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

learn more… | top users | synonyms (1)

11
votes
3answers
2k views

Is using a predictable IV with CFB mode safe or not?

While writing this answer, I noted that NIST SP 800-38A says that (emphasis mine): "For the CBC and CFB modes, the IVs must be unpredictable. In particular, for any given plaintext, it must not be ...
10
votes
3answers
2k views

In what way is XXTEA really vulnerable?

I'm looking at using the XXTEA algorithm to encrypt a small amount of data (say, less than 32KB) in the context of a software licensing algorithm. That is, we wish to make it difficult (not ...
10
votes
4answers
12k views

What is the difference between known-plaintext attack and chosen-plaintext attack?

I am very confused between the concept of known-plaintext attack and chosen-plaintext attack. It seems to me that these two are the same thing, but it definitely is not. Can anyone explain to me how ...
7
votes
1answer
377 views

Importance of block size in CBC mode

Why is it necessary to use a sufficiently long block size when implementing a CBC block cipher with a truly random initialization vector? In ECB mode it's easy to get information about the message if ...
6
votes
1answer
741 views

If RSA is only used to encrypt symmetric keys which are random, what's wrong with textbook RSA?

IND-CPA is used to protect against frequency analysis AFAICT. But if RSA is only used to encrypt symmetric keys (AFAICT) then what's wrong with using only textbook RSA because random keys are very ...
6
votes
1answer
2k views

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 ...
6
votes
2answers
6k views

How does a chosen plaintext attack on RSA work?

How can one run a chosen plaintext attack on RSA? If I can send some plaintexts and get the ciphertexts, how can I find a relation between them which helps me to crack another ciphertext?
6
votes
1answer
423 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
2answers
431 views

For public-key encryption, why does COA resistance imply CPA resistance?

My professor told me: If a public-key crypto-system is secure against ciphertext-only attacks, then it is also secure against a chosen-plaintext attack. Why is this true? Is there a proof that ...
6
votes
1answer
342 views

Message space in security definitions

What is the message space in the following example and how does a message space relate to a security definition? I mean, what difference does it make to such a definition if your message space is ...
6
votes
1answer
466 views

Proof that padded RSA is CPA-secure

I'm referring to page 383 of J. Katz and Y. Lindell's Introduction to Modern Cryptography. The book presents a padded RSA: ${\bf Key Generation:}$ same as Textbook RSA (given security parameter ...
5
votes
3answers
266 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 ...
5
votes
2answers
741 views

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 say if the encryption is not CPA secure i.e , the adversary can ...
5
votes
1answer
279 views

Attacking a hardware AES implementation if it leaks the intermediate round states

Let's say that we have a hardware AES implementation that, on request, will encrypt or decrypt a 16-byte block of data in ECB mode using a fixed key, but refuses to reveal its fixed key. In other ...
4
votes
2answers
773 views

What is the smallest plaintext/ciphertext size for an algorithm like?

When I read about a choosen plaintext attack, for example on AES the block size is always 128 bits, does it mean the attacker will only supply 128 bits data words as "plain text" ?
4
votes
3answers
70 views

Why is it good to split a CTR-mode counter into nonce and counter?

When discussing the CTR mode of block ciphers, Wikipedia says the following: Simply adding or XORing the nonce and counter into a single value would completely break the security under a ...
4
votes
1answer
613 views

Why is asymmetric encryption IND-CPA?

According to Wikipedia, the IND-CPA game is: The challenger generates a key pair PK, SK based on some security parameter k (e.g., a key size in bits), and publishes PK to the adversary. The ...
4
votes
2answers
160 views

Is the reduction from left-or-right IND-CPA to real-or-random IND-CPA tight?

A modern trend in cryptography consists of defining security as rigorously as possible, and then designing schemes which are secure according to those definitions. Proving security comes in the form ...
4
votes
1answer
102 views

Why are security against eavesdropping and security against CPA equivalent in the setting of public-key cryptography?

Why are security against eavesdropping and security against CPA equivalent in the setting of public-key cryptography?
4
votes
1answer
1k views

Figuring out key in hill cipher (chosen-plaintext attack)

I have been wondering what approach to take in order to figure out what key was used to encrypt a message using the hill cipher. I know it is possible to obtain it even if it were just a ...
3
votes
1answer
160 views

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, ...
3
votes
1answer
516 views

How much data can I encrypt with AES before I need to change the key in CBC mode?

