CSE 351: The Hardware/Software Interface

Autumn 2012 Course Website Return home »

Lab 2: Disassembling and Defusing a Binary Bomb

Assigned Wednesday, October 10, 2012
Due Date Friday, October 19, 2011 at 5:00p
Files Available on attu at /projects/instr/12au/cse351/bombs/$USER/lab2-bomb.tar


The nefarious Dr. Evil has planted a slew of "binary bombs" on our 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 everyone a bomb to defuse. Your mission, which you have no choice but to accept, is to defuse your bomb before the due date. Good luck, and welcome to the bomb squad!


The bombs were constructed specifically for 64-bit machines. You should do this assignment on a lab Linux machine or on a 64-bit CSE Linux VM and be sure it works there or on attu (at least test your solution there before submitting it!), to make sure it works when we grade it. In fact, there is a rumor that Dr. Evil has ensured the bomb will always blow up if run elsewhere. There are several other tamper-proofing devices built into the bomb as well, or so they say.

Everyone gets a unique bomb to defuse. On attu or a lab machine, get yours by extracting the relevant tar file:

tar xvf /projects/instr/12au/cse351/bombs/$USER/lab2-bomb.tar

Your shell will automatically replace $USER with your username when it runs the command (or you can replace it with your username yourself if you want). This command will create a directory called bombi (where i is the ID of your bomb) with the following files:

Your job is to defuse the bomb. You can use many tools to help you with this; please look at the tools section for some tips and ideas. The best way is to use a debugger to step through the disassembled binary.

The bomb has 5 regular phases. The 6th phase is extra credit (worth half as much as a regular phase), and rumor has it that a secret 7th phase exists. If it does and you can find and defuse it, you will receive additional extra credit points. The phases get progressively harder to defuse, but the expertise you gain as you move from phase to phase should offset this difficulty. Nonetheless, the latter phases are not easy, so please don't wait until the last minute to start. (If you're stumped, check the hints section at the end of this document.)

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

./bomb defuser.txt

then it will read the input lines from defuser.txt until it reaches EOF (end of file), and then switch over to stdin (standard input from the terminal). 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. One of the nice side-effects of doing the lab is that you will get very good at using a debugger. This is a crucial skill that will pay big dividends the rest of your career.


There are a number of online resources that will help you understand any assembly instructions you may encounter while examining the bomb. In particular, the programming manuals for x86-64 processors distributed by Intel and AMD are exceptionally valuable. They both describe the same ISA, but sometimes one may be easier to understand than the other.

Useful for this Lab

Not Directly Useful, but Good Brainfood Nonetheless

x86-64 Calling Conventions

As a reminder, the x86-64 ISA passes the first six arguments to a function in registers. Registers are used in the following order: rdi, rsi, rdx, rcx, r8, r9. The return value for functions is passed in rax.

Tools (Read This!!)

There are many ways of defusing your bomb. You can examine it in great detail without ever running the program, and figure out exactly what it does. This is a useful technique, but it not always easy to do. You can also run it under a debugger, watch what it does step by step, and use this information to defuse it. This is probably the fastest way of defusing it.

We do make one request, please do not use brute force! You could write a program that will try every possible key to find the right one, but the number of possibilities is so large that you won't be able to try them all in time.

There are many tools which are designed to help you figure out both how programs work, and what is wrong when they don't work. Here is a list of some of the tools you may find useful in analyzing your bomb, and hints on how to use them.

Looking for a particular tool? How about documentation? Don't forget, the commands apropos and man are your friends. In particular, man ascii is more useful than you'd think. If you get stumped, use the course's discussion board.


If you're still having trouble figuring out what your bomb is doing, here are some hints for what to think about at each stage: (1) comparison, (2) loops, (3) switch statements, (4) recursion, (5) pointers and arrays, (6) sorting linked lists.

Submitting Your Work

Please submit your completed defuser.txt file through the Catalyst Drop Box for this assignment.