This lab should be done with a partner. Here is the lab4 partner list.

Lab 4 Goals:

• reading and tracing through the execution of IA32 assembly
• understanding how IA32 translates to C instructions, data structure access, and function calls
• practice with tools for examining binary files
• gain expertise in gdb

In this assignment, you and your partner are going to be given a binary bomb program. Your bomb has 5 phases, each on needs to be defused by entering a correct phrase on stdin (you can also have your bomb read phrases from a file given as a command line argument).

Your goal is to defuse all phases of your bomb, limiting the number of times you explode it in the process.

Your bomb will automatically notify me if it explodes and when you have defused a phase. In addition, you must run your bomb on one of the CS lab machines (your assigned machine or any machine in the overflow or main labs).

You will submit your lab4 solution in two parts:

Part 1: The Checkpoint (due before Fall break):

• defusing the first two phases of your bomb (I encourage you to try to get 3 phases defused before break).

Part 2: The complete solution (due after Fall break):

• defusing the first four phases of the bomb
• submitting (via handin31) a file contianing the 4 (or 5) solution strings to defuse your bomb, and a file containing your write-up of how you sovled each of the phases. update31 copies over starting point files for both of these into your cs31/labs/04/ subdirectory.
• phase 5 is not required. If you solve it, you will receive extra credit. Include a description of how you solved it in your write-up.

1. First, both you and your partner should run update31, which will create the directory structure for your lab 4 assignment and copies over some starting point files:

     $ update31
     $ cd cs31/labs/04
     $ pwd
     /home/your_user_name/cs31/labs/04
     $ ls
     README  bomb_ID  bomb_how_we_solved_it  soln.txt
   

2. Next, follow these steps to get your bomb:
   1. In firefox on the CS system, one of you or your partner should enter this url:

      http://zucchini.cs.swarthmore.edu:15213/
      

   2. Enter your user name and email address on our system (username@cs.swarthmore.edu), and choose Submit.
   3. Choose to save the bombX.tar file in the dialog box that pops up.
   4. move the file into your labs/ directory (do not put it in your labs/04 directory as I do not want you to submit your bomb with your lab 4 solution):

      mv ~/Desktop/bombX.tar ~/cs31/labs/
      

   5. cd into your labs directory, and scp the tar file to your partner

      cd ~/cs31/labs
      ls
      scp bombX.tar partner_username@cs.swarthmore.edu:cs31/labs/.
      # partner enters his/her password, and the bombX.tar file should be scp'ed 
      # into their cs31/labs/ subdirectory
      

   6. both you and your partner can untar the bomb.tar file in your private labs/ subdirectory:

      cd ~/cs31/labs
      ls     # you should see your bombX.tar file
      tar xvf bombX.tar
      

   7. if you cd into your bombX subdirectory, you will see 3 files:

      README  bombX*  main.c
      

      Do not run the bomb program yet!

From firefox running on the CS system, enter this url (you cannot connect to this url from outside our network):

http://zucchini.cs.swarthmore.edu:15213/scoreboard

You will see your bomb in this list. The scoreboard displays how many stages you have defused, how many total explosions your bomb has had, and your score so far.

This information is updated about every 40 seconds. Re-load the page to see the latest information.

Introduction

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 and terminates. The bomb is defused when every phase has been defused. You will be penalized for explosions (1/4 a point for each one), so you need to be careful not to explode your bomb too many times.

You can receive up to 85 points total on this lab:

1. Defusing phases 1-4 are each worth 10 points (40pts total)
2. Defusing phase 5 is worth 15 points
3. Your write-up of how you solved each phase is worth 4 points for each phase (20 points total). Your write-up should be in the file bomb_how_we_solved_it in your cs31/labs/04 subdirectory.
4. The checkpoint is worth 10 points.

In addition, I will take up to 5 points off total for explosions. You will lose a point for each 4th explosion (number 4, 8, 12, ...), so you get a few for "free". I will not take off more than 5 points total for explosions, unless it is clear that you are trying a brute force approach, in which case I will. Also, you may find some opportunity to gain a few additional points as you go.

Defuse Your Bomb

You must run your bomb on one of the class machines; the bomb will always blow up if run elsewhere. There are several other tamper-proofing devices built into the bomb as well. In particular, do not use the gdb set command while trying to defuse your bomb.

To kill your bomb executable (to make it exit without exploding), type CNTRL-C. This way you can run your bomb, solve a phase, and exit and come back later to try the next phase.

