Alice and Bob both use a (Textbook) RSA public key encryption scheme.

Suppose Bob has public key and private key .

Suppose Alice wants to encrypt a message to send to Bob using this scheme, so that only Bob can decrypt and read the message.  Pick the correct statement.  (We know that , and you may assume that all the arithmetic is correct!)

Suppose Alice wants to generate a public-private key pair for public key encryption.  

Pick all the desirable properties of the public key encryption system.  (Marks may be deducted for wrong choices.)

Suppose we have a block cipher with block size and 3-bit keys .  Encryption is as follows:

where is binary bitwise addition.  (So , where , .  For example, . )

Alice wishes to send a message to Bob with data origin authentication, where 

Suppose Alice and Bob agreed to use the above block cipher to compute CBC-MACs without the optional processing and truncating, and suppose they agree on the key .

What is the message authentication code MAC on Alice's message?

Suppose a hash function has the property of second preimage resistance.  Which other property does this also imply?

Pick the correct statements.  Marks may be deducted for wrong choices.

Suppose  is a block cipher which uses 128-bit blocks and 192-bit keys.  Suppose   is a repeated use of :  

In a "meet-in-the-middle" attack on , how many ciphertext are required to be stored?

Suppose we have a plaintext string to encrypt.  

Suppose we have a block cipher with block size and 4-bit keys .  Encryption is as follows:

where is binary bitwise addition.  (So , where , .  For example, . )

We can write the plaintext as , so , .

Encrypt the plaintext using the CBC mode of operation, with key and initialisation vector .

Pick all the correct statements.  Marks may be deducted for wrong answers.

A secure stream cipher should have keystreams with large linear equivalence.

Suppose you intercepted 8 bits of ciphertext: .

Suppose you know that this ciphertext is obtained by encrypting some plaintext using a stream cipher, and suppose you know that the keystream has period 4.  Suppose further that you managed to obtain some bits of the plaintext:

What can you say about the keystream?