Part A: Password Cracking Solution

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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



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 and (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 testkeyfile)

echo $CT

python3 testkeyfile $CT


Your job is to assess the security of this encryption scheme. Your solution will be a Python program 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 The file 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

We will test your solution with original versions of and and with different encryption keys than the test key provided.


To ensure that produces the correct formatted output, you can run from the command line:

CT=$(python3 testkeyfile)

MODCT=$(python3 $CT)

python3 testkeyfile $MODCT


  1. Complete the attack program (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.




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()


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.