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.

Filter by
Sorted by
Tagged with
1
vote
1answer
60 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
votes
1answer
98 views

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 ...
3
votes
1answer
730 views

Indistinguishability versus semantic security?

I found some foundations of crypto course notes that mention that these two are equivalent statements of their own degrees of security and was given the following definitions: I was always under the ...
1
vote
1answer
129 views

How the message is decrypted in an RSA chosen ciphertext attack when it's a modulo?

During the last stage of decryption of a chosen ciphertext attack we are left off with this equation $$c = m \cdot t \pmod n$$ Removing all exponents like $d$ and $e$ where $c$ is the decrypted ...
6
votes
0answers
236 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 ...
1
vote
0answers
50 views

CCA Security/Decryption of PRF scheme

Let $F : \{0,1\}^n \times \{0,1\}^n \rightarrow \{0,1\}^n $ be a PRF. And let the encryption function be $Enc{_k}(m) = r\mathbin\| (F{_k}(m\bigoplus r)$ , r being a random value. Im trying to proof ...
0
votes
0answers
81 views

RNG to hide some bits

Suppose I have a sequence of bits of length M. I need to hide N out of M bits (hiding means forcing them to be 0) using RNG with a key. How I can relate the output of RNG to the number and positions ...
6
votes
2answers
333 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 ...
0
votes
1answer
76 views

How to prove that RSA is not IND-CCA2?

RSA is homomorphic wrt multiplication and therefore it is not IND-CCA2. But how to show it. What are the steps to win IND-CCA2 game? How is the probability of winning calculated?
1
vote
1answer
47 views

Knowledge of discrete log is needed in the proof of Cramer-Shoup public key scheme?

In the proof of the Cramer-Shoup public key scheme [1], I understand that the adversary's view can be seen as equations such as $\log c = x_1 + w x_2, \log d = y_1 + w y_2$ and so on (equation 1 and 2 ...
3
votes
0answers
38 views

Markov ciphers - generalized group operation

Iโ€™m reading this paper of Lai and Massey where they give a very nice perspective on differential attacks in general http://www.isiweb.ee.ethz.ch/papers/arch/xlai-mass-inspec-1991-3.pdf They do their ...
1
vote
1answer
91 views

How can I show RSA-OAEP is not IND-CCA secure if G always outputs 0 (i.e. there is no hash function)?

Here is what I understand the algorithm to be: Let $G, H$ be one-way functions, where G outputs the 0 string. ๐ธ๐‘›๐‘๐‘œ๐‘‘๐‘’ Select a random $๐‘˜$-bit integer $๐‘Ÿ$. Pad out $๐‘š$ with $0$s to length $๐‘™...
1
vote
0answers
45 views

Proofs that NM-CPA implies IND-Cpa and $\text{IND-CCA} \leftrightarrow \text{NM-CCA}$

I am looking for a proof for the following two claims from "Serious Cryptography" by Aumasson. NM-CPA implies IND-CPA. [...] IND-CCA and NM-CCA are equivalent notions that imply the presence of the ...
3
votes
2answers
112 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 ...
1
vote
1answer
57 views

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 ...
0
votes
0answers
169 views

Strong pseudo random permutation implies CCA-security

Let $F$ be a strong pseudo random permutation and define a fixed-length encryption scheme $(\operatorname{Enc,Dec})$ as follows: On input $m \in \{0,1\}^{\frac{n}{2}}$ and key $k\in \{0,1\}^{n}$, ...
3
votes
2answers
339 views

Security of CFB on a public channel

Given a server that broadcasts all received data to all connected peers (except the sender), two participants can have an encrypted conversation using a self-synchronizing method like CFB and some ...
1
vote
1answer
568 views

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 ...
2
votes
1answer
517 views

AES-GCM secure against IND-CCA2

How would you prove that AES-GCM is secure against a chosen ciphertext attack (IND-CCA2), assuming that the attacker cannot forge AES-GCM MAC tags in less than brute force?
46
votes
1answer
20k 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. ...
2
votes
1answer
202 views

How many required known plaintexts for an attack are considered insecure?

