Questions tagged [chosen-plaintext-attack]

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

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Intuitive Explanation for why IND-CPA Security does not imply EKRp-CPA (Existential Key Recovery)

In this paper, one of the facts stated was that $\text{IND-CPA} \not\implies \text{EKR}_p$, in which $\text{EKR}_p$ is defined as the property that for a polynomial number accessible of oracles p with ...
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How to find modulus and decrypt a message with a CPA on textbook RSA. The encryption oracle only allows two queries

The encryption oracle only allows you to encrypt a custom message $m$ or the secret message $m_s$. In both cases it asks you to choose a public exponent $e>16$. Of course, $n$, is not available. If ...
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220 views

Figure out chosen-plaintext-attack in Zerologon attack

Actually, I'm reading the description about Zerologon attack from the original whitepaper document. In there, Tom Tervoort mentions these sentences: So I tried to come up with some chosen-plaintext ...
2 votes
2 answers
116 views

Hard to guess ciphertext

Is there an encryption scheme where it's hard to guess a valid ciphertext without knowing the key and the scheme is CPA secure?
10 votes
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3k 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 ...
68 votes
1 answer
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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. ...
3 votes
1 answer
330 views

Is the secret key kept constant in IND-CPA game?

I read the Wikipedia Page on ciphertext indistinguishability. Here it gives the following outline of the IND-CPA game: The challenger generates a key pair PK, SK based on some security parameter k (e....
6 votes
1 answer
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A modern rotor machine, could it be any safe?

I wonder if a rotor machine similar to enigma can be considered secure by today's standards under four conditions: A rotor machine which consists of 50 rotors picked out of a set of 100. A rotor ...
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1 answer
197 views

Are modes like CBC, OFB, CFB subject to chosen plaintext attacks?

I haven't found much info on the internet about the weakness of modes to chosen plaintext attacks, but from what I understand of them, there seem to be some trivial attacks, so I'm a bit confused. For ...
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1 answer
91 views

Bit flipping attack in hash function for message authentication

In this picture we have a use of a hash function for message authentication. M is plaintext message. H is hash function. E is encryption block with K symmetric key. || is concatenation of plaintext M ...
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1 answer
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What schemas are possible for the following encryption algorithms?

I'm trying to practice and to understand possible scenarios and the basics of symmetric encryption schemes. For the scenarios I listed below, I am not 100% sure of scenarios 1 and 3. A symmetric ...
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How to give a hybrid proof that IND CPA secure implies multiple query IND CPA secure and vice versa? [closed]

For a public-key encryption scheme (Gen,Enc,Dec), the textbook definition of IND-CPA security is the following: The challenger runs (pk,sk)←Gen and sends pk to the adversary. The adversary performs ...
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1 answer
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How is Crystal Kyber CPA-secure algorithms converted into a CCA-secure algorithm

In Crystal Kyber specification, page no10, there are 3 algorithms namely KYBER.CCAKEM for making the CPA-secure Kyber into CCA-secure one. Q1: How is the shared key K being generated in KYBER.CCAKEM....
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Are there any papers talking about generating the pairs used in differential cryptanalysis or impossible differential Cryptanlysis?

I'm trying to implement impossible differential cryptanalysis on IDEA cipher based on the Miss in the middle attack on IDEA and Khufu paper. I'm trying to implement the attack on 3.5-Round IDEA and I ...
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When could CPA work against non-deterministic encryption schemes?

Is there a non-deterministic encryption scheme that is vulnerable to a CPA? I got asked this in an interview and I answered with no but the interviewer said that my answer is wrong. Would somebody ...
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Microsoft exchange server data breach relevance to IND-CPA, IND-CCA1, IND-CCA2

I was studying about the Microsoft exchange server data breach attack of 2021 as part of assignment case study. One of the tasks of the assignment it to find its relevance with IND-CPA, IND-CCA1, IND-...
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Showing that CPA encryption schemes cannot preserve the length of a message

I am self studying "A Graduate Course in Applied Cryptography" by Boneh-Shoup. I am stuck on the following problem. Let $\mathcal{E}$ be be an encryption scheme where messages and ...
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Solving DDH from an ElGamal adversary

