More AES

Question 1

What encryption standard was used before AES and why did it need to be changed?

Answer 1

Data Encryption Standard was used before AES. The encryption standard needed to be changed because advances in computing power made DES vulnerable to exhaustive keysearch, as 2^56 keys are not enough nowadays.

Question 2

Describe the CBC (Cipher block chaining) mode of operation for a block cipher (you can draw a diagram if you prefer).

Answer 2

The first block is XORed with an initialisation vector IV and then encrypted. (IV can be made public or kept secret; it’s usually random changed each time CBC is used).
Each subsequent block of plaintext is XORed with the previous block of ciphertext, and then encrypted using the same key K.
Repeated blocks of plaintext are not exposed.