Breaking A Synchronous Stream Cipher - Reused Keystream - IT/Computer Science Assignment Help

Assignment Task :

1. Objectives
By the end of this lab, you should be able to:
• Gain an understanding of synchronous stream ciphering
• Understand that keys should never be reused for stream ciphering
• Attack a ciphertext with reused keystream System
• Any Linux machine, you can use the Kali-Nessus from Lab 1

2. You are given several files that contain ciphertexts (and in one case a corresponding
plaintext), which were encrypted by an unknown stream cipher. Your main goal is to recover
the secret string (i.e., seed) that was used to generate the keystream. The secret string has
form INS{...23 characters...}

3. Leverage the fact that the keystream is being reused in order to obtain the first part of the keystream. With this keystream, you may (partially) decrypt the remaining files and see the first part of the cipher that was used in this laboratory.
You will see that a secret seed is passed as an argument to the cipher and function next() generates an unlimited keystream using its previous output. Thanks to that you can recover the full encrypted files – full super_cipher.py and hint.gif.
Recovered picture outlines you how the cipher was used and demonstrates what you have to do with the function next() that generates keystream in order to obtain the secret seed.

4. In your solution, write a python3 script that will recover the secret seed used for generating the stream cipher. Hint: “transform” the function next() and realize that triplets of bits are mapped to a single bit (with significant overlap). Here, you will exercise bit operations in python, such as bit shifts, AND, OR.

5. Use python3 language and hand in one zipped file on Dimension. The archive will include
1) Solution.py, containing your programmed solution
2) doc.pdf with your write-up
3) Bash script installs .sh, which will install all non-standard dependencies required for the running of your solutions. This script will be run as the first.

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