Week 5 - Encryption

Question 1

Define encryption [1]

Answer 1

The process of converting plaintext into ciphertext, which cannot be understood without the key to decrypt it.

Question 2

State what is meant by plaintext and ciphertext [1]

Answer 2

Plaintext: Data in human-readable form
Ciphertext: Data that has been encrypted

Question 3

Explain the difference between symmetric and asymmetric encryption [2]

Answer 3

Symmetric encryption uses the same key to encrypt and decrypt. Asymmetric encryption does not (it uses a public-private key pair).

Question 4

State what kind of cipher the Caesar Cipher is, and the role of the encryption key. [2]

Answer 4

1.Substitution cipher
2. The key is the number of positions the plaintext characters are shifted in the alphabet

Question 5

Explain why the Caesar Cipher is not typically used [2]

Answer 5

• Letter frequency analysis can be used to determine the shift
• There are only 25 possible different shifts

Question 6

Explain what is meant the term ‘key exchange problem’

Answer 6

We need to send the key to the recipient without anybody intercepting it

Question 7

One-time pad

Answer 7

A key that is only used once to encrypt and decrypt a message, then it is discarded (in case of interception)

Question 8

Which logical operation is used in the Vernam cipher?

Answer 8

XOR

Question 9

Under what conditions is Vernam perfectly secure? [4]

Answer 9

• The key must be truly random
• The key length must be at least the same as the plaintext length
• The key must be used once then discarded (one time pad)
• The key must only be shared with the intended recipient

Question 10

One limitation of Vernam cipher

Answer 10

Perfect security if the key is only known to the intended recipient.
However, the key must also be sent with the message. It it’s intercepted the ciphertext can be decrypted.