Question

[Predictable IV] Let us consider why predictable IV is a problem for CPA security.

Specifically, consider the CBC mode of encryption and suppose that in the semantic security game with many-time key, the adversary knows IV in advance (that is before deciding on the plaintexts m0 and m1). Explain how the adversary can win the game.

Note: You need to assume that IV is still chosen from a large set. Hence, the ciphertexts, which the adversary sees in the game, are unlikely to repeat.

Answer 1

Predictable IV is a problem in CPA security, especially in the CBC mode of encryption. If the adversary knows the IV in advance, they can perform chosen plaintext attacks and potentially deduce information about the encryption scheme or plaintexts.

Step-by-step explanation:

When it comes to CPA security in the CBC mode of encryption, predictable IV can be a problem. In the semantic security game, if the adversary knows the IV in advance before deciding on the plaintexts, they can exploit this knowledge to win the game.

The adversary can perform chosen plaintext attacks by carefully selecting plaintexts m0 and m1 and observing the corresponding ciphertexts c0 and c1. Since the IV is known, the adversary can encrypt m0 and m1 by XORing them with the IV, and obtain c0 and c1, respectively. By analyzing these ciphertexts, the adversary can potentially deduce information about the encryption scheme or the plaintexts themselves, compromising the security of the encryption.

It is important to note that this scenario assumes the IV is still chosen from a large set and the ciphertexts the adversary sees are unlikely to repeat. This is to ensure the adversary does not exploit any patterns or repetitions in the IV to gain an unfair advantage.