COMP201 - Assignment 4 - Defusing a Binary Bomb - Solved

Dear Bomb Squad!

The nefarious Dr. Evil has planted a slew of “binary bombs” on our class machines. A binary bomb is a program that consists of a sequence of phases. Each phase expects you to type a particular string on stdin. If you type the correct string, then the phase is defused and the bomb proceeds to the next phase. Otherwise, the bomb explodes by printing ”BOOM!!!” and then terminating. The bomb is defused when every phase has been defused.

There are too many bombs for us to deal with, so we are giving each student a bomb to defuse. As the Dr. Evil targeted our school, you are only able to work on your bombs while you are connected to the school network. Your mission, which you have no choice but to accept, is to defuse your bomb before the due date. We are eagerly looking at the scoreboard that reflects how you are moving forward with the bombs. In the followings, you can find how to obtain your bomb and check the scoreboard.

Finally and most preciously, we are sharing information that our agents sent to us from Dr. Evil’s lab which can be your tools and maps in this dangerous mission.

Good luck, and welcome to the bomb squad!

Step 1: Get Your Bomb
You can access the following URL and by typing your KU ID and email you can obtain your bomb package.

http ://ku−bomblab . eastus . cloudapp . azure .com:15213
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•    bomb: The executable binary bomb file.

•    bomb.c: Source file with the bomb’s main routine.

•    README: Identifies the bomb and its owners.

Accept the assignment link: https://classroom.github.com/a/fQIo7tEm, and open the generated private Github repository and add your files to it.

For copying file from your own machine to the linuxpool accounts you may want to look at ”scp” (copying files and folders over ssh connection) command.

WARNING: Don’t forget that you can only work on your bombs on the Linuxpool machines.

You can use the following URL to check the progress for each bomb.

http ://ku−bomblab . eastus . cloudapp . azure .com:15213/ scoreboard
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Step 2: Defuse your Bomb
You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas. The best way is to use your favorite debugger to step through the disassembled binary. Each time your bomb explodes it notifies the bomblab server, and you lose 1/2 point (up to a max of 20 points) in the final score for the lab. So there are consequences to exploding the bomb. You must be careful! The first four phases are worth 10 points each. Phases 5 and 6 are a little more difficult, so they are worth 15 points each. So the maximum score you can get is 70 points. Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty. However, the last phase will challenge even the best students, so please don’t wait until the last minute to start.

The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,

$ ./bomb psol . txt
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then it will read the input lines from psol.txt until it reaches EOF (end of file), and then switch over to stdin. In a moment of weakness, Dr. Evil added this feature so you don’t have to keep retyping the solutions to phases you have already defused. To avoid accidentally detonating the bomb, you will need to learn how to single-step through the assembly code and how to set breakpoints. You will also need to learn how to inspect both the registers and the memory states.

1. Hints
1.1 gdb
To keep the bomb from blowing up every time you type in a wrong input, you’ll want to learn how to set breakpoints and use gdb. Here is a short video that shows how to use gdb and commands to go through a disassembled code. The basics are the same as what you learned during your gdb lab. There are treasures commands in the mentioned video, so watch it carefully.

1.2 objdump -t filename
This will print out the bomb’s symbol table. The symbol table includes the names of all functions and global variables in the bomb, the names of all the functions the bomb calls, and their addresses. You may learn something by looking at the function names!



1.3 objdump -d filename
Use this to disassemble all of the code in the bomb. You can also just look at individual functions. Reading the assembler code can tell you how the bomb works. Although objdump -d gives you a lot of information, it doesn’t tell you the whole story. Calls to system-level functions are displayed in a cryptic form. For example, a call to sscanf might appear as:

8048c36 :
e8 99 fc
f f
f f
call
80488d4 <  init+0x1a0>
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To determine that the call was to sscanf, you would need to disassemble within gdb.

1.4 strings filename
This utility will display the printable strings in your bomb maybe something can be found there.

m CMU CS15-213 course contents.