idekCTF 2025 constructor

The category of the challenge is rev. At first, I tried to execute the binary and here’s the output.

As you can see that, it only showed the emoji. But, what about passing the arguments to it?

It said wrong!. To figure out what’s going on here, I opend up the IDA to reverse it.
To effeciently track the binary, I started to look at the address of “Wrong!”. It appeared at the 0x40301C.
To trace back the function that use “Wrong!” string, and I found it at the address of 0x40111E.
This function function might be the main function since it used the “Wrong!”. Let’s move on to the first part of this function.
I found that there’s a for loop here. To understand what it is, I opend up the gdb to understand it.
Let’s set the breakpoint at the address of 0x4010AF to understand the registers info. After serveral run, we found the format of flag!

Time to leak out the flag!

flag: idek{he4rd_0f_constructors?_now_you_d1d!!}