I stumbled over this problem when looking at symmetrical block ciphers with a relatively small block size like 32 bit. When I was looking at more common block ciphers with a 64 bit, I learned that ...
0
votes
1answer
126 views

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 ...
0
votes
1answer
569 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}\...
3
votes
2answers
298 views

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 ...
1
vote
1answer
195 views

Is Hybrid Encryption CCA secure?

If Enc is a PKE(Public Key Encryption) with key generation algorithm KG and message space $M = \{0, 1\}^n$, and SE is a Symmetric Key Encryption(SKE) with key space $\{0, 1\}^n$ and message space $M' =...
0
votes
1answer
418 views

Hash ElGamal CCA approach

With this Elgamal variant defined below, we have covered in lecture but I do not still understand how a CCA could work against it. Is the use of the homomorphic nature of the encryption something you ...
9
votes
1answer
1k views

AES-CTR mode and the Biclique Attack

The biclique attacks that break AES (Biclique Cryptanalysis of the full AES) appear to require decryption oracles to work, presumably because the key schedule of AES is weaker in the decryption ...
6
votes
1answer
325 views

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 ...
2
votes
0answers
85 views

Intuition for Cramer Shoup Encryption Scheme?

I was reading the Cramer Shoup CCA2 secure encryption scheme. The scheme is as follows. Public key = $(g_1, g_2, c, d, h, hk)$, where $c = g_1^{x_1}g_2^{y_1}$, $d = g_1^{x_2}g_2^{y_2}$, $h = g_1^z$, ...
1
vote
2answers
284 views

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 ...
1
vote
1answer
595 views

Unforgeable CPA-secure private-key encryption scheme that is not CCA-secure

This is exercise with 4.26 in Katz and Lindell's introduction to cryptography 2nd ed. The question is: Show a CPA-secure private-key encryption scheme that is unforgeable but is not CCA-secure. ...
4
votes
3answers
10k views

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?
0
votes
0answers
125 views

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 ...
3
votes
1answer
862 views

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 ...
1
vote
0answers
436 views

Cryptanalysis affine cipher and substitution cipher

1) Consider the chosen-plaintext attack on the simple substitution cipher. How many chosen plaintext characters suffice to uniquely determine the key? My answer was: 26! 2) Consider the chosen-...
4
votes
1answer
1k views

Security proof of FO(Fujisaki-Okamoto) hybrid encryption

The proof of FO hybrid encryption is hard to understand. $\:$ Especially, how does the challenger respond to the decryption queries when the challenger can only have some encryption queries?
1
vote
0answers
115 views

Patarinโ€™s H-Coefficient Technique

I have come across many paper that applies Patarinโ€™s H-Coefficient Technique in various cryptographic securities (KPA, CPA, CCA,...). I am asking kindly if someone could explain it simple with ...
1
vote
1answer
132 views

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 ...
2
votes
1answer
93 views

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 ...
2
votes
1answer
72 views

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 ...
4
votes
1answer
3k views

RSA Blind Signature Attack

In Information Security Class I learned a non-adaptive chosen ciphertext attack using "blinding" but I just can't follow the proof and therefore can't reconstruct it. From what I've found out, it ...
1
vote
0answers
170 views

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 โˆˆ $ {$0, 1$}$^l$ with respect to the public key $(N, e)$ by ...
4
votes
1answer
288 views

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 ...
3
votes
1answer
179 views

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?
21
votes
2answers
19k views

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?
8
votes
1answer
503 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 ...
1
vote
1answer
422 views

CPA-secure of $\Pi$ implies CPA-security of $\Pi||\Pi||\ldots||\Pi$

Katz and Lindell book claim that from the theorem: Any private-key encryption scheme that is CPA-secure is also CPA-secure for multiple encryptions. Follows the fact that: If $(Gen,...
5
votes
1answer
584 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 ...
2
votes
1answer
136 views

GCM decryption and clear text processing

When using GCM mode decryption, is it necessary to wait for the decryption to complete, (and thus being able to see if the integrity of cipher text is OK) before beginning processing the clear text ...
4
votes
0answers
242 views

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 ...