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Questions tagged [chosen-plaintext-attack]

The attacker can specify his own plain text and encrypt or sign it.

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1answer
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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 or IND-CPA or IND-CCA secure. I know what CPA ...
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111 views

Given partial key exposure and encryption oracle, can we recover full AES key?

Suppose I have an encryption oracle for AES with some key $k$ (16 bytes) and that I know $n$ bytes of it. Is it possible to recover the rest ($16 - n$) in complexity less than $256^{16-n}$?
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Indistinguishable encryptions and CPA-secure example

Let $ F $ be a PRF and $ G$ be a PRG with expansion factor $n \to n +1 $ For the following encryption scheme, decide whether it has indistinguishable encryptions and whether it is CPA-secure. ...
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On the probability of a collision in RFO-rCTR

My slides show that assuming a polynomial number $q(n)$ of blocks have been encrypted by a random function oracle in the context of a CPA-game, the probability of having a collision in the input to ...
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1answer
64 views

Is the scheme $(r, Mac_k(r) \oplus m)$ CPA-secure?

I honestly admit this question is taken from a h.w assignment: Prove or refute: the following encryption scheme is CPA-secure: Let $(\text{Gen}, \text{Mac}, \text{Vrfy})$ be a secure MAC with tags ...
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54 views

Building a CPA-secure sCTR from a PRF

Let $F$ be a PRF with $n = l_{in}(n) = l_{out}(n)$. For any PPT-encoding $[\;\;]:\mathbb{Z}_{2^n} \to \{0,1\}^n$ and any polynomial $l(n)$, $G_l(x) = F_k([1])\|\ldots\|F_k([l(n)])$ is a PRG ...
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1answer
67 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,...
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1answer
139 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?
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132 views

CPA-secure private-key encryption scheme that is unforgeable but 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. ...
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2answers
122 views

Motivation of Adversarial Indistinguishability Experiment

I had a question about the motivation behind this definition provided in Katz and Lindell's cryptography book. I copied the paragraph in question along with the definition of the experiment. ...
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Prove the advantage of a ROR-CPA is less than or equal to the advantage of a IND-CPA

Note that this is a homework problem assigned in Comp 655, UNC-CH. Please do not give an explicit answer as the instructor prohibited consulting outside resources. The problem is as follow: Prove ...
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1answer
45 views

Recovery of private key in AES-CBC from two ciphertexts with different IV, but identical plantext

I have a system that encrypts messages for a given user using key K using AES-CBC in response in an external stimulus. Each message has a randomly generated IV. The user never decodes these messages,...
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7answers
3k views

Possibility of Chosen Plaintext Attack (CPA) in real-world scenario?

In CPA, it is said that the adversaries get ciphertext for the choice of plaintext of adversaries through an encryption oracle. Is this a realistic assumption in real-world, in which the adversaries ...
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0answers
193 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-...
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0answers
46 views

IND-CPA game with a state-dependent Encryption Function that is insecure once

in the context of another setting I was asked the following question. Suppose the IND-CPA game is played with a symmetric encryption function $E$ that keeps a state of how many encryptions have ...
5
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1answer
169 views

How to encode messages in $\Bbb Z_p^*$ to be encrypted with ElGamal scheme?

In ElGamal encryption scheme, in order to achieve IND-CPA security, one must use a group where the DDH problem is assumed to be hard. As this answer suggests, one way to achieve that is the following: ...
3
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1answer
158 views

security against CPA attack?

Let $\mathcal{E} = (E,D)$ be a CPA secure cipher. Let's define $\mathcal{E'} = (E', D')$, where $E'(k,m) = E(k, E(k,m))$. How can I prove that $\mathcal{E'}$ will be CPA secure if $\mathcal{E}$ is? ...
3
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2answers
286 views

Proving that there is no scheme which would be perfectly CPA-secure

A private-key encryption scheme Π = (Gen, Enc, Dec) has perfectly indistinguishable encryptions under a chosen-plaintext attack, if for all probabilistic polynomial-time adversaries A it holds that Pr[...
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1answer
113 views

Relationship between CCA/CPA-security and PRFs

I'm a bit confused about the relationship between CCA/CPA-security and PRFs and particularly when do we think of encryption and decryption as a PRF. Assume we have an encryption scheme $\Pi = (Enc, ...
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1answer
50 views

Is there a property comparable to IND-CPA for hash functions taking a random value?

