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InterIUT 2021 Reverse

We are given a file containing assembly code to analyze. Actually, for this challenge, having x64 assembly knowledge was not even necessary: comments indicating the equivalent code in C are everywhere, and are more than enough to solve the challenge.

We can see that the program starts here, displays a small message asking for a PIN code, then performs a user input in the user_input variable.

Then we can see a comparison between the first character of the input and 1, so the first character of our PIN code is 1.

Then follows a comparison of the second character with 0x33, which is the ASCII value of 3.

Then, a comparison between the 2nd and 3rd character. They must therefore be equal. We deduce that the 3rd character of the PIN code is once again 3.

Last line, we take the 4th character of our input, shifted once to the right, then we compare it to 0x1b. So we just have to take 0x1b and shift it to the left to know the last character of our PIN code. Since we shift it to the left, there are 2 possibilities: 110110 or 110111, respectively 6 and 7.

All we have to do is test both and that’s it, we have the flag: CTFIUT{1337} (the flag is a reference to the leet speak)