Chapter 2: Symmetric Encryption

Question 1

What are the two requirements for secure use of symmetric encryption?

Answer 1

Strong encryption algorithm: it should not be possible to derive the key by using the ciphertexts and the plaintexts

Secure key exchange

Question 2

What are the different types of attacks?

Answer 2

The encryption algorithm is always known.

1. Ciphertext only: The ciphertext to be decoded.
2. Known plaintext: Ciphertext and one or more plaintext-ciphertext pairs formed with the secret key.
3. Chosen plaintext: ciphertext and a chosen plaintext message together with its corresponding ciphertext generated with the secret key.
4. Chosen ciphertext: ciphertext and purported (and chosen) ciphertext, together with its corresponding decrypted plaintext.
5. Chosen text: ciphertext and chosen plaintext message together with its corresponding ciphertext and purported (and chosen) ciphertext together with its corresponding decrypted plaintext.

Question 3

What criteria does a computationally secure encryption scheme meet?

Answer 3

1. The cost of breaking the cipher exceeds the value of the encrypted information.
2. The time required to break the cipher exceeds the useful lifetime of the information

Question 4

What is a Feistel cipher structure?

Answer 4

A general structure used by most symmetric block ciphers.

Question 5

Why do we want it to be easy to analyse encryption algorithms?

Answer 5

It makes it easier to find vulnerabilities in the algorithm which helps in developing a higher level of assurance in the strength of the algorithm