Greetings everyone!
This writeup is going to be about one of the reversing challenges from THCon CTF 2021.
Let’s start!!!!
Here, Name of the binary is chall.bin.

First, Trying to run the binary without any input prints that we need to provide an argument and exits the program.So, let’s try by passing an argument and here it prints Nope sorry, try again and exits the program.

1 So, I loaded the binary in write mode using radare2 with command:

r2 -w chall.bin

I got the binary details, and strings using iz and iI commands respectively. It seems that binary is not stripped, but has stack protection turned on as canary is set to true.
Also, after checking strings, it seems like the binary prints [+] Well done ! Here is the flag : once we provide it correct flag. 2 Then, I analysed the binary using aaa command, and listed all functions with afl command. I saw main function in there, so I used s main command to seek to the main function. 3 As radare2 has an awesome Graph Mode which can use with VV command, I switched to graph mode to get an overview of how the given binary works.
There were so many assembly instructions which weren’t of much importance to us. But, I was able to know how the program works by looking at graph. Such graphs are very helpful while analyzing complex binaries.
4 In this challenge, first comparision checks if a parameter is provided, and if a parameter is provided jumps to 0x1c0b and continues execution, otherwise the program jumps to 0x1c81 and exits. Then, in another box within the graph, we can see another comparision of qword [var_1c8h] with rax register, and if they are equal, the programs gives us the flag, otherwise it exits. 5 Here, its important to focus on the instruction jne 0x1c6e

It simply jumps to 0x1c6e if the value in rax register doesn’t match with the value of qword [var_1c8h]. And 0x1c6e is the location which prints Nope Sorry, Try Again and exits.


So, how about we change the instruction jne 0x1c6e to je 0x1c6e which will cause the program to jump to 0x1c45 if we enter wrong input, and thus it will print us the flag.

So, I exited graph mode by pressing q two times. Then, I printed disassembled code of the main function using pdf @ main command.

Most of the assembly code wasn’t of much importance. All I needed was to find the location where rax and qword [var_1c8h were being compared.

At location 0x00001c3c the comparision was being done, and at 0x00001c43 the jump was executed, So, I wrote reverse jump using wa je 0x1c6e @ 0x00001c43 command. 7

Finally, I can exit the program with q command. Then, I ran the binary with random argument, and boom!, I got the flag in hex format:


8 Then, decoding the hex gave me base32 encoded string, which after decoding gave base64 string, and finally by base64 decoding that string, I got the final flag. I used bash to complete the decoding part.

echo "4B5A4357515244434749324853544B594F524C45344D44514A424B455157535A4D455957593654424746424559544B454D524B5536524C504F354957595753454C4959575936535A4B354A464B57544B4C4532564533525148553D3D3D3D3D3D" | xxd -r -p | base32 -d | base64 -d

9 And there is our flag. THCon21{U7JGLvXkYskPK07T8J0BVCgYsadTf69F}
Hope you learned something!
Happy Hacking!! :)

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