HW1

Click here to download HW1 files: HW1.zip

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").hexdigest();

The same process was used to generate the challenge hash ee688ca24c201a27fcc94ebd46e87ae6a7c4f54b445fccfc0727a70332353f7f for user hugh and salt 20193833.

1. Recover the password used to generate the challenge hash above. Hint: The password is an ASCII string consisting only of numeric 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 in the file solutions.txt.

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=$(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. 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=$(python badencrypt.py testkeyfile) 
 MODCT=$(python attack.py $CT) 
 python 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.)

Deliverables

• Put all the files (attack.py, solutions.txt) in a directory named your wisc ID and compress it.

Grading

• Parts A and B are worth up to 50 points for a total of 100 points for this assignment. The extra credit below is worth up to 25 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, but don't 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.

Extra credit: More password cracking

Yet another colleague, to make the password cracking hard, uses hash iteration: SHA256 is iterated 256 times. Something like the following code:

h=hashlib.sha256(m).digest()
for i in range(254):
    h = hashlib.sha256(h).digest()
print(hashlib.sha256(h).hexdigest())

For example, the input ironman,password,84829348943 processed with SHA256 iterated 256 times produces the hash 5483d76bc214a60e35a8a068a28912c168ea5aea8d1441559e3568135185d636. While using the same technique, for the username bucky with salt 8934029034, the challenge hash is f8e2ab8b1678cc51b68f866e4ef7c477c7ba33a731fce412de60c2f8f5e21c11.

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.

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

Hints

• The website has a password policy that requires that the password must have at least 6 characters and atleast 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 for some help.
• It is wise to estimate the running time of your solution before starting it.