**Ciphertext indistinguishability** is a property of many encryption schemes. Intuitively, if a cryptosystem possesses the property of indistinguishability, then an adversary will be unable to distinguish pairs of ciphertexts based on the message they encrypt. The property of indistinguishability under chosen plaintext attack is considered a basic requirement for most provably secure public key cryptosystems, though some schemes also provide indistinguishability under chosen ciphertext attack and adaptive chosen ciphertext attack. Indistinguishability under chosen plaintext attack is equivalent to the property of semantic security, and many cryptographic proofs use these definitions interchangeably.

A cryptosystem is considered *secure in terms of indistinguishability* if no adversary, given an encryption of a message randomly chosen from a two-element message space determined by the adversary, can identify the message choice with probability significantly better than that of random guessing (1/2). If any adversary can succeed in distinguishing the chosen ciphertext with a probability significantly greater than 1/2, then this adversary is considered to have an "advantage" in distinguishing the ciphertext, and the scheme is *not* considered secure in terms of indistinguishability. This definition encompasses the notion that in a secure scheme, the adversary should glean no information from seeing a ciphertext. Therefore, the adversary should be able to do no better than if it guessed randomly.

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