Harvey Mudd CS 105 - Lab : Defusing a Binary Bomb

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CS 105Lab : Defusing a Binary BombDue Date: Check the Calendar1 IntroductionThe nefarious Dr. Evil has planted a slew of “binary bombs” on our machines. A binary bomb is a programthat consists of a sequence of phases. Each phase expects you to type a particular string on stdin. If you typethe correct string, then the phase is defused and the bomb proceeds to the next phase. Otherwise, the bombexplodes by printing "BOOM!!!" and then terminating. The bomb is defused when every phase has beendefused.There are too many bombs for us to deal with, so we are giving each group (continue in in your two-person group) a bomb to defuse. Your mission, which you have no choice but to accept, is to defuse yourbomb before the due date. Good luck, and welcome to the bomb squad!Step 1: Get Your BombEach group of students will attempt to defuse their own personalized bomb. Each bomb is a Linux binaryexecutable file that has been compiled from a C program. To obtain your group’s bomb, one (and only one)of the group members should point your Web browser to the bomb request daemon athttp://wilkes.cs.hmc.edu:15213/or to make things easier:http://www.cs.hmc.edu/˜bomb/or to make things consistent:http://www.cs.hmc.edu/courses/2010/spring/cs105/index.htmland follow the labs link.Fill out the HTML form with the email addresses and names of your team members, and then submit theform by clicking the “Submit” button. The request daemon will build your bomb and return it immediatelyto your browser in a tar file called bombk.tar, where k is the unique number of your bomb.Save the bombk.tar file to a (protected) directory in which you plan to do your work. Then give thecommand: tar xvf bombk.tar. This will create a directory called ./bombk with the following files:• README: Identifies the bomb and its owners.1• bomb: The executable binary bomb.• bomb.c: Source file with the bomb’s main routine.If you make any kind of mistake requesting a bomb (such as neglecting to save it or typing the wronggroup members), simply request another bomb.Step 2: Defuse Your BombYour 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 approach is to use your favorite debugger to step through the disassembled binary.Each time your bomb explodes it notifies the staff, and you lose 1/32 point (up to a max of 2 points) inthe final score for the lab. So there are minimal consequences to exploding the bomb... So experiment!!Each phase is worth 10 points, for a total of 60 points.The phases get progressively harder to defuse, but the expertise you gain as you move from phase tophase should offset this difficulty. However, the last phase will challenge even the best students, so pleasedon’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.txtthen it will read the input lines from psol.txt until it reaches EOF (end of file), and then switch overto stdin. In a moment of weakness, Dr. Evil added this feature so you don’t have to keep retyping thesolutions to phases you have already defused.To avoid accidently detonating the bomb, you will need to learn how to single-step through the assemblycode and how to set breakpoints. You will also need to learn how to inspect both the registers and the memorystates. One of the nice side-effects of doing the lab is that you will get very good at using a debugger. Thisis a crucial skill that will pay big dividends the rest of your career.LogisticsAs usual, you ARE to continue to work in your group.Any clarifications and revisions to the assignment will be posted on the class Web page and email.You should do the assignment on wilkes, which is an X86 machine. In fact, there is a rumor thatDr. 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-InThere is no explicit hand-in. The bomb will notify your instructor automatically after you have successfullydefused it. You can keep track of how you (and the other groups) are doing by looking athttp://www.cs.hmc.edu/courses/2010/spring/cs105/index.htmland following the labs link. You can also go directly to the statistics:2http://www.cs.hmc.edu/˜bomb/stats/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 theprogram, and figure out exactly what it does. This is a useful technique, but it is not always easy to do. Youcan also run it under a debugger, watch what it does step by step, and use this information to defuse it. Thisis 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 everypossible key to find the right one. But this is no good for several reasons:• You lose some 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 thenetwork with these messages, and cause the system administrators to come find you....• We haven’t told you how long the strings are, nor have we told you what characters are in them. Evenif you made the (wrong) assumptions that they all are less than 80 characters long and only containletters, then you will have 2680guesses for each phase. This will take a very long time to run, and youwill not get the answer before the assignment is due—or the semester ends.. .There are many tools which are designed to help you figure out both how programs work, and what iswrong 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.• gdbThe GNU debugger, this is a command line debugger tool available on virtually every platform. Youcan trace through a program line by line, examine memory and registers, look at both the source codeand 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 learnhow to set breakpoints.– The CS:APP Student Site at http://csapp.cs.cmu.edu/public/students.htmlhas a very handy


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