Defusing a Binary Bomb Assigned

1 Introduction
Step 0: Get on the Engineering School Network
Step 1: Get Your Bomb
Once you are on the school network, you can obtain your bomb by pointing your Web browser at:
This will display a binary bomb request form for you to fill in. Enter your user name and email address and hit the Submit button. The server will build your bomb and return it to your browser in a tar file called bomb.tar, where is the unique number of your bomb.
If you have done this within a Linuxlab desktop VM, your bomb is already on one of the linuxlab machines. If you have done this on your local machine while on the VPN, your bomb will be downloaded onto your local machine. In the latter case, copy the file onto the shell server / the linuxlab machines, since the binary can only be executed there.
Save the bomb.tar file to a (private) directory in which you plan to do your work (I suggest your cse361 folder).
Then give the command: tar -xvf bomb.tar. This will create a directory called ./bomb with the following files:
• README: Identifies the bomb and its owners.
• bomb: The executable binary bomb.
• bomb.c: Source file with the bomb’s main routine and a friendly greeting from Dr. Evil.
If for some reason you request multiple bombs, this should not be a problem, though it won’t help you either. Choose one bomb to work on and delete the rest. (Note that all the bombs you download will be associated with your WUSTL key, so DO NOT cause them to explode!)
Step 2: Defuse Your Bomb
Your job for this lab is to defuse your bomb.
You must do the assignment on one of the class machines (i.e., for = 001 — 012. 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 we hear.
You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas. The best strategy is to use your favorite debugger to step through the disassembled binary.
Each time your bomb explodes it notifies the bomblab server and you lose 1/2 point (up to a max of 20 points) off your final score for the lab. So, from the very beginning there are consequences to exploding the bomb. Handle it with care!
The bomb must be defused in 6 phases. The first four phases are worth 10 points each. Phases 5 and 6 are a little more difficult, so they are worth 15 points each, giving a total of 70 points for defusing all six phases.
Although phases get progressively harder to defuse, 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 accidentally 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 for the rest of your career.
This is an individual project. All handins are electronic.
This writeup can always be found under Resources in Piazza.
There is no explicit handin. The bomb will notify the 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:
This web page is updated continuously to show the progress for each individual bomb. In addition to showing the progress of each bomb, it also shows a progress summary at the bottom of the page — how many bombs have been defused past phase i, where i = 1–7. “Wait, I thought there are only 6 phases?” you ask. Well, Dr. Evil has his plan, and there is only one way to find out … 🙂
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 (up to a max of 20 points) every time you guess incorrectly and the bomb explodes.
• Every time you make a wrong guess, a message is sent to the bomblab server. kou could very quickly saturate the network with these messages, and cause the system administrators to revoke your com- puter 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 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.
FYI: Once again, the explosions of all bombs given to you are recorded under your name. Asking for a new bomb will not clear your history of past explosions. This is not clear from viewing the scoreboard, since the scoreboard displays scores via Bomb IDs, but all Bomb IDs downloaded by you will be counted towards your grade. So any explosion on these bombs are on you!
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. In gdb 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, among other things.
The CS:APP web site
has a very handy single-page gdb summary that you can print out and use 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
set breakpoints.
You might find the following gdb commands helpful:
– display/i $pc: this command causes gdb to print out the current assembly instruction every time you advance an instruction via stepi or nexti.
– x/s $rbx: this command treats the value in $rbx as a char* and prints out the content as a string (i.e., a null terminated char array).
– x/xb : this command examines bytes starting at and prints out their hex values.
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 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 <_init+0x1a0>
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.
If you get stumped, feel free to ask your TAs or instructor for help.


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