代写Introduction to Architecture coursework II: Defusing a Binary Bomb代写R语言

Introduction to Architecture coursework II: Defusing a Binary Bomb

1    Introduction to defusing Binary Bomb problem

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 need a VPN connection or an ssh connection to access DoC machines.  Information about VPN and SSH connections are available at:

• http://www.doc.ic.ac.uk/ ˜cg1710/rd/csgssh/

• http://www.doc.ic.ac.uk/ ˜cg1710/rd/ictvpn/

If you use the SSH connection, please make sure you do a “second hop” to work on a DoC machine rather than a shell server. Otherwise, CSG may lock your account.  The DoC machines that support this exercise include:

•  ash[01-41].doc.ic.ac.uk

• beech[01-20].doc.ic.ac.uk

• texel[01-44].doc.ic.ac.uk

• oak[01-38].doc.ic.ac.uk

• pixel[01-40].doc.ic.ac.uk

• vertex[01-22].doc.ic.ac.uk

• ray[01-26].doc.ic.ac.uk

You may invoke /vol/linux/bin/freelabmachine to find a free machine.  You also need a web  browser to get your unique bomb.  If you are using a text-based SSH connection, you may use the ‘elinks’ browser pre-installed on DoC machines. Please point your web browser at:

http://cg1710-pi.doc.ic.ac.uk: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.  If you use ‘elinks’, please make sure that you hit ’submit’ to fetch the bomb.  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. It may take few seconds to generate the bomb, please be patience.

Save the bombk .tar file to a (protected) directory in which you plan to do your work.  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.

Step 2: Defuse Your Bomb

Your job for this coursework is to defuse your bomb.

The bomb only works on the lab machines listed in Step 1, which have already the environment that is needed for doing the first problem of the coursework. However, through VPN College service and SSH, you can also use your own laptop/machine.  Please refer to the urls provided in step1 to know how to do it, if you are unsure. 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 coursework.  So there are consequences to exploding the bomb.  You must be careful!

The 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.

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 coursework 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 and all hand-ins are electronic. Each of you gets a unique bomb binary, so even if you work in teams everyone should submit their own solution.

Submission

Please submit two text files to Scientia: one file called psol .txt that contains all the input lines that you used for the successfully diffused phases and one file called bombid .txt which contains your DoC login and your bomb ID k separated by a space. For example,

xyz20  43

If you mistakenly downloaded multiple bombs, please pick exactly one of them to work on.  This will not affect your score. However, if you started working on multiple bombs, please contact Dr Ce Guo to explain the reason. Otherwise, Dr Evil will give you a score according to the bomb with the poorest performance. In addition, 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://cg1710-pi.doc.ic.ac.uk: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 and we recommend 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 20 points) everytime you guess incorrectly and the bomb explodes.

• Everytime 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.

• Do not try to work on more than one bomb as each guess wrong in every bomb counts towards the final mark.

• 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 2680 guesses for each phase. This will take a very longtime to run, and you will not get the answer before the assignment is due.

• Do not try to hack the bomblab server as it is detectable and you will automatically lose 40 points.

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.

–  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.

–  You can find two different gdb summaries that you can print out and use a reference on Sci- entia, or you can find several gdb tutorials like this one: http://www.yolinux.com/ TUTORIALS/GDB-Commands.html. We uploaded the two summaries of dgb commands in Scientia under other resources.

•  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.