CSCE 231: Computer System Engineering
Lab 4: Defusing a Binary Bomb
Due: See Canvas
Zach Warneke()is the lead person and bomb squad chief for this lab.
1 Introduction
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 each person 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!
Step 1: Get Your Bomb
Each person will attempt to defuse one own personalized bomb. Each bomb is a Linux binary executable
file that has been compiled from a C program. To obtain your bomb, you need to visit the bomb request
daemon webpage at
http://csce.unl.edu:15213/
Fill out the HTML form. with the your CSE login name and email address, and then submit the form. by
clicking the “Submit” button. The request daemon will build your bomb and return it immediately to your
browser in a tar file called bombk.tar, where k is the unique number of your bomb. Because your
bomb only runs on csce.unl.edu(will be referred to as CSCE from this point onward), your next step
is to move your bomb to CSCE. To login to CSCE, you will use the same login information as logging in to
cse.unl.edu. Create a directory to store your bomb on CSCE then give the command:
tar xvf bombk.tar
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This will create a directory called ./bombk with the following files:
• README: Identifies the bomb and its owner (your CSE’s login name).
• bomb: The executable binary bomb.
• bomb.c: Incomplete source file with the bomb’s main routine.
Step 2: Defuse Your Bomb
Your job is to defuse the bomb. You can use many tools to help you with this; please look at the hintssection
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 staff, and you lose 1/2 point (up to a max of 25 points) in the
final score for the assignment. So there are consequences to exploding the bomb. You must be careful!
Each phase is worth 10 points, for a total of 60 points (with a 10-point bonus).
The phases get progressively harder to defuse, but 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,
linux> ./bomb psol.txt
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 accidently 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.
A sample bomb is provided as part of this assignment. You can obtain the practice bomb (bomb0.tar) from
Canvas or CSCE at.
http://csce.unl.edu/˜cse296/bomb0.tar
The idea is to give you an opportunity to practice defusing a bomb without affecting your score.
Logistics
Any clarifications and revisions to the assignment will be posted on the class Web page.
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You have to do the assignment on CSCE. In fact, there is a rumor that Dr. Evil really is evil, and 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.
Hand-In
There is no explicit hand-in. The bomb will notify your instructor automatically after you have successfully
defused it. You can keep track of how you (and the other groups) are doing by visiting:
http://csce.unl.edu:15213/scoreboard
Note that it reports progress based on the bomb id. This web page is updated continuously to show the
progress of each group.
Hints (Please 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 this is no good for several reasons:
• You lose 1/2 point every time you guess incorrectly and the bomb explodes.
• Every time you guess wrong, a message is sent to the staff. You could very quickly saturate the
network with these messages, and cause the system administrators to revoke your computer access.
• We haven’t told you how long the strings are, nor have we told you what characters are in them. Even
if you made the (wrong) assumptions that they all are less than 80 characters long and only contain
letters, then you will have 2680 guesses for each phase. This will take a very long time to run, and
you will not get the answer before the assignment is due.
Because the bomb is compiled to run on any x86, 32x-bit processor running Linux, you need to have a better
understanding of x86 assembly. Appendix E in our text book provide an overview of x86-32. Please use it
as a reference for this assignment.
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.
• gdb
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The GNU debugger, this is a command line debugger tool available on virtually every platform. You
can trace through a program line by line, examine memory and registers, look at both the source code
and assembly code (we are not giving you the source code for most of your bomb), set breakpoints,
set memory watch points, and write scripts. Here are some tips for using 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.
– Thecourse website has twouseful documents ongdbavailable underContent/References
and Tutorials.
– For other documentation, type “help” at the gdb command prompt, or type “man gdb”, or
“info gdb”at a Unixprompt. Some people also like to rungdbundergdb-modeinemacs.
• objdump -t
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!
• objdump -d
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 ff ff call 80488d4
To determine that the call was to sscanf, you would need to disassemble within gdb.
• strings
This utility will display the printable strings in your bomb.
Looking for a particular tool? How about documentation? Don’t forget, the commands aproposand man
are your friends. In particular, man asciimight come in useful. Also, the web may be a treasure trove of
information. If you get stumped, feel free to ask your TA for help.