I would like to have a function $F(x, rnd)$, where $rnd$ is a fresh random value, such that it is hard, given $x_0$ and $x_1$, to distinguish $F(x_0, rnd)$ from $F(x_1, rnd)$, EDIT: but the values ...
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0answers
232 views

Why is counter mode encryption with a 16-byte cipher block not broken?

We determine a system IND-CPA secure when an adversary has a negligible advantage after any feasible amount of queries. AES256-GCM uses a 128bit block cipher. We know that the distinguishability ...
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1answer
98 views

Security of OFB mode if PRP is used instead of PRF

In the cryptography lecture at my university, we had the theorem that (randomized) OFB mode is IND-CPA secure if the used pseudorandom function (PRF) is IND-PRF secure. Afterwards, we investigated ...
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0answers
86 views

CCA secure Homomorphic encryption?

Do We need a homomorphic encryption scheme to be CCA secure ? In homomorphic encryption scheme we usually store data in cloud environment and process there. After processing we sent data to the ...
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1answer
114 views

Does CPA-secure means security against differential cryptanalysis?

I've read on differential cryptanalysis where it traces the differences on the changes of ciphertexts of the same plaintext, except for some bits. However, I wonder is the differential cryptanalysis ...
4
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1answer
477 views

Encrypt-and-mac IND-CCA security

I have been stuck all day trying to understand how to show that an Encrypt-and-Mac ($SE_{EaM}$) scheme is not IND-CCA secure. I have seen this similar post, however it is slightly different and the ...
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1answer
183 views

What is security of stream cipher against chosen plaintext attack?

I have been looking for image encryption after compression using Ax+e, where A is a random matrix, x is plaintext and b is a bitstream generated using a stream cipher. Suppose choosing plaintext as ...
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1answer
50 views

Semantic security IND-CPA of Affine Hill Cipher

The ciphertext,c corresponding to plaintext,m is given by c= mK + w, K is the invertible matrix and w is the uniformly distributed random vector. If w is changed during every encryption, will this ...
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0answers
176 views

If an encryption scheme is IND-CPA, can we construct an anonymous PKE?

Is the following statement true? Given an IND-CPA secure public key encryption , one can always construct an anonymous PKE in a blackbox way, i.e., call the algorithms of as subroutines. Why? ...
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1answer
145 views

weakness of a DES enhancement

The following key enhancement to DES was proposed in order to increase the complexity of finding the keys by exhaustive search. $$\text{DES}^V_{k,k_1}(M)=\text{DES}_k(M)\oplus k_1$$ where the keys’ ...
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1answer
50 views

The differences between the attackers in the definitions of IND-ATK and NM-ATK

I read the definitions of IND-ATK and NM-ATK in a paper. Let $\Pi = \left( Gen, Enc, Dec \right)$ be a public key scheme. And let $A = \left( A_{1}, A_{2} \right)$ be an adversary attacking $\Pi$ in ...
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1answer
274 views

Is AES-CTR-NoPadding resistant to variable-length chosen-plaintext attack?

Consider the following scenario: an adversary $A$ with access to an encryption oracle $\mathcal{O}$ is able to choose $n \in \mathbb{N}$ and pick $p_1,\dots,p_n$ such that $\forall i \in \{1,\dots,n\}:...
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1answer
213 views

Attacking AES without transposition using ShiftRow and Mixcolumn layers

How can we attack by using chosen plaintext attack (and which plaintext should we choose) if AES didnt have the ShiftRow and Mixcolumn layers. I know each byte would be independent in the 16 byte ...
2
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1answer
140 views

What happens if five messages are generated in IND-CPA security?

I am wondering why in IND-CPA security game, the adversary generates two random messages. What happens if for example 5 messages are generated? What would be the advantage for the adversary?
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2answers
321 views

Is there such a thing as perfect CPA security?

