HW1 Solution

Prerequisites:

You will need to set up and use a specified python environment for this homework. Please refer to this instruction.

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 challenge hash

61ef437ca1493baf5ce815a8ca13ec1fba31645f7d85edebac7c60e0aa98b5c6  for user bucky  and salt 20200128 .


    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 and suggest how your attack might be made intractable (harder).

    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=$(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

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


h = m.encode()

for i in range(256):

h = hashlib.sha256(h).digest()

print(h.hex())

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


1b2ebfab6e70dcb13f3ff4750d065bab8474dac4dc611df339446071ae3e7977 .


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 (https://crackstation.net/crackstation-


wordlist-password-cracking-dictionary.htm) for some help.

It is wise to estimate the running time of your solution before starting it.


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.


HW1 Rubric

Criteria
Ratings
Pts





Correct Password in Part A

20.0 pts





Pseudo Code for Part A

10.0 pts





Running time

10.0 pts





Discussion and improvement

10.0 pts





Part B: code for the attack.py

30.0 pts





Problem with the scheme

10.0 pts





Fix that scheme

10.0 pts





Bonus part: correct password

20.0 pts





Bonus part: correct pseudocode

5.0 pts






Total Points: 125.0