSTY

Another cool crackme for beginners, this time from C/C++ section from crackmes.de.



TL;DR:

For a while I stuck on where this crackme is checking if serial is ok... then I found TEST ECX,ECX. Check it out:

I decide to change the JNZ to JZ:

...and that's how I found a GoodBoy message:

P.S. 

I know that the idea here was 'no patching'. Sorry. ;)