Consider the following experiment. If we require that $$\operatorname{P}\left( \mathcal A \text{ succeeds} \right) = \frac{1}{2}$$ for any adversary $\mathcal A$ in order to call the scheme $\Pi$ ...
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0answers
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How can I show the new crypto scheme is still IND-CCA1 in proof of “IND-CCA1 does not imply NM-CPA”?

I am trying to show IND-CCA1 does not imply NM-CPA. From what I have read the "classical" proof goes by taking an IND-CCA1 scheme $E$ and modifying it so that the encryption of the inverted plaintext ...
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1answer
244 views

How to use CPA on AES128-CBC with constant Key and IV to leak info?

I'm doing a challenge in which I need to login to a server as an admin to capture a flag. I can enter an $id$ and a $password$ only consists of 'a-z', '0-9','-', '_' characters. The flow is as follows:...
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1answer
262 views

Why do the messages in IND-CPA have to be of same length

I understand the general idea of IND-CPA, however I fail to understand why the messages the adversary chooses have to be of same length. I guess that with variable length messages there would always ...
0
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1answer
195 views

Selective Forgery of a Digital Signature - Goldwasser et al

I am working though the paper of Goldwasser, Micali and Rivest ''A Digital Signature Scheme Secure Against Adaptive Chosen- Message Attacks'' In the hierarchy of the severity of breaking the ...
2
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1answer
210 views

Attack on AES-CBC with small pool of IV

Let's assume that for encryption with AES in CBC mode we have $IV\in \{0\}^{113}||\{0,1\}^{15}$. I know that if we can predict next IV with 100% probability there is attack that completely breaks ...
5
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5answers
253 views

How to rigorously prove that $Enc_{\Pi'(k)}(M) = Enc_{\Pi(k)}(M) || LSB(k)$ is CPA secure assuming $\Pi$ is?

Assume $\Pi$ is a CPA secure scheme. Let $\Pi'$ be a derived scheme, such that the encryption of a message $M$ is as follows: $Enc_{\Pi'(k)}(M) = Enc_{\Pi(k)}(M) || LSB(k)$, where $LSB(k)$ is the ...
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0answers
88 views

Why does the DBDH simulator have half the advantage of an adversary?

Literature that includes the DBDH assumption such as this paper or this paper has a formal security proof that states: Suppose an adversary with non-negligible advantage $\varepsilon = Adv$. A ...
6
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3answers
859 views

Understanding the definition of polynomially CPA (IND-CPA) security

I just started on encryption schemes and have some trouble in understanding the following definition of IND-CPA security: $E$ is polynomially CPA (IND-CPA) secure, if for all adversaries $A$ and all ...
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1answer
2k views

Why plain RSA encryption does not achieve CPA-security?

Why plain RSA encryption does not achieve CPA-security?
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1answer
151 views

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 ...
8
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1answer
724 views

Is CBC mode with a fixed IV secure, if a counter is prepended to the plaintext?

In this answer to an earlier, related question I noted that encrypting a nonce, such as a sequential counter, using the same block cipher and key as used for the message encryption itself is one of ...
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0answers
298 views

Semantically secure, but not CPA secure?

I'm just starting to read a cryptography book and I'm having trouble understanding this. I know that semantically secure means that the adversary cannot learn any significant information seeing the ...
2
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3answers
300 views

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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1answer
124 views

Who defined semantic security?

In my opinion the most important development in cryptography in the last 30 years is the precise definition of semantic security of a cipher. This definition captures the intuitive ideas of Martin ...
4
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1answer
140 views

BEAST Attack: from guessing to decoding

I understood the XOR trick that allows an attacker to guess a block of a previously sent message from the client to the server: the attacker forces the client to send the message $\tilde{m} = c_n \...
3
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0answers
88 views

Chosen Plaintext Attack and Chosen Multiple-Plaintext Attack

The original CPA game: The adversary gives two arbitrary messages $m_0$ and $m_1$ to a challenger; The challenger encrypts an arbitrary message $m_u$, $u∈\{0,1\}$ , and sends the ciphertext to the ...