Adaptive chosen-ciphertext attack

An adaptive chosen-ciphertext attack (abbreviated as CCA2) is an interactive form of chosen-ciphertext attack in which an attacker sends a number of ciphertexts to be decrypted, then uses the results of these decryptions to select subsequent ciphertexts. It is to be distinguished from an indifferent chosen-ciphertext attack (CCA1). Charles Rackoff and Dan Simon defined CCA2 and suggested a system adapting the CCA1 definition and system of Moni Naor and Moti Yung.

The goal of this attack is to gradually reveal information about an encrypted message, or about the decryption key itself. For public-key systems, adaptive-chosen-ciphertexts are generally applicable only when they have the property of ciphertext malleability — that is, a ciphertext can be modified in specific ways that will have a predictable effect on the decryption of that message.

Adaptive-chosen-ciphertext attacks were largely considered to be a theoretical concern until 1998, when Daniel Bleichenbacher of Bell Laboratories demonstrated a practical attack against systems using RSA encryption in concert with the PKCS#1 v1 encoding function, including a version of the Secure Socket Layer (SSL) protocol used by thousands of web servers at the time.

The Bleichenbacher attacks, also known as the million message attack, took advantage of flaws within the PKCS #1 function to gradually reveal the content of an RSA encrypted message. Doing this requires sending several million test ciphertexts to the decryption device (e.g., SSL-equipped web server). In practical terms, this means that an SSL session key can be exposed in a reasonable amount of time, perhaps a day or less.

In order to prevent adaptive-chosen-ciphertext attacks, it is necessary to use an encryption or encoding scheme that limits ciphertext malleability. A number of encoding schemes have been proposed; the most common standard for RSA encryption is Optimal Asymmetric Encryption Padding (OAEP). Unlike improvised schemes such as the padding used in the early versions of PKCS#1, OAEP has been proven secure in the random oracle model. OAEP was incorporated into PKCS#1 as of version 2.0 published in 1998 as the now-recommended encoding scheme, with the older scheme still supported but not recommended for new applications.

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