Question 1

Question 2

Question 1

Distinguish between the following: 
Substitution and transposition cipher
Monoalphabetic and polyalphabetic cipher

Substitution cipher replaces one symbol or character with another, while transposition cipher changes the location of the symbol or character. 

Monoalphabetic cipher is a one-to-one encipherment between a symbol in a plaintext to a symbol in the ciphertext. 
The symbol in a plaintext is always enciphered into the same symbol in the ciphertext regardless of its position.

Polyalphabetic cipher is a one-to-many encipherment between a symbol in a plaintext to a symbol in the ciphertext. 
The symbol in a plaintext may be enciphered into different symbols in the ciphertext, depending on its position and the key stream used.

Question 2

State the maximum number of characters that will be changed in the ciphertext if only a single character is changed in the plaintext, 
for each of the following ciphers. Briefly explain the reason for your answer.

i) Affine
ii) Auto-key
iii) One-time pad
iv) Rotor

Affine: one, Affine cipher is a monoalphabetic cipher that encrypts character independently 

Auto-key: two; Since the plaintext will become part of the keystreams, when a character is changed, 
          that ciphertext is changed, and the next ciphertext will changed too this character is the key for next encipherment 

One-time pad: one. Each character is enciphered independently with specific key in the keystream. 

Rotor: one. Each character is mapped to its corresponding ciphertext from the rotor.

Question 3

State the two problems with the one-time pad.
    
Producing large quantities of random keys becomes a challenging task.
Distributing and protecting these keys becomes a problem.

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