There is an error in Module 3 Quiz 3. The question should be:

Which of the following is a desirable property for an encryption function Enc_K(M), where K is the key and M is the message?

A. Given Enc_K(M), M and Enc_K(M2), it should be hard to find M2.

A is correct. This is desirable because we do not want an attacker to be able to decrypt any other text if they can guess one text. Note that an IV is required to achieve this property.

B. Given Enc_K(M) and K, it should be hard to find M.

This is the decryption scheme, which is public.

C. Given K and M, it should be hard to find Enc_K(M).

This is the encryption scheme, which is public.

D. Given M, but not K, it should be hard to find Enc_K(M).

While this property is true, it is not relevant to security.