ForbiddenBITS CTF 2013 Write Up : Misc 150 Invisible

In this problem, a file is provided. Dump the file, we can observe it only contain 0x20 and 0x09.

So we guess it is program call Whitespace. So we find the compiler and disassembler of Whitespace.
After executing the program, we get the following result.

We know that this program only accept some input. Then we disassemble the program and get it's instructions.

This program check if the first character is 'w' and second one is 's'. Then we collect the characters this program reads. And executes the program with expected input "wslang", the key will be shown.