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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48
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1answer
23k views

Easy explanation of “IND-” security notions?

There are many schemes that can advertise themselves with certain security notions, usually IND-CPA or IND-CCA2, for example plain ElGamal has IND-CPA security but doesn't provide IND-CCA security. ...
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1answer
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Can you explain Bleichenbacher's CCA attack on PKCS#1 v1.5?

I've studied that the Bleichenbacher's CCA attack on PKCS#1 v1.5. is a base to many versions of attacks in the area. I'm trying to understand that attack, but every explanation I saw starts with the ...
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1answer
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The difference between these 4 breaking Cipher techniques?

I'm trying to understand the difference between the following and what they actually mean : Known plaintext attack Known ciphertext attack Chosen ciphertext attack Chosen plaintext attack Any ...
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How does a padding oracle attack work?

I am unsure of how a padding oracle attack works. What I am not getting is how changing one bit at one time allows one to exploit(get keys) ASP.NET machines. Can anyone explain this?
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1answer
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Can a homomorphic encryption scheme be made CCA2 Secure?

Is it possible to modify a homomorphic encryption scheme so that it can be CCA2 secure? From the definition of a homomorphic scheme, it seems that it is malleable, which would result in lack of CCA2 ...
6
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1answer
2k views

Is ElGamal IND-CCA1?

We all know that textbook ElGamal falls due to chosen ciphertext attacks, because of its multiplicative homomorphic property ($E(A)*E(B)=E(AB)$). However these attacks require the ciphertext ($E(A)$ ...
6
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0answers
253 views

IND-CCA1 RSA padding?

I've found a way to complete a task which I'd solve with passwords or by sending keys over the wire (otherwise) by using RSA's homomorphic property. I'm restricted to RSA (any padding; for hardware ...
13
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1answer
4k views

Why do we encrypt-then mac but sign-then-encrypt?

This question was asked in comments on my answer to should we sign then encrypt or encrypt then sign. I think posing the question as a separate entity is best, so: The general advice for applying ...
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2answers
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How does a chosen ciphertext attack work, with a simple example?

Can someone please explain - using a simple example - how a chosen ciphertext attack works?
8
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1answer
529 views

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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3answers
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Is the CBC weakness in XML Encryption a new discovery? Are other applications vulnerable?

The RUB in Germany reports that XML encryption is broken. This is essentially the W3C standard for protecting XML documents from prying eyes. Does this mean that an attacker can only see a single ...
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1answer
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Common modulus attack on RSA when the 2 public exponents differ by a single bit

This is an exam question an i have no idea how to recover the message m. John wants to send an encrypted message to mary who has a pair of RSA keys, However, John does not know Mary's public key and ...
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1answer
391 views

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 ...
5
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1answer
635 views

Secure ElGamal with OAEP

Is it possible to make ElGamal IND-CCA2 using OAEP or OAEP+? (OAEP+ from: "OAEP Reconsiderd" by Shoup) The reason I ask is that I recently answered this question and it came to my mind that OAEP or ...
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IES: What security has the asymmetric primitive to provide?

I think most of us know the notion Shoup introduced of KEM/DEM (Key Encapsulation Mechanism / Data Encapsulation Mechanism) which is used for example by the (famous) ECIES, where the key is the hash ...
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Chosen plaintext attack on textbook RSA decryption

Let $(n,d)$ be an RSA private key and $(n,e)$ the corresponding public key. Generate a secret random $r$ from $(\mathbb{Z}/n\mathbb{Z})^*$ Compute $C’ = C (r^e) \bmod n$ Submit $C’$ as a chosen ...
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Is there a partially homomorphic quantum secure public key cryptosystem with IND-CCA1 security?

I recentely asked "IND-CCA1 RSA padding?" about whether there is a IND-CCA1 secure variant of RSA. The original version of the question also allowed usage of ECC which would allow usage of ElGamal, ...
6
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2answers
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Creating own ciphertext after a padding oracle attack?

I've written a script that breaks a cipher text based on a padding oracle for an assignment, but was wondering how I would continue on to create my own cipher text with any plain text I desired? ...
4
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3answers
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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?
4
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2answers
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AES-CTR vulnerability to cryptographic oracle

I am new to cryptographic issues and from what I googled so far I could not retrieve the information I need. Consider the use of AES-128 in CTR mode. Let M be the set of possible plaintexts, for ...
3
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2answers
126 views

Bleichenbacher-attack test vectors

I was trying to understand the Bleichenbacher-attack. I went through the paper of Bleichenbacher to understand it bit more. However I tried to get some test vectors to simulate this attack locally but ...
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1answer
639 views

Breaking One Time Pad with CCA

I was taught that: although OTP is CPA secure, it is not the case with CCA. I'm trying to figure how to break OTP with CCA by showing how it fails the IND test. ...
8
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1answer
477 views

Can RSA be securely used for “blind decryption”?

Assume we have the following setup: A client with trusted storage and computing capabilities (e.g. a smartcard) A server with trusted computing and short-term storage capabilities (e.g. RAM + CPU, ...
6
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2answers
472 views

Why does Encrypt-Then-Sign not yield a CCA-secure public-key encryption scheme?

In symmetric cryptography, combining an IND-CPA secure symmetric encryption scheme with a secure MAC with the encrypt-then-MAC method yields a IND-CCA secure symmetric encryption scheme. I am trying ...
6
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3answers
818 views

What are the practical implications of ciphertext distinguishability?

Commonly there are four ways to "break" a secrecy-focused cryptosystem: Recover the secret key Recover the message Distinguish an encryption from random noise Distinguish the encryption of two ...
4
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1answer
2k views

How can a Encrypt-And-MAC scheme be insecure if the encryption is CCA secure?

I was presented with this seemingly paradoxical information, and am hoping someone can explain what I'm missing here. I have an encrypt-and-mac scheme here.If I want to transmit message m to Bob, I'd ...
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1answer
700 views

Does RSA-OEAP have integrity and authenticity properties?

RSA-OAEP is IND-CCA2 secure (indistinguishable under an adaptive chosen ciphertext attack). Does it also have the INT-CTXT (integrity of ciphertext) and INT-PTXT (integrity of plaintext) properties? ...
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1answer
314 views

To which game-based security definition is Perfect Secrecy equivalent?

We all know the classic definitions of perfect secrecy, being $$\Pr[M=m|C=c]=\Pr[M=m]$$ and $$H(M|C)=H(M)$$ But now what I've asked myself: If we were to remove the polynomial restriction on the ...
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1answer
113 views

Finding RSA private exponent with a chosen ciphertext attack?

Let's say there's this blackbox app that has a known modulus $N$, private exponent $e$, and takes ciphertext $C$ encrypted against $N$ and $e$ input and outputs plaintext $M$. The encryption is done ...
0
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1answer
949 views

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 ...
0
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1answer
1k views

Proving the CCA-insecurity of El Gamal encryption scheme

I am trying to prove that El Gamal Encryption is not CCA-Secure. If an adversary queried the encryption oracle for the encryption of $m=1$, he'll get a pair $(c_1,c_2)=(g^y,g^{xy}\cdot m)=(g^y,g^{xy}\...