This binary reversing challenge seems deceptively simple. Can you figure out the key that it accepts?

Binary available on the shell machine in /problems/mildly_evil/ and available for download here[1]
Hint: You probably want to reverse this dynamically, with a tool like gdb[2]

[1]https://2013.picoctf.com/problems/mildly_evil
[2]http://www.dirac.org/linux/gdb/