# Tagged Questions

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.

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### Why Boneh-Franklin BasicIdent IBE is not chosen-ciphertext secure? Why use random oracle?

I don't know why BasicIdent is not chosen-ciphertext secure. If there are anybody who knows well, please explain it to me with example. Moreover, I don't know random oracle and its usage for security ...
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### LR-Oracle Experiment in Lindell and Katz

In reference to the LR-Oracle experiment in “Introduction to Modern Cryptography” (2nd edition) by Lindell & Katz, Definition 3.23 states a scheme $\pi = (Gen,Enc_K,Dec_K)$ is CPA secure for ...
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### What is the difference between RCCA and CCA2?

Well, I know it's easy to tell CCA1 from CCA2, but I failed to find the difference between RCCA and CCA2. What is the difference?
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### What do you mean by Ciphertext indistinguishability [duplicate]

Wiki definition : 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 ...
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### How do I prove that a bit by bit PKE scheme (that is IND-CPA secure) for an n-bit message is not IND-CCA-2 secure?

Gen generates Pk and Sk (l bits each), Enc encrypts m (length l) but by bit using each bit of Pk respectively, Dec decrypts ciphertext bit by bit using each bit of Sk respectively. How do I prove this ...
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### Confused about how chosen ciphertext attacks work?

When performing a CCA, the adversary gets the public key. At some point, he will send a message m to the challenger, and the challenger will send back either $E_{pk}(m)$ or $E_{pk}(r)$, where r is ...
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### Cracking a hash which descends to a single value after repeated hashes

Assume I have a hash function (such as the one below) and a cipher, and I know that if I repeatedly hash any string over and over again, it will eventually descend to one unique value. Is there any ...
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### Definition of ciphertext security

If I got it right, chosen ciphertext security implies also CPA security. In other words, attacker can submit plaintexts to the challenger (along with ciphertexts). I do not understand why ...
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### Definition of the Decryption oracle

In the context of public-key encryption, what would be a formal definition of the decryption oracle? I know the informal definition (i.e., a function that is available to the adversary and that ...
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### Chosen plaintext vs Chosen ciphertext

What is the difference between chosen plaintext and chosen ciphertext attack? In both of these adversary has access to paintext-ciphertext pairs.Then where lies the difference?
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### Secure double encryption using CPA and CCA

Do you mind if you give me any hints, links or ideas about how to improve the security of double regular encryption and decryption, by using CPA game and CCA game, it sounds interesting question, and ...
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### What is the difference between IND-CCA2 and NM-CCA2? And how does one imply another? [closed]

Is there any proof that shows the equivalence of these two?
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### CCA security of a system that splits messages and encrypts each packet

Propose a symmetric key based crypto-system for implementing a secure email system. This system is based on AES and CCA secure. Suppose that you have to encrypt a large message and that this message ...