Assignment 3

Due on 20/03/2023

Problem(s)

Use the following plaintext for encryption in the following questions:

Plaintext: 2210ModernCrypto

NOTE: You need to print all of the following for Part-1 (Q2) & Part-2 (Q1)

1. Plaintext or original message (provided above)
2. Key/Shift Value
3. Ciphertext
4. Decrypted text

Part-1:

1. Define a function LinearCongruentialGenerator() to generate Pseudo-Random Numbers (PRNs) with the following properties:

   • Length of PRN: 16
   • Number of zeros (0) in PRN: N
   • Here N is taken as input. Keep the default value (if no input) of N = 5
   • No other digit should repeat more than twice (i.e., maximum occurrence allowed = 2)

2. Define Vernam Cipher for encrypting and decrypting the message of length 16 as per the following steps:

   • You may use week2.py for implementing the XOR on strings
   • Generate a random number PRN using LinearCongruentialGenerator() (also print the `PRN` as proof)
   • Encrypt the given plaintext (above) using key = PRN

Part-2:

1. Define a modified Caesar Cipher as per the given instructions:

   • There is no restriction of only ‘26’ alphabets. Hence, DO NOT use value rotation.
   • e.g., encryption of Z with shift value 3 should not be C. It can be any number/special characters/lower-case letter as well.
   • Encrypt the given plaintext (above) using key/shift value = digital sum of your roll number (week1.py or week2.py can be used for finding the digit/digital sum)

2. You got a securely encrypted message using a classical substitution cipher contained in secret.enc. Download the secret.enc 📥 and:

   • Read the content of secret.enc (i.e., ciphertext) using the python file-handling concept
   • Decrypt and provide the actual textual information i.e. stored in secret.enc (it should look like M3022201_FLAG{xxxxxxxxxxxxxxxxx})
   • Provide the substitution key K, used for encrypting the message and name the algorithm.
   • Define a function decipher() in Python to decrypt any ciphertext CT that is encrypted using this algorithm.

* NOTE: the ‘classical substitution cipher’ used in Part-2 (Q2) is one of the basic ciphers and has already been discussed in one of the previous 3 lab sessions.

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