# cs24-21faProject 02a: bomb

## Introduction to Computing Systems (Fall 2021)

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 sshing to labradoodle and running the download-bomb command. 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

You must do this question on the labradoodle machine. 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 bomb server and your ranking on the server goes down. In previous iterations of this assignment, we have taken off points for explosions, but we won’t do that this year.

The phases are worth varying amounts of points:

• Phase 1 is worth 20 points
• Phase 2 is worth 20 points
• Phase 3 is worth 30 points
• Phase 4 is worth 30 points

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

Terminal

then it will read the input lines from psol.txt until it reaches EOF (end of file), and then switch over to stdin. Make sure to have an empty line at the end of this file or it won’t work. 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 project 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.

# Handin

There is no explicit handin for this question. The bomb will notify the course staff automatically about your progress as you work on it. You can keep track of how you are doing by looking at the scoreboard:

http://goldendoodle.caltech.edu/scoreboard

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

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:

• Every time you guess wrong, a message is sent to the bomb server. You could very quickly saturate the network with these messages, and cause the system administrators to revoke your computer 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 $$26^{80}$$ 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.

Use this to disassemble all of the code in the bomb and redirect the output into a file called code. 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. (By the way, sscanf might be a function you haven’t seen before. If so, you can read documentation on sscanf).

• strings : This utility will display the printable strings in your bomb.

# Suffixes on Instructions

Most of the instructions we’ve seen in class (and the ones listed on the x86-64 cheatsheet are written as just the instruction (e.g., cmp). Sometimes, gdb and objdump will output suffixes on these instructions to indicate how big the arguments are (e.g., cmpl or cmpq). These suffixes can be mostly ignored, but you should know they all indicate the same instruction.