密码学原理_学习笔记

Varieties of chosen-ciphertext attacks

Lunchtime attacks

A specially noted variant of the chosen-ciphertext attack is the “lunchtime”, “midnight”, or “indifferent” attack, in which an attacker may make adaptive chosen-ciphertext queries but only up until a certain point, after which the attacker must demonstrate some improved ability to attack the system. The term “lunchtime attack” refers to the idea that a user’s computer, with the ability to decrypt, is available to an attacker while the user is out to lunch. This form of the attack was the first one commonly discussed: obviously, if the attacker has the ability to make adaptive chosen ciphertext queries, no encrypted message would be safe, at least until that ability is taken away. This attack is sometimes called the “non-adaptive chosen ciphertext attack”; here, “non-adaptive” refers to the fact that the attacker cannot adapt their queries in response to the challenge, which is given after the ability to make chosen ciphertext queries has expired.

Adaptive chosen-ciphertext attack

Main article: Adaptive chosen-ciphertext attack
A (full) adaptive chosen-ciphertext attack is an attack in which ciphertexts may be chosen adaptively before and after a challenge ciphertext is given to the attacker, with only the stipulation that the challenge ciphertext may not itself be queried. This is a stronger attack notion than the lunchtime attack, and is commonly referred to as a CCA2 attack, as compared to a CCA1 (lunchtime) attack. Few practical attacks are of this form. Rather, this model is important for its use in proofs of security against chosen-ciphertext attacks. A proof that attacks in this model are impossible implies that any realistic chosen-ciphertext attack cannot be performed.

A practical adaptive chosen-ciphertext attack is the Bleichenbacher attack against PKCS#1.

Numerous cryptosystems are proven secure against adaptive chosen-ciphertext attacks, some proving this security property based only on algebraic assumptions, some additionally requiring an idealized random oracle assumption. For example, the Cramer-Shoup system is secure based on number theoretic assumptions and no idealization, and after a number of subtle investigations it was also established that the practical scheme RSA-OAEP is secure under the RSA assumption in the idealized random oracle model.