CS220-Project 3 Defusing a Binary Bomb Solved

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 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 student 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
You can obtain your bomb by pointing your Web browser at:

http://zdu.binghamton.edu:15213/

[This URL can only be accessed from one of the lab machines.]

This will display a binary bomb request form for you to fill in. Enter your user name or remote.cs and Binghamton University email address and hit the Submit button. The server will build your bomb and return it to your browser in a tar file called bombk.tar, where k is the unique number of your bomb. Note that the tar file will be downloaded directly and there is no success feedback; you should find the downloaded tar file in your Downloads directory.

Save the bombk.tarfile to a (protected) directory in which you plan to do your work, like your ˜/i220X/work directory. Then give the command: tar -xvf bombk.tar. This will create a directory called ./bombk 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 is not a problem. Choose one bomb to work on and delete the rest.

Step 2: Defuse Your Bomb
Your job for this lab is to defuse your bomb.

You must do the assignment on one of the remote.cs machines. 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.

Note that when the bomb requires you to type in an input string, it silently pauses without any prompt.

You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas. The best way is to use objdump to disassemble your binary combined with using 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) in the final score for the lab. So there are consequences to exploding the bomb. You must be careful!

The bomb requires a 6-phase defusing. However, in this project we will defuse only the first 4 steps. Phase 1 will be worth 19 points and the remaining 3 phases are worth 27 points each. Hence if you correctly defuse all 4 phases but have the bomb go off 4 times, you will obtain a grade of 98.

Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty.

The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,

$ ./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 the rest of your career.