Suppose an adversary wins IND-CPA against ElGamal, They're given public key $h=g^x$, Give a pair of messages $m = [m0,m1]$, Get back ciphertext $(a,b) = (g^r, g^{xr} \cdot g^{m[b]})$, from which ...
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Protecting against CPA for AES128/ECB by using padding

I was going to encrypt with AES, But I noticed ECB mode is not safe for CPA. So I thought about preventing CPA by padding input text to a multiple of 16 bytes. First, perform custom padding system: ...
5 votes
2 answers
236 views

In a PGP like Application , would compress and encrypt leak information

would compress and encrypt on data comromise security for PGP like application, would it be secure for use in Encrypted Messaging? I have heard its safe for data at rest encryption , i have heard ...
2 votes
1 answer
112 views

Why do the game-hops in Kyber and related papers contain 2 steps at a time?

In the Kyber paper in section 3 about the Kyber IND-CPA Encryption there is a proof by sequence of games containing three games. I understand that in the first game hop the M-LWE advantage is used to ...
1 vote
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138 views

Probability of getting a collision using chosen plaintext attacks

For university I am doing a piece of coursework right now. This question is focusing on CPA and collisions using CPA. Question: I have attempted to answer part 3, but am not very confident in the ...
2 votes
1 answer
799 views

Chosen Plain text attack [closed]

I have a course work for university, the question is: Consider a symmetric encryption scheme with its encryption operation written as $$C = E(K, R||P)$$ where $E$ is a block cipher encryption ...
2 votes
1 answer
450 views

CPA secure to CCA secure encryption

Can we use a MAC to transform a CPA secure encryption scheme into CCA secure one?
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Semantic Security of Modified Textbook/Raw RSA

Here's a modification of the textbook RSA scheme, in an attempt to achieve semantic security. Key generation: chooses public key $pk = (N,e)$ and secret key $sk = d$ as in any RSA-based encryption ...
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1 answer
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Can we force a chosen ciphertext to be decrypted to a chosen plaintext while controlling only $e(=3)$ in RSA? [duplicate]

I have bumped into this challenge from a well known CTF site. I don't want to make a reference to it because I don't want this to be a hint for anyone. And also to avoid giving out the source code of ...
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CBC Operation with Known IV [duplicate]

I am trying to understand how XOR operation can lead us to know the plaintext if we can predict the iv using the chosen plaintext attack game.
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Difference between the eavesdropping and the CPA experiment in the case of multiple messages

while reading Introduction to modern cryptography by Katz and Lindell, I came across these two experiments: For me, they look like it's the same thing. Can you please explain me the difference? ...
2 votes
1 answer
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what is IV predictable?

What is IV predictable in the CBC mode? We only need one IV once for CBC mode, does the IV-predictable means we know the initialization vector before we start a CBC mode? Or it means we know each ...
2 votes
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Regev PKE not CPA secure for specific $A$?

I encountered notes stating that, for certain fixed $A$, such as $A \in M_{n\log(q)\times n}$ as follows: \begin{bmatrix} 1 & 0 & 0 &\dots\\ 2 & 0 & 0 &\dots\\ 4 & 0 & ...
3 votes
1 answer
763 views

AES with a different implementation of byte substitution step

I have tried to answer this question for quite some time now. But a complete intuitive understanding still eludes me: suggested a new implementation for AES: The byte substitution step will be ...
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Can the proof only be the stateful?

Let SE be stateful encryption. Then, it is well known that the oracle for the security proof becomes stateful too. (We only consider the IND-CPA security game.) On the other hand, assume that E is ...
4 votes
1 answer
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Is this really the correct way to do an IND-CPA test?

In a description of IND-CPA (indistinguishability under the chosen plaintext attack), I have been reading the following, simple test: The adversary can generate as many messages as he wants. Then, ...
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Is AES with random bitstring IND-CPA?

