# Questions tagged [chosen-ciphertext-attack]

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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### Is Cramer-Shoup Lite IND-CCA secure?

Recently, I studied about Cramer-Shoup encryption scheme. Then, I read a book which mentioned a simpler version of this scheme, the Cramer-Shoup Lite encryption scheme. I was wondering if there is ...
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### Why does this attack on a simplified version of the Naor-Yung scheme not work for the original scheme?

tl;dr: We were given a homework, in which we should discuss, whether encrypting twice in the Naor-Yung scheme is actually necessary. I could prove that it is, by constructing an attacker on a scheme ...
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### CCA security of this scheme

Let $k$ be uniformly sampled from $\{0,1\}^\lambda$, $F$ be a secure PRP with block length $\lambda$ and let $Enc(k, m)$ be such that it returns $c = (F(k,r), r \oplus F(k,m))$ with $r$ uniformly ...
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### How to prove an encryption scheme is not CCA secure

Given the following context below, how does one show or prove the encryption scheme is NOT IND-CCA secure The IND-CPA secure private-key encryption scheme for n-bit messages can be constructed from a ...
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### Attack on security model for RSA-FDH

I recently read in Cryptography textbooks and online videos that compared to Textbook RSA which is insecure against Chosen Plaintext Attacks (CPA) and Chosen Ciphertext Attacks (CCA), RSA-FDH is ...
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### Efficient attack on RSA with known padding scheme and public key

I am having some trouble with an assignment problem. I hope I can get some guidance here. The question provides us with a public key, $N$ and the padding scheme. This is the padding scheme "Assume ...
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### Formal security-definition reference

Is there a reference where all common formal security-definitions are summarized (such as OW-CPA, IND-CPA, IND-CCA1, IND-CCA2, EUF-CMA and so on)? I need a PUBLISHED reference (paper, book) which ...
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### Is it possible to mathematically extract an AES key from black-box encrypt/decrypt hardware?

I presented our mathematician with an idea: If you have a black box that encrypts or decrypts AES with the same 128 bit key (you don't have any direct access to the key), and you control the input ...
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### Patarin’s H-Coefficient Technique

I have come across many paper that applies Patarin’s H-Coefficient Technique in various cryptographic securities (KPA, CPA, CCA,...). I am asking kindly if someone could explain it simple with ...
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### Is chosen ciphertext attack used to obtain a key? [duplicate]

I don't understand what the goals are of a Chosen Ciphertext Attack. What I've read from Wikipedia is that it is an attack one can use to obtain the key that the crypto algorithm uses. However, I ...
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### If TDP exist then CCA-secure PKES exist?

My cryptography slides describe several relations between cryptographic problems. I don't still have a good justification on the following: If trapdoor permutations exist then CCA-secure public key ...
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### If DDH is hard then CCA-secure PKES exist?

My cryptography slides describe several relations between cryptographic problems. I don't still have a good justification on the following: If decisional Diffie-Hellman problem is hard then there ...
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### CCA attack on RSA padded with random oracle

I'm trying to do the ex 11.15 of the Katz-Lindell book. Consider the RSA-based encryption scheme in which a user encrypts a message $m ∈$ {$0, 1$}$^l$ with respect to the public key $(N, e)$ by ...
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### Does Elliptic Curve Integrated Encryption Scheme (ECIES) provide IND-CCA2 security?

I am looking for a faster alternative to RSA with OAEP as a IND-CCA2 public key scheme. Elliptic Curve Integrated Encryption Scheme might be a candidate, but I am not sure if it provides IND-CCA2 ...
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### Differences between NewHope-CPA-KEM and NewHope-CCA-KEM

According to the web page for NewHope, an R-LWE post-quantum key encapsulation mechanism (KEM) candidate for standardization, it comes in types that are IND-CPA or IND-CCA secure. I know what CPA and ...
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### Symetric key encryption is not Authentication?

Is there any way to show CCA-secure symmetric-key encryption does not have to be an authenticated encryption? I know you can create a setup that isn't authentication, but how will you go about it?
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### Is there a way of maintaining malleability in a homomorphic encryption system while making it infeasible to perform chosen ciphertext attacks?

Is there a way of maintaining malleability in a homomorphic encryption system while making it infeasible to perform chosen ciphertext attacks? I have been reading about homomorphic encryption and ...
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### RSA-CCA security of $m || H(m)$ as input to modular exponentiation

If we encrypt a message $m$ as $m || H(m)$ using textbook RSA encryption algorithm, $H$ is random oracle modeled. In this case the message can be recovered. Is this encryption CCA secure? If so how ...
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### Why is the lite version of Cramer-Shoup not IND-CCA2 secure?

In the lite version of Cramer-Shoup we have a group $G$ with generators $g_1$ and $g_2$, private key $a_1, a_2, b_1, b_2$, and public key $A = g_1^{a_1} g_2^{a_2}$, $B = g_1^{b_1} g_2^{b_2}$. ...
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### CCA2-secure encryption based on LWE/Ring-LWE

As I understand it, cryptosystems based on LWE/Ring-LWE are typically only IND-CPA, and fail to satisfy IND-CCA2. In fact, it appears that chosen ciphertext attacks could allow secret key recovery (!)...
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### Can I overcome ElGamal malleability with AES preprocessing?

So textbook ElGamal is known to be vulnerable to CPA, and although there are several proposals (DHAES/EIES) for modifications none of those are formally accepted or standardized. My question is ...
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### Does security against a CCA1 attacker imply security against a CPA attacker?

I've seen it mentioned multiple times, but have not seen a formal proof of how security of an encryption scheme against a non adaptive chosen-ciphertext attacker (under non-malleability or ciphertext ...
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### Invalid challenge-ciphertext in IND-CPA and IND-CCA games

Consider the IND-CPA game between a challenger $C$ and an adversary $A$ for a given public-key encryption scheme $PKE$: $C$ generates a key pair $(pk,sk)$ based on some security parameter $n$, and ...
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### many time pad attack [duplicate]

I am a beginner in cryptography. I studied many time pad attack and now I want to solve a popular exercise, It consists 11 ciphertexts (s1,s2,...,s11), that encrypted by a same key and I want to find ...
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### Are common (secure) stream ciphers CCA1-secure?

My latest question and especially Ricky Demer's comment on the answer got me thinking: This homomorphic tranformation of RSA is most likely IND-CPA and maybe even IND-CCA1, but if it could be IND-CCA1,...