Breaking A Synchronous Stream Cipher - Engineering Assignment Help

Assignment Task :

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 system with Python 

Notes 
Download all needed files.
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...} 

Note that all the files were encrypted with the same keystream (i.e., using the same input  key), resulting into reuse of keystream, which is the first security vulnerability you should  exploit. Then, you may consider XOR function used for encryption/decryption, which has 2  inputs: keystream and plaintext/ciphertext.
Leverage the fact that the keystream is being reused in order to obtain the first part of  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.
 

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