Legendoflegacy — crackthekey02

Introduction

Language: C/C++

Platform: Windows

Arch: x32

Crackmes.one: https://crackmes.one/crackme/5ab77f5f33c5d40ad448c7cb

Debugging/Reversing

Alright, after some time, I finally found some time to focus on crackMes yay!

So when I do initial analyze, I see that this new crackMe looks simple as it is shown below.

Ok, let’s load our exe to our debugger and let is start at entry point.

It seems, the entry point is the point where the code start, let’s scroll around to see what we have because we are trying to find couple strings in the code like “Enter key:” or “Invalid Key!!!”.

Little bit below, we found what we looked for. let’s leave some breakpoints around.

And we can see that the method ‘pass.404DFC” reads the key as we put breakpoint around dll read.

Let’s follow our input what will happen over it with algo.

Alright, after looking a bit, found out that algorithm has 3 sections

Section -1:

Does calculation as if we input ‘reverseThis’ and it sum up all like

r + e + v + e + r + s + e + T + h + i + s and it gets last 8 bits of the result like xxxxxx94

Section — 2:

The result of first calculation (94) goes through all letters in input and sum up each letter with number of 94.

Section — 3:

It always produce the same key for further comparison

Actual comparison happens below function;

This function compares generated key (section — 3) and result of section 2 and the result should be that EAX is set 0 (zero).

Then the rest is just test-ing eax like

Done!

Let’s bypass!

Conclusion

Thanks to writer, Algo was a bit boring to solve!

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