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A chosen-ciphertext attack (CCA) is an attack model for cryptanalysis in which the cryptanalyst gathers information, at least in part, by choosing a ciphertext and obtaining its decryption under an unknown key. In the attack, an adversary has a chance to enter one or more known ciphertexts into the system and obtain the resulting plaintexts. From these pieces of information the adversary can attempt to recover the hidden secret key used for decryption.
A number of otherwise secure schemes can be defeated under chosen-ciphertext attack. For example, the El Gamal cryptosystem is semantically secure under chosen-plaintext attack, but this semantic security can be trivially defeated under a chosen-ciphertext attack. Early versions of RSA padding used in the SSL protocol were vulnerable to a sophisticated adaptive chosen-ciphertext attack which revealed SSL session keys. Chosen-ciphertext attacks have implications for some self-synchronizing stream ciphers as well. Designers of tamper-resistant cryptographic smart cards must be particularly cognizant of these attacks, as these devices may be completely under the control of an adversary, who can issue a large number of chosen-ciphertexts in an attempt to recover the hidden secret key.
For a while in the late 1970s and 1980s it was not clear at all that public key cryptosystems can withstand the chosen ciphertext attack, until the initial work of Moni Naor and Moti Yung in 1990, which suggested a mode of dual encryption with integrity proof (now known as the "Naor-Yung" encryption paradigm).
When a cryptosystem is vulnerable to chosen-ciphertext attack, implementers must be careful to avoid situations in which an adversary might be able to decrypt chosen-ciphertexts (i.e., avoid providing a decryption oracle). This can be more difficult than it appears, as even partially chosen ciphertexts can permit subtle attacks. Additionally, other issues exists and some cryptosystems (such as RSA) use the same mechanism to sign messages and to decrypt them. This permits attacks when hashing is not used on the message to be signed. A better approach is to use a cryptosystem which is provably secure under chosen-ciphertext attack, including (among others) RSA-OAEP secure under the random oracle heuristics, Cramer-Shoup and many forms of authenticated symmetric encryption when one uses symmetric encryption rather than public key cryptography.
Varieties of chosen-ciphertext attacks
Chosen-ciphertext attacks, like other attacks, may be adaptive or non-adaptive. In a non-adaptive attack, the attacker chooses the ciphertext or ciphertexts to decrypt in advance, and does not use the resulting plaintexts to inform their choice for more ciphertexts. In an adaptive chosen-ciphertext attack, the attacker makes their ciphertext choices adaptively, that is, depending on the result of prior decryptions.
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
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.
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