CS 213 Fall 2000 Lab Assignment L2 Defusing a Binary Bomb Assigned Sept 21 Due Wed Oct 4 11 59PM September 20 2000 Dave O Hallaron droh cs cmu edu 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 the fish cluster 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 group 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 group of students will attempt to defuse their own personalized bomb Each bomb is an Intel Red Hat Linux 5 2 binary executable file that has been compiled from a C program Each bomb consists of 6 phases To obtain your group s bomb follow these steps carefully and in order 1 IMPORTANT First each group member must login to an Andrew machine not a fish machine and type the following aklog cs cmu edu 2 Next one of the group members should send mail to droh cs cmu edu with the string Lab 2 bomb request in the subject header and the names and Andrew ID s of the group members in the message body Don t do this until each group member has completed Step 1 Dave will then send you the location of your bomb and your group s ID number 1 Step 2 Defuse Your Bomb Once you have received your bomb from Dave copy it to your personal directory Your job is to defuse the bomb You can use many tools to help you with this 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 staff and you lose 1 4 point up to a max of 10 points in the final score for the lab So there are consequences to exploding the bomb You must be careful Each phase is worth 10 points for a total of 60 points Remember that each lab is worth about 8 of your final grade 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 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 Logistics As usual you may work in a group of up to 2 people Any clarifications and revisions to the assignment will be posted on the class bboard and Web page You should do the assignment on the class machines the fish cluster 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 Hand In There is no explicit hand in The bomb will notify your TA automatically after you have successfully defused it You can keep track of how you and the other groups are doing by looking at http www cs cmu edu afs cs academic class 15213 f00 www bombstat html This web page will be updated every 5 minutes or so 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 2 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 4 point up to a max of 10 points 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 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 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 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 The Documents link on the course Web page has a very handy single page gdb summary For other 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 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 …
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