You can use many tools to help you defuse your bomb. Please look at the hints section below for some tips and ideas. The best way is to use ddd or gdb to step through the disassembled binary.

Each time your bomb explodes it notifies the bomblab server, and you lose 1/4 point (up to a max of 5 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. Phase 5 is a little more difficult, so it is worth 15 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.

Once you have solved a phase, I encourage you to run your bomb with a soln.txt file containing the input phrases for the phases you have solved. The format of the file should be one phrase per line, in order of bomb phases. Using an input file will help to prevent you from accidentally exploding the bomb on a previously solved phase. For example:

./bomb soln.txt

will read the input lines from soln.txt until it reaches EOF (end of file), and then switch over to stdin for the remaining input. This feature is so you don't have to keep retyping the solutions to phases you have already defused. The bomb ignores blank input lines, both in the file and on stdin.

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.

The Write-up

Edit the file cs31/labs/04/bomb_how_we_solved_it in vim (or emacs) to include a short explanation of how you defused each phase and a short explaination of what the phase is doing.

Describe at a high-level what the original C code is doing for each phase. For example, is it doing some type of numeric computation, string processing, function calls, etc. and describe the specific computation it is doing (i.e. what type of string processing and how is that being used?).

Do not list registers and assembly code for this part, but describe what the phase is doing at a higher-level in terms of C sematics. You do not have to reverse engineer the IA32 code and translate every part of every phase to equivalent C code. Instead, give a rough idea of equivalent C or psuedo code for the main part of each phase. For example, something like "uses an if-else to choose to do X or Y based on the input value Z" is an appropriate right level of explaination. Something like "moves the value at %ebp-8 into register %eax" is way too low-level.

This part of the lab should not be onerous; you should be able in a short paragraph or two (maybe with a few lines of C or psuedo code to help explain), describe to me what each phase is doing. I recommend doing the write-ups for each phase as you solve them.

Excessively verbose, low-level descriptions will be penalized, as will vague descriptions; you want to clearly demonstrate to me that you figured out what that phase is doing by examining the IA32 code for each phase in your bomb executable.

If you are unable to solve a phase, you can still receive partial credit for it in the write-up part by telling me what you have determined about that phase.

Hints

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:

1. You lose 1/4 point (up to a max of 5 points) every time you guess incorrectly and the bomb explodes.

2. Every time you guess wrong, a message is sent to the bomblab server. You could very quickly saturate the network with these messages, and cause the system administrators to revoke your computer access.

3. 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 26^80 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.

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. And refer to the week04 and week05 weekly lab pages for information on using gdb and tools for examining binaries:

• weekly lab week05: resources for examining binary files,
• weekly lab week 4: gdb examples.

• ddd (or gdb) bomb The GNU debugger will be your most useful tool. 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.

  See weekly lab pages about using ddd and gdb, my gdb Guide, and figure 3.30 of the text book for a summary of useful gdb commands.

• draw the stack and register contents as you are tracing through code in gdb, and take notes as you go (this will also help you with the write-up part of the lab assignment).
• objdump -t bomb
  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 bomb
  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 bomb
  This utility will display the printable strings in your bomb.

Looking for documentation about a particular tool? Don't forget, the commands apropos, man and in gdb the info command.

Whether or not you have defused a bomb phase, and how many times you have exploded your bomb is automatically submitted to the bomb server by your bomb program.

Checkpoint: For the checkpoint, you need do nothing to "submit" it. I'll check the scoreboard at the due date to verify whether or not you met the checkpoint.

Complete Solution: For the complete solution you will run handin31 to submit the contents of your cs31/labs/04 directory. In particular, make sure that it contains these three files:

1. bomb_how_we_solved_it: the write-up part for your bomb.
2. soln.txt: your bomb's solution input file: ./bombX soln.txt
3. bomb_ID: this file just lists your bomb ID

You do not need to submit your binary bomb file (I have a copy of it). I'll also check the scoreboard at the due date to determine which phase you sovled and how many times you exploded your bomb.

Once you are satisfied with your solution, hand it in by typing handin31 at the unix prompt.

Only one of you or your partner should run handin31 to submit your joint solutions If you accidentally both run it, send me email right away letting me know which of the two solutions I should keep and which I should discard (you don't want the grader to just guess which joint solution to grade).

You may run handin31 as many times as you like, and only the most recent submission will be recorded. This is useful if you realize, after handing in some programs, that you'd like to make a few more changes to them.