I've seen some papers drawing a connection between IND-CPA and passive attacks and IND-CCA and active attacks. Can someone explain how IND-CPA relates to passive attacks and IND-CCA relates to active attacks if this is indeed the case?

The most recent example of such a paper that I can recall is this: https://eprint.iacr.org/2020/1533

Another example is this excerpt from the preface of Introduction to Modern Cryptography, 3rd edition (http://www.cs.umd.edu/~jkatz/imc.html):

"We have divided our treatment of symmetric-key encryption into two parts: Chapter 3 deals with security against “passive” attacks (i.e., CPA-security), while Chapter 5 addresses “active” attacks (i.e., CCA-security and authenticated encryption)."


1 Answer 1


IND-CPA and IND-CCA are formal security models. "Active" and "passive" are informal descriptive terms.

"Passive" usually refers to an adversary who simply observes a system under its normal operation by honest users. "Active" refers to an adversary who "interferes" in some way, causing situations that cannot arise under honest usage, usually by providing "unexpected" or "impossible" cryptographic data.

CPA is a "passive" attack since the adversary just observes honestly generated ciphertexts. The adversary chooses the plaintexts, but that is not considered an "active" attack -- plaintexts themselves are not cryptographic, and the encryption algorithm is still applied honestly to those plaintexts.

CCA is an "active" attack since the adversary can cause maliciously generated ciphertexts to be decrypted. This is something that can never happen under the normal operations of a system by honest users.

The paper that you reference considers itself a "passive" attack because it only involves decrypting ciphertexts that are the result of honestly running (an adversarially chosen sequence of) the encryption scheme's algorithms.


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