EEL4930/5934Cross-Layer Computer Security,Spring2015
Assignment1:Defusing a Binary Bomb
Due:Feb1711:59pm
1Introduction
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 quence of phas.Each pha expects you to type a particular string on stdin. If you type the correct string,then the pha is defud and the bomb proceeds to the next pha.Otherwi, the bomb explodes by printing"BOOM!!!"and then terminating.The bomb is defud when every pha has been defud.
There are too many bombs for us to deal with,so we are giving each student a bomb to defu.Your mission,which you have no choice but to accept,is to defu your bomb before the due date.Good luck, and welcome to the bomb squad!
Step1:Get Your Bomb
You can obtain your bomb by pointing your Web browr at:
ufl.edu:15213
This will display a binary bomb request form for you tofill in.Enter your ur name and email address and hit the Submit button.The rver will build your bomb and return it to your browr in a tarfile called bombk.tar,where k is the unique number of your bomb.
Save the bombk.tarfile to a(protected)directory in which you plan to do your work.The directory is sup-pod to be under a Linux environment.Then give the command in terminal:tar-xvf bombk.tar. This will create a directory called./bombk with the followingfiles:
•README:Identifies the bomb and its owners.
•bomb:The executable binary bomb.
•bomb.c:Sourcefile with the bomb’s main routine and a friendly greeting from Dr.Evil.
If for some reason you request multiple bombs,this is not a problem.Choo one bomb to work on and the highest score will be your grade.Remember that you could run your bomb on your own linux machine, but you have to be connected to the network for your score to be recorded.
Step2:Defu Your Bomb
Your job for this lab is to defu your bomb.
You can u many tools to help you defu your bomb.Plea look at the hints ction for some tips and ideas.The best way is to u your favorite debugger to step through the disasmbled binary.
Each time your bomb explodes it notifies the bomblab rver,and you lo0.05point(up to a max of3 points)in thefinal score for the lab.So there are conquences to exploding the bomb.You must be careful! The point from pha1to6is1point,1point,1.5points,2points,2points and2.5points respectively. There are also extra2points for the cret pha.So the maximum score you can get is12points. Although phas get progressively harder to defu,the experti you gain as you move from pha to pha should offt this difficulty.However,the last pha will challenge even the best students,so plea 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>./
then it will read the input lines until it reaches EOF(end offile),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 phas you have already defud.
To avoid accidentally detonating the bomb,you will need to learn how to single-step through the asmbly code and how to t 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.
Logistics
This is an individual project.All handins are electronic.Clarifications and corrections will be posted on the cour message board.
Handin
There is no explicit handin.The bomb will notify your instructor automatically about your progress as you work on it.You can keep track of how you are doing by looking at the class scoreboard at: ufl.edu:15213/scoreboard
This web page is updated continuously to show the progress for each bomb.
Hints(Plea read this!)
There are many ways of defusing your bomb.You can examine it in great detail without ever running the program,andfigure out exactly what it does.This is a uful technique,but it not always easy to do.You can also run it under a debugger,watch what it does step by step,and u this information to defu it.This is probably the fastest way of defusing it.
We do make one request,plea do not u brute force!You could write a program that will try every possible key tofind the right one.But this is no good for veral reasons:
•You lo0.05point(up to a max of3points)every time you guess incorrectly and the bomb explodes.
•Every time you guess wrong,a message is nt to the bomblab rver.You could very quickly saturate the network with the messages,and cau 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(incorrect)assumptions that they all are less than80characters long and only contain letters,then you will have2680guess for each pha.This will take a very long time to run,and you will not get the answer before the assignment is due.
There are many tools which are designed to help youfigure out both how programs work,and what is wrong when they don’t work.Here is a list of some of the tools you mayfind uful in analyzing your bomb,and hints on how to u them.
•gdb
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 asmbly code(we are not giving you the source code for most of your bomb),t breakpoints, t memory watch points,and write scripts.
The CS:APP web site
u.edu/public/students.html
has a very handy single-page gdb summary that you can print out and u as a reference.Here are some other 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 t breakpoints.
–For online documentation,type“help”at the gdb command prompt,or type“man gdb”,
or“info gdb”at a Unix prompt.Some people also like to run gdb under gdb-mode in
emacs.
•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 address.You may learn something by looking at the function names!
•objdump-d
U this to disasmble all of the code in the bomb.You can also just look at individual functions.
Reading the asmbler 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:e899fc ff ff call80488d4<_init+0x1a0>
To determine that the call was to sscanf,you would need to disasmble 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 apropos,man, and info are your friends.In particular,man ascii might come in uful.info gas will give you more than you ever wanted to know about the GNU Asmbler.Also,the web may also be a treasure trove of information.
For tho of you who have not ud linux before,you should download Vmware Player/Fusion/Workstation or VirtualBoxfirst and then install linux environment(ubuntu recommended)in it.Thanks to Abrar Polani, there is a detailed guide on VirtualBox download and Ubuntu installation that you can follow.Now start to defu your bombs!Good luck!
