Easy Crack
After the program starts, enter any character to display a MessageBox Of Incorrect Password.

open OllyDbg, After loading the program, find the target string Incorrect Password, Then go to the program , The code before and after the string is as follows :


Procedure passed GetDlgItemTextA Get the input string , After a period of algorithm to determine right or wrong . Let's analyze the algorithm :

First determine esp+5 The position character is 0x61, namely a.

After judgment esp+A Whether the string at is 5y. Here because push 2 Instructions , The stack needle will move up 4byte, So now esp=esp-4, So now esp+A It's equivalent to the last time esp-4+A(esp+6), So it's the same as last time esp+5 Become a continuous string , That is to say 3 The first consecutive character is a5y.

Then match the string R3versing, And at the head of the string (esp+4 It's about ) Add the character as E. So the answer is Ea5yR3versing.

The algorithm here involves esp Changes , therefore OD It's better to follow than IDA Static analysis is simpler . You can run the program directly , When the program encounters the relevant judgment ( It's usually cmp) Will change the flag bit ( It's usually ZF), Just change the value of the flag register and let it execute according to the logic you want , Of course, you need to record these characters at the same time to keep the logic correct , You can go straight to the stack ( Or data area ) Make changes in . Then when the program is finished , Your stack ( Or data area ) Is a correct value .

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