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CS 642: Computer Security - Homework Three

This homework assignment covers topics in cryptography. You must work with a partner. There are two parts labeled A and B and one extra credit part described at the end of this document.

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 and password 12345 and salt 999999 is c50603be4fedef7a260ef9181a605c27d44fe0f37b3a8c7e8dbe63b9515b8e96.

For example, the Python code to generate this is:

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

The same process was used to generate the challenge hash 83c02558e533b6051e6d40e84bd03d19193b7090fa016b9b247f86d129d5f608 for user ace and salt 8593018378.

• Recover the password used to generate the challenge hash above. Hint: The password consists only of numbers.
• Give a pseudocode description of your algorithm and the worst case running time for it.
• Discuss the merits of your colleague’s proposal. Suggest how your attack might be made intractable.
• Put your solutions in the file solutions.txt.

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

CT=$(python badencrypt.py testkeyfile)
echo $CT
python 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. attack.py must do this without access to the keyfile 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=$(python badencrypt.py testkeyfile)
MODCT=$(python attack.py $CT)
python baddecrypt.py testkeyfile $MODCT

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.

Put the files attack.py and solutions.txt into a tarball with the following command (where UWID1/2 are your group members' UWID #s):

tar -cf UWID1_UWID2_hw3.tar attack.py solutions.txt

Upload UWID1_UWID2_hw3.tar to the hw-3 folder on the Desire2Learn website for this course: https://uwmad.courses.wisconsin.edu/d2l/home/3199130

Use Assignments > Dropbox to find the hw-3 submission folder.

Parts A and B are worth up to 5 points for a total of 10 points for this assignment. The extra credit below is worth up to 2 additional points.

You are encouraged to use the internet, the Piazza discussion board for this class, and classmates for information about tools and setup. Please help your fellow classmates with setup and understanding Python, but don't discuss solution specifics with anyone beyond your project partner.

The input ace,password,2953983556 processed with SHA256 iterated 256 times produces the hash 1f449b8b05967da7b3c069012174d587e788a8ecd20a5d6a62746c9ed9a2d6d1. For the username ldecarli with salt 0556927495 the challenge hash is f6bf5d2b5a7c85e2bcf4d183fc7ba9db335cf6f177913328c43cf85123f1b18c.

The password is representative of real-world passwords: something complex enough that the person that selected this password would consider using it for a website login, but easy enough to be memorable.

Hints

• The password is based on a dictionary word.
• The password has a small number (not more than 4) of common transformations, all of which are simple character substitutions. (Think: changing capitalization,swapping symbols and numbers for letters like '@' for 'a', or '1' for 'i').
• Knowing something about the real-world user associated with this password might help finding the underlying dictionary word.

Find the password used to produce the challenge hash. Submit the code you used to solve the hash and add to solutions.txt the correct password and a description of how you found the correct password.