CS 105 Lab Defusing a Binary Bomb Due Date Check the Calendar Your professor 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 group continue in in your twoperson 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 a Linux binary executable 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 at http 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 2009 spring cs105 index html and follow the labs link Fill out the HTML form with the email addresses and names of your team members 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 Save the bombk tar file to a protected directory in which you plan to do your work Then give the command tar xvf bombk tar This will create a directory called bombk with the following files 1 README Identifies the bomb and its owners 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 wrong group members simply request another bomb 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 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 in the 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 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 Logistics As 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 that Dr Evil really is evil and the bomb will always blow up if run elsewhere There are several other tamperproofing 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 looking at http www cs hmc edu courses 2008 spring cs105 index html 2 and following the labs link You can also go directly to the statistics http 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 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 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 the network 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 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 or the semester ends 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 …
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