Lab2 bomblab solution

Repository into which students should add their bomb lab. You should commit the changes to your attack string as you finish each phase of your bomb lab.




In order to add your bomb lab to this repository, do the following:

* Check out your git repository on the [coding server](https://coding.csel.io) or on your own linux system

* Go the [bomblab server link](http://cs2400.cs.colorado.edu:15213/) and download your bomb.

* Move your bomb file to your git repo - for example `mv bomb42.tar ~/lab2-bomblab` but replace '42' with your bomb number

* Untar your bomb - `tar xvf bomb42.tar` but replace '42' with your bomb number

* Add all the items to your git repo - `git add *`

* Do your initial commit - `git commit -a -m'initial commit'`

* And push your changes - `git push`




Lab Details

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://cs2400.cs.colorado.edu:15213

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 bombk.tar, where k is the unique number of your bomb.




Save the bombk.tar using the directions at the start of this README. This will contain 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.




There are several 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 way 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) in the final score for the lab. So there are consequences to exploding the bomb. You must be careful!




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. So the maximum score you can get is 70 points.




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 the rest of your career.




Logistics

This is an individual project. All handins are electronic. Clarifications and corrections will be posted on the course message board.




Handin

You must check in your bomb solution to github. The bomb will also notify us 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:




http://cs2400.cs.colorado.edu:15213/scoreboard




This web page is updated continuously to show the progress for each bomb.




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 will take more time than just learning how to defuse the bomb. In any case, you're going to have to explain what each phase does during your grading interview.




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.




The CS:APP web site http://csapp.cs.cmu.edu/public/students.html 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.

* 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.




Online diassembler




https://onlinedisassembler.com/static/home/index.html The online disassembler provides a browser-based disassembler for different computer architectures. It’s primary benefit is that it will automatically extract a flow-chart for the program structure.




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.




Looking for a particular tool? How about documentation? Don’t forget, the commands apropos, man, and info are your friends. In particular, man ascii might come in useful. info gas will give you more than you ever wanted to know about the GNU Assembler. Also, the web may also be a treasure trove of information. If you get stumped, feel free to ask your instructor for help.