Questions tagged [chosen-plaintext-attack]

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

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Define a Decryption algorithm on a given group-based Cramer Shoup lite scheme

I am currently working on public key encryption schemes and I want some help to figure out how decryption algorithms work. Suppose we have a public key $pk = (G,p,g,e)$ with $e \in Z^*_p$ . (where $G$ ...
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IND-CPA for permutation based encryption

My encryption function is some permutation using a permutation key. An input to the encryption function is encoding of my plain text where one plaintext is mapped to many encodings. Can we prove ...
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Security properties of ElGamal encryption variants

I'll use Taher ElGamal's A Public Key Cryptosystem and a Signature Scheme Based on Discrete Logarithms (July 1985 in IEEE Transactions on Information Theory, formerly in proceedings of Crypto 1984) as ...
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Is non-adaptive CPA strictly weaker than standard CPA security?

Today during a cryptography lecture an interesting question came up: Whether non-adaptive CPA security is equivalent to adaptive (FtG / LOR) CPA security. Now for a short description of what this ...
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Security of key bits under indistinguishability for randomized encryption

My question is about symmetric randomized encryption. Let c=E(k,m,r) be a randomized block encryption function where r denotes a random input chosen for each message block m. k and c denote the ...
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Is it possible to sample HE ciphertexts using pseudo-random generator?

I'm trying to sample HE ciphertexts which serve as ciphertexts encrypting some random values. I have realized a example program using Microsoft SEAL, which implements a variant of BFV scheme. ...
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How to prove that ECB mode is not CPA secure?

I am new to cryptography and I do not know how to prove the ECB mode is not chosen plaintext attack (CPA) secure.
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Does AES-ECB with random padding added to each block satisfy IND-CPA?

I present a modified version of AES-ECB. Suppose I want to encrypt the string hello, world. this is a test. For each block, I use the first eight bytes as my actual ...
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202 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 ...
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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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Basic CPA walkthrough

My professor put this on the slides but gave us very little context. It is meant to be an overview of how it works at a high level. I have a programming background and I get the general idea of ...
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173 views

DES - Weakness in Sbox

In the book "Introduction to Cryptography with Coding Theory ", in the chapter about the Differential Cryptanalysis for Four Rounds for DES algorithm, the authors say: There is a weakness in the ...
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How do I prove that an encryption based on a multiplicative cyclic group is IND-CPA?

Consider the following encryption and decryption functions where $\mathbb{Z}_{N}^{*}$ is a cyclic multiplicative group, $g$ is a generator for the group and $F$ is a keyed PRF. $E(m, i, k) = m \times ...
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how to prove that given cryptosystem is not IND-CCA secure?

Let $m$ and $k$ be positive integers ($m$ bit plain text) ; let $Q$ be a family of trapdoor one-way permutations such that $f : \{0,1\}^k \rightarrow \{0,1\}^k$ for all $f$ in $Q$; and let $G : \{0,1\}...
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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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Is it possible to build a CPA-secure encryption scheme which remains secure even when the encryption of secret key is given?

How can I design a CPA-secure encryption scheme which is secure even after the encryption of secret key is given in the training phase? I.e., in the training phase, $\mathit{Enc}_{\mathit{pk}}(\...
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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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Proving a scheme's EAV-insecurity

EDIT: The instructor clarified that part (a) was wrong. He declined to indicate why. I can share more details of the prompt if need be. I apologize again if this question is too low-level; it comes ...
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How to figure out this encryption? [closed]

I can provide input to an application that encrypts plaintext via an unknown algorithm, which I would like to reverse. I can obtain plaintext:ciphertext pairs, and the length of the cipher depends on ...
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Is AES resilient to chosen plaintext with access to related keys (key expansion primitives)?

I'm interested in knowing whether a cryptosystem is broken given access to a few primitives. $\DeclareMathOperator{\KEYEXP}{KEY\_EXP}$ $\DeclareMathOperator{\E}{E}$ Suppose that you have access to an ...
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Proof that secret sharing based scheme is CPA secure as long as one of the scheme is CPA secure

I want to construct a CPA-secure schemes using two given schemes $\prod_1$ and $\prod_2$ if only one of them is CPA secure. Taking suggestions from this answer, I am able construct a scheme as ...
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262 views

AES oracle with bad S-box

Say you run in to a messed up AES (128) implementation which has the s-box configured as [0x0, 0x1, ..., 0xFE, 0xFF] and where you can query a few plaintexts to get the corresponding ciphertexts. How ...
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116 views

Show that CPA-security implies security against plaintext recovery

Consider the following experiment: $k = Gen(1^n)$ $m \stackrel{u}{\in} \{0,1\}^{l(n)}$ $c = Enc_k(m)$ $m' = B(1^n,c)$ Define the winning event as $m' = m$. I would like to show that if the ...
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What is the difference between Forgery and Chosen-Message Attacks?

I did some researches about forgery attack and chosen-message attack. For me, forgery attacks are more focused on the verifiability whereas chosen-message attacks are similar to brute-force attacks. ...
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Is it necessary to study the KPA security for asymmetric encryption algorithms in addition to CPA security? [duplicate]

In a KPA attack model, an adversary $\mathcal{A}$ has access to pairs of messages $(m_i,c_i)$ which are encrypted using some asymmetric algorithm. However, assuming either El-Gamal or RSA algorithms, ...
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158 views

Give a CPA-attack on Elgamal when used with $\mathbb{Z}_p^*$

We know that if the decisional Diffie-Hellmann problem is hard then the Elgamal encryption is CPA-secure. One show that if the chosen group is $\mathbb{Z}_p^*$ then the decisional Diffie-Hellman ...
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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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216 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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87 views

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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114 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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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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268 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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388 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. ...
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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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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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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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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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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 ...
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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: ...
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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? ...
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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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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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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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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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148 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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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 ...
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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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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 ...