Sunday, March 1, 2015

Boston key party 2015 - Community College Reversing 300 Writeup

The binary is a c++ compiled code under MIPS architecture that takes the flag as a command line argument. It uses a c++ list to store the whole flag in binary form and a class called Wires to store 3 'bits' (words in fact) in 3 different fields. In order to access those field the class has 4 different functions, one to initialize the 3 fields, and others to retrieve the value of the each one of them.
A vector of type Wires is created in order to store the flag , e.i : the previously created list is converted to that vector. The difference is that each field of the vector stores 3 'bits' now and each new field is pushed onto the back of the vector.
After setting up the vector, the binary start to somehow shuffle (check script) the bits around using a recursive function that calls itself 8196 times. Finally, the result vector is converted to a string a compared to another string in memory :

Here's a C script automating the process and printing the flag :
flag : myheadmateisafredkin!

binary download : here
See you again soon.
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