HW1

Click here to download HW1 files: HW1.zip Download HW1.zip

Prerequisites

You will need to set up and use a specified python environment (python 3.8) for this homework. Please refer to the instructions below:

There are subtle differences between versions of Python, and between versions of Python packages. We need to make sure we’re all using the same versions. Otherwise, your code may run differently while the TAs are grading it.

In the following instructions, we assume you have access to a computer with the following programs installed on it.

• python3
• pip
• virtualenv

Note: the CS lab Linux computers satisfy this requirement.

We’re going to do the following:

• create a virtual environment for running your cs642 hw1 code
• install specific version of crypto library for that virtual environment.

Open a bash terminal and run the following commands:

# Make a directory where you can keep your virtual environments (if you don’t already have one)
$ mkdir ~/envs

# Make a virtual environment called ’cs642hw1’
$ virtualenv --python=python3.8 ~/envs/cs642hw1

# activate the virtual environment
$ source ~/envs/cs642hw1/bin/activate

# install the allowed packages for your environment (You will need the “requirements.txt” file located in HW1.zip)
(cs642hw1) $ pip install -r requirements.txt

You now have a virtual environment identical to the one used by the TAs. You’ll know the virtual environment is active because its name will appear in parentheses to the left of the bash prompt, so run code files by:

(cs642hw1) $ python your_code.py

When you don’t need the virtual environment, just ‘deactivate’ it

(cs642hw1) $ deactivate

Packages in the virtual environment should be sufficient for HW1. If you want to use a Python package that isn’t included in the official virtual environment, contact the TAs for approval.

Part A: Password Cracking

A colleague has built a password hashing mechanism. It applies SHA-256 to a string of the form "username,password,salt", where salt is a randomly chosen value. For example, the stored value for username user, password 12345 and salt 999999 is c50603be4fedef7a260ef9181a605c27d44fe0f37b3a8c7e8dbe63b9515b8e96. The Python code to generate this is:

import hashlib
print(hashlib.sha256("user,12345,999999".encode()).hexdigest())

The same process was used to generate the following challenge hashes: 

a) 53b8da235e6ab04edfe2d73dfd976d4ab26e2bf4e356840ca8104c24a22af139 for user suleman and salt 20202293.

b) c9808f6d88ffb8089d44b903aed1e09be2d7432be46db8c06c273ca65a0e6fe7 for user mazharul and salt 20193833.

Tasks:

1. Recover the password for both challenge hashes above. Hint: Both the passwords are an ASCII string consisting only of numeric digits up to 8 digits.
2. Give a pseudocode description of your algorithm and the worst-case running time for it.
3. Discuss the merits of your colleague’s proposal. Suggest how your attack might be made intractable.
4. Put your solutions under the correct section in the file solutions.txt.
5. Also, upload the pwcrack.py containing the code to crack the hashes, with clear instructions about how to run it.

Part B: Encryption

Another colleague decided to build a symmetric encryption scheme. These are implemented in badencrypt.py and baddecrypt.py (see attached .zip file) and are designed to encrypt a sample message to demonstrate the encryption scheme. To use these demo programs, run:

  CT=$(python3 badencrypt.py testkeyfile)
  echo $CT
  python3 baddecrypt.py testkeyfile $CT

Your job is to assess the security of this encryption scheme. Your solution will be a Python program attack.py that takes as input a ciphertext and modifies the ciphertext so that the decrypted message has a different (and more lucrative to the recipient) AMOUNT field and still passes the verification in baddecrypt.py. The file attack.py must do this without access to the key file or knowledge of the key. You can assume the ciphertext contains the sample message hardcoded in badencrypt.py.

We will test your solution with original versions of badencrypt.py and baddecrypt.py and with different encryption keys than the test key provided. To ensure that attack.py produces the correct formatted output, you can run from the command line:

 CT=$(python3 badencrypt.py testkeyfile) 
 MODCT=$(python3 attack.py $CT) 
 python3 baddecrypt.py testkeyfile $MODCT

1. Complete the attack program attack.py (feel free to make modifications to the pre-filled content. The skeleton is provided just to help you out)
2. In solutions.txt, describe what is wrong with your colleague's scheme and how it should be fixed so that it will be more secure.

(Your attack script will not have direct access to the key file and should not attempt to gain access to the process memory of baddecrypt or any other files to steal the key directly.)

Extra credit: More password cracking

594b32011f597e921b07be213b469a94492ddcdeea84ffea27e2e0392e77f6c59690f1f85b22b8fcb9f551f6613880ef1dc1cc855d600165b8a285c9a342ad8f

0b5445eb04edda3dbe3ce55368908703aec9056efcebb490c75f92afdc4e326c39d4e9fa39c86a2d35252235ffbd86d0f66808abc8ef5678ef32bede12e7d050

Find the password used to produce the challenge hash. Give a pseudocode description of your algorithm and the correct password in solutions.txt.

Hints for Extra Credit:

• The website has a password policy that requires that the password must have at least 6 characters and at least three of the four character classes: uppercase letters (A-Z), lower case letters (a-z), symbols (~`!@#$%^&*()+=_-{}[]\|:;”’?/<>,.), and digits (0-9).
• You can look at CrackStation's password cracking dictionaries (Links to an external site) for some help.
• It is wise to estimate the running time of your solution before starting it.

Deliverables

• Submit all three files attack.py, pwcrack.py, solutions.txt.

Grading

• Parts A and B are worth up to 50 points each for a total of 100 points for this assignment. The extra credit below is worth up to 20 additional points.

Collaboration Policy

This assignment is to be done individually. You are encouraged to use the internet or talk to classmates for information about tools and setup. Please help your fellow classmates with setup and understanding Python. However,  do NOT discuss solution specifics with anyone. Remember, searching for homework solutions online is academic misconduct. If two students' submissions are very similar --- for some definition of similarity --- both students will get zero points for this assignment.