Network and Information Security

  1. Military, diplomatic and commercial messages have for centuries been protected by encryption. Nowadays most Internet communications are similarly protected.
  • Most cryptosystems involve the use of keys.
  • Explain the concept of a cryptographic key. When designing a cryptosystem, what is the major advantage of including a key?



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  • What is the difference between a “symmetric” and an “asymmetric” cryptosystem? Describe one significant advantage the latter has over the former.



  • In a “monoalphabetic” cipher, each plaintext character is always represented by the same ciphertext character. An example is the generalised Caesar cipher, although with only 25 possible keys this cipher is easily broken by “brute force”.


  • The Caesar cipher can be strengthened by using a “key word” to add randomness to the substitution. Use the Caesar cipher with the key word “victory” to encrypt the message “Ides of March”


  • With around 4×1026 possible keys, a random substitution cipher is hard to break by brute force. Explain how such an encryption could be broken.



  • An early example of a polyalphabetic cipher is the Vigénere cipher. Explain the operation of this cipher, and what makes it more secure than a monoalphabetic cipher.


Continuation of Question 2:


  Plaintext Ciphertext


Figure 1


(c)          Digital-age communications employ symmetric block ciphers such as the Advanced Encryption Standard (AES).


  • Figure 1 shows two images encrypted using the AES block cipher operating in Electronic Codebook (ECB) mode. Explain why the outline of image A is visible in the ciphertext image, but not the outline of image B.


  • Explain how the use of Cipher Block Chaining (CBC) could have prevented the leakage of information about Image A.


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