Guess - CTF 72

CTF Name: Guess

Topic: Reverse Engineering

Flag Format: ICTF{}

Description:

Do you guess the result? This is a simple mathematical operation.

Solution:

Open up the source, and we see that it is a guessing game. It takes 2 number from user add them if the guess of user is right then it gives the flag.

But there is a problem in this binary file it doesn’t work properly so we need to check this using IDA PRO software or other reversing tool.

Here I use IDA PRO

I find a portion of flow chart which are related to flag

In first box we see that

Cmp: Here a comparision occurring based on addition value with preset value

Jnz: It means jump if value is not equal to zero, If our given value is not zero then it jumps to loc_1254 . Otherwise it jumps to the other function

Now let’s little bit fun with the Jnz instruction. I modified the jump instruction with Jz which means if

current amount is zero then it moved me into the loc_1254 function otherwise it moves me into the flag portion

After saved this we run again the binary

We found our flag.

~ write-up by mum1n

Flag:

ICTF{Y0U_GU355_17}

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