ctf-writeup/creator/internal-efiensctf-2020

I created 2 challenges for the Internal Efiens CTF 2020

• Nodejs
• Offsets

For the nodejs challenge, I modified the code of nodejs to add 2 functions, one at js layer and one at C++ layer. Both can be accessed from the global environment of Node by require('flag').

The js code is a simple state machine, by reading the source code it is trivial to find the flag.

The C++ code is harder to read though. I give pointers by printing out debug information, thus it is trivial to find the function. Then a simple xor operation is done on the input array, we can easily read the key and encrypted value to decrypt the flag.

There is a mistake when I made this challenge, I built the binary with a debug statement to print the expected character after the xor.

Offsets challenge are made using LLVM and O-LLVM. I added another transform operation. This transform converts array[i] in assembly into array[f()] where f() = i. In this challenge I apply simple math and can be read through easily. One can use angr to solve this challenge. For more information, read StructOffset.cpp.

The source code for the challenge is lost, try to solve with only the binary.