Let $E:\{0,1\}^{128}\times\{0,1\}^{128}\to\{0,1\}^{128}$ be the AES encryption and $R\gets\{0,1\}^{128}$ uniform random bitstring. Would $E'(K,P):=R\mathbin\|E(K,P)\mathbin\|E(K,R\oplus P)$ be IND-CPA?...
2 votes
1 answer
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Security definition for IND-CPA of public key encryption

In the security game between the challenger and the adversary for the security definition of public key encryption, the challenger creates and gives the public key pk to the adversary. The adversary ...
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Practical necessity of semantic security under chosen plain text attack (CPA) in CBC mode

I was not able to understand why we practically need a CPA security in Cipher Block Chaining, which insist on having a random IV. Let's say if the encryption is not CPA secure, i.e. the adversary can ...
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Show that block cipher with CTR is a good PRNG

Assume that block cipher scheme $(KeyGen, Enc, Dec)$ is CPA-secure. Show that $CTR_{Enc_k(0^n)}$ is a good PRNG. Counter mode works as follows: $Y_i = Enc_k(IV + f(i))$ $C_i = Y_i \bigoplus P_i = Y_i \...
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How can we measure the security levels of post-quantum cryptographic algorithms? Is there a standard way of this measurement?

How do we measure the security levels of Post Quantum Cryptographic algorithms such as: NTRU Prime, Saber, Kyber,...that are submited to NIST PQC Standardization Process(Competition) in general? I ...
5 votes
1 answer
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The essential differences between IND-CCA1 and IND-CCA2?

For some encryption scheme $(\mathcal{E}, \mathcal{D})$: In the definition of IND-CCA, the adversary $\mathcal{A}$ can access the decryption oracle $\mathcal{D}$. The deep reason of this setting is to ...
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119 views

Do attackers need to know what ciphers are being used when conducting ciphertext-only, known plaintext, chosen plaintext, and chosen ciphertext attack

I am trying to understand the basic ingredients needed to conduct various types of cryptanalytical attacks. For instance, I understand that for Ciphertext-Only Attacks (COA) an attacker only has ...
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Adversaries knowledge of the encryption algorithms internal workings in CPA

If I want to prove that an encryption algorithm is CPA secure (or that it isn't), then how much am I allowed to assume the attacker to know about the internal workings of the algorithm? Do I need to ...
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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 ...
1 vote
1 answer
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OFB mode discussion

In OFB mode, I understand that a bit flip in $c_i$ for $i > 0$ only causes a bit flip in message block $m_i$. However, how is it possible for a bit flip in $c_0$ (i.e, in IV) will result in all the ...
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Is it possible to perform CPA attack against CBC changing IV by last ciphertext block?

I was trying to do a simple CPA attack against this scheme, to understand better the concept. Instead of using a new 𝐼𝑉 each time, we decide to use the last block of the previous ciphertext as an ...
2 votes
1 answer
239 views

Does Enc and Dec need to be a pseudo-random function for a scheme to be CPA secure?

I am currently going through past finals' questions as exercises for my exam and there are no solutions provided. The question I am currently doing is: Let ∏ = (Enc, Dec, Gen) be a CPA-secure ...
1 vote
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Prove CPA security [closed]

Assume $(Gen, Enc, Dec)$ is a public-key encryption scheme with message space M that is CPA-secure. Prove that the encryption scheme $(Gen^2, Enc^2, Dec^2)$ is CPA-secure. $Gen^2=(pk_0, sk_0) \...
32 votes
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What do the signature security abbreviations like EUF-CMA mean?

From time to time, one stumbles across formal security definitions. This includes security definitions for signature schemes. The most common ones are the *UF-* ...
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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 would be an O(n) time adversary for this kind of scheme

Suppose we have a block cipher $$E:\{0,1\}^k \text{ x } \{0,1\}^{2k} \rightarrow \{0,1\}^{2n} \quad \text{ with } \quad k,n\geq128$$ K is the key generation algorithm that returns a random k-bit key. ...
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Possible for a different key to decrypt properly?

I am thinking about CPA-security for symmetric encryption. So $A$ gets access to an encryption oracle, and it can keep asking queries (training phase). In this training phase, he asks $m_i$ and ...

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