This is a model of  stream cipher.  X is the input to a keystream generator, Y is the output.  Y and W are inputs to an XOR operator, and Z is the output.  What is X?  

Consider the following cryptosystem:

Let the plaintext and ciphertext space be .  The secret key is , where each for .

To encrypt a string of plaintext symbols using key  , we perform the following operation to obtain ciphertext   :

, if ;

,  if ;

,  if .

In other words, we add (mod 5) to the first position of the plaintext, to the second position of the plaintext, to the third position of the plaintext, and continue with adding to the fourth position of the plaintext, to the fifth position of the plaintext and so forth.

Suppose you intercepted the ciphertext 0 1 4 4 0 0 3 2 1, and you know that the plaintext in positions 4, 8 and 9 are 3, 4, 4, that is, the plaintext is 3   , 4, 4, where indicates that these symbols are unknown.  

Determine the key.

The following substitution cipher is to be used to encrypt and decrypt some plaintext and ciphertext:

Decrypt the ciphertext RLPJ. 

Enter your answer without spacing.

Match the types of attack to the amount of information the adversary Oscar has.

A block cipher uses binary plaintext blocks of length 64 and binary keys of length 128.  What is the size of the key space? (Marks may be deducted for a wrong choice.)

An encryption algorithm outputs a ciphertext.  What is/are the input(s)?  Pick all the correct answers.  (Marks may be deducted for wrong answers.)

Pick all the descriptions of active attacks. (Marks may be deducted for wrong choices.)

Pick a security property that has been compromised in the following scenario.  (There may be more than one correct answer.  You only need to pick one.)

Alice found that she could not send messages in her instant messaging app.