Formal Definitions
Security in terms of indistinguishability has many definitions, depending on assumptions made about the capabilities of the attacker. It is normally presented as a game, where the cryptosystem is considered secure if no adversary can win the game with significantly greater probability than an adversary who must guess randomly. The most common definitions used in cryptography are indistinguishability under chosen plaintext attack (abbreviated IND-CPA), indistinguishability under (non-adaptive) chosen ciphertext attack (IND-CCA), and indistinguishability under adaptive chosen ciphertext attack (IND-CCA2). Security under either of the latter definition implies security under the previous ones: a scheme which is IND-CCA secure is also IND-CPA secure, and a scheme which is IND-CCA2 secure is both IND-CCA and IND-CPA secure. Thus, IND-CCA2 is the strongest of the three definitions of security.
Read more about this topic: Ciphertext Indistinguishability
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