Defusing Bomb

I've completed step 1, how do I complete the rest of the assignment?

1 Introduction 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!

Dr. Evil's Insidious Bomb, Version 1.1

* Copyright 2011, Dr. Evil Incorporated. All rights reserved.

*

* LICENSE:

*

* Dr. Evil Incorporated (the PERPETRATOR) hereby grants you (the

* VICTIM) explicit permission to use this bomb (the BOMB). This is a

* time limited license, which expires on the death of the VICTIM.

* The PERPETRATOR takes no responsibility for damage, frustration,

* insanity, bug-eyes, carpal-tunnel syndrome, loss of sleep, or other

* harm to the VICTIM. Unless the PERPETRATOR wants to take credit,

* that is. The VICTIM may not distribute this bomb source code to

* any enemies of the PERPETRATOR. No VICTIM may debug,

* reverse-engineer, run "strings" on, decompile, decrypt, or use any

* other technique to gain knowledge of and defuse the BOMB. BOMB

* proof clothing may not be worn when handling this program. The

* PERPETRATOR will not apologize for the PERPETRATOR's poor sense of

* humor. This license is null and void where the BOMB is prohibited

* by law.

***************************************************************************/

#include

#include

#include "support.h"

#include "phases.h"

/*

* Note to self: Remember to erase this file so my victims will have no

* idea what is going on, and so they will all blow up in a

* spectaculary fiendish explosion. -- Dr. Evil

*/

FILE *infile;

int main(int argc, char *argv[])

{

char *input;

/* Note to self: remember to port this bomb to Windows and put a

* fantastic GUI on it. */

/* When run with no arguments, the bomb reads its input lines

* from standard input. */

if (argc == 1) {

infile = stdin;

}

/* When run with one argument , the bomb reads from

* until EOF, and then switches to standard input. Thus, as you

* defuse each phase, you can add its defusing string to and

* avoid having to retype it. */

else if (argc == 2) {

if (!(infile = fopen(argv[1], "r"))) {

printf("%s: Error: Couldn't open %s ", argv[0], argv[1]);

exit(8);

}

}

/* You can't call the bomb with more than 1 command line argument. */

else {

printf("Usage: %s [] ", argv[0]);

exit(8);

}

/* Do all sorts of secret stuff that makes the bomb harder to defuse. */

initialize_bomb();

printf("Welcome to my fiendish little bomb. You have 6 phases with ");

printf("which to blow yourself up. Have a nice day! ");

/* Hmm... Six phases must be more secure than one phase! */

input = read_line(); /* Get input */

phase_1(input); /* Run the phase */

phase_defused(); /* Drat! They figured it out!

* Let me know how they did it. */

printf("Phase 1 defused. How about the next one? ");

/* The second phase is harder. No one will ever figure out

* how to defuse this... */

input = read_line();

phase_2(input);

phase_defused();

printf("That's number 2. Keep going! ");

/* I guess this is too easy so far. Some more complex code will

* confuse people. */

input = read_line();

phase_3(input);

phase_defused();

printf("Halfway there! ");

/* Oh yeah? Well, how good is your math? Try on this saucy problem! */

input = read_line();

phase_4(input);

phase_defused();

printf("So you got that one. Try this one. ");

/* Round and 'round in memory we go, where we stop, the bomb blows! */

input = read_line();

phase_5(input);

phase_defused();

printf("Good work! On to the next... ");

/* This phase will never be used, since no one will get past the

* earlier ones. But just in case, make this one extra hard. */

input = read_line();

phase_6(input);

phase_defused();

/* Wow, they got it! But isn't something... missing? Perhaps

* something they overlooked? Mua ha ha ha ha! */

return 0;

}

Step 1: Get Your Bomb You can obtain your bomb by pointing your Web browser at: http://gzimmer373.cdm.depaul.edu:15213 This will display a binary bomb request form for you to fill in. Enter your class server user name and preferred 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 file to your local system and then upload the file to the class server using an sftp client. Then login to your class server account; verify the file was correctly uploaded and 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 bombs 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. 1 Step 2: Defuse Your Bomb Your job for this lab is to defuse the first three phases of your bomb. You must do the assignment on one of the class 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. 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 10 points) in the final score for the lab. So there are consequences to exploding the bomb. You must be careful! Each phase is worth 15 points each for a total of 45 points. Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty. Expect the learning curve to be steep at first: please dont 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 dont 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 There is no explicit handin. The bomb will notify your 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: http://gzimmer373.cdm.depaul.edu:15213/scoreboard

This web page is updated continuously to show the progress for each bomb. 2 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 1/2 point (up to a max of 10 points) every time you guess incorrectly and the bomb explodes. 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. We havent 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. There are many tools which are designed to help you figure out both how programs work, and what is wrong when they dont 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, youll 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. objdump -t This will print out the bombs 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! 3 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 doesnt 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? Dont 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.