In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted. It seems that ...
3
votes
2answers
1k views

Which categories of cipher are semantically secure under a chosen-plaintext attack?

I am trying to get my head around the circumstances under which a cipher is (or is not) semantically secure under a chosen-plaintext attack. I can't seem to find a good reference explaining this. I ...
3
votes
2answers
44 views

Chosen plain text and chosen ciphertext definitions clarification

I am trying to understand a concept of CPA and CCA attack and I cannot get one thing from the definition. If an attacker can execute a CPA attack for something like Send 100 dollars to attacker that ...
3
votes
1answer
543 views

Is CBC mode encryption vulnerable to a reordering attack?

I know that, if CBC-MAC is used in such a way that the tag is the concatenation of all the blocks' outputs (and not just the last one), it's insecure under CPA for the simplest case of 2 blocks ...
3
votes
1answer
39 views

Are deployed MACs IND-CPA?

There are a number of message authentication codes (MACs) used in practice. They are highly used in practice (f.ex. in TLS), but there's (at least) one application where they can't be used: Full Disk ...
3
votes
2answers
178 views

Security model for privacy-preserving aggregation scheme.

Suppose that $S=(E,D)$ is an additively homomorphic encryption scheme. Now I want to design a protocol $P$ such that given inputs $x_1,x_2,..,x_n$, the adversary $A$ (who can decrypt) can only learn ...
3
votes
1answer
460 views

ECB weakness and its exploitation

Lets say that I have a server which encrypts information at the server side using ECB mode with the blocksize of 16. If the server appends data that I control with a secret and sends back the ...
3
votes
0answers
28 views

Proving existence of an encryption scheme that has indistinghuishable multiple encryptions in the presence of an eavesdropper, but is not CPA-secure [duplicate]

I got stuck in trying to find a solution to the 3.7 exercise of the Katz-Lindell book. The exercise also assumes the existence of a pseudorandom function. The problem is that a multiple messages ...
2
votes
2answers
91 views

Are all stream-ciphers IND-CPA?

A stream cipher usually generates a stream of pseudo-random bits which get XORed with the plaintext to form the ciphertext. The stream is generated using a given IV / nonce and a secret key. CTR-mode ...
2
votes
2answers
656 views

Why is an Encrypt-and-MAC scheme with deterministic MAC not IND-CPA secure?

I'm preparing myself to exam, but I have a lot of troubles with rigorous proofs. It's the task from two-years ago exam. At the bottom I reminded one definition. Let $(Gen_E,Enc,Dec)$ be an ...
2
votes
1answer
173 views

Building a combined encryption scheme from two encryption schemes that's secure if at least on of them is secure

Any thoughts on how this can be done? Let $\Pi_1 = (\mathrm{Gen}_1, \mathrm{Enc}_1, \mathrm{Dec}_1)$ and $\Pi_2 = (\mathrm{Gen}_2, \mathrm{Enc}_2, \mathrm{Dec}_2)$ be two encryption schemes for ...
2
votes
1answer
48 views

On composition of encryption schemes

If we composed (as in function composition) multiple CPA-secure encryption schemes would the result also be CPA-secure?
2
votes
2answers
701 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?
2
votes
1answer
147 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
666 views

Ciphertext indistinguishability (IND-CPA) for symmetric cryptography

My understanding are these steps are necessary for symmetric cryptography to be IND-CPA: The adversary submits two distinct $M_0$, $M_1$ plain-texts to the challenger. The challenger selects one of ...
2
votes
1answer
87 views

Is MCrypt's 8-bit OFB mode secure?

I just stumbled across a Stack Overflow post which points out that the libmcrypt library (notably used in PHP) implements a somewhat unusual set of block cipher modes: it calls the usual CFB and OFB ...
2
votes
1answer
373 views

How can an encryption scheme be indistinguishable with multiple messages but vulnerable to an adaptive chosen-plaintext attack?

In order to be indistinguishable with respect to multiple messages, wouldn't the scheme need to be nondeterministic? How would this then possibly be vulnerable to an adaptive CPA?
2
votes
0answers
33 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 ...
2
votes
2answers
228 views

How to construct a variable length IND-CPA cipher from a fixed length one?

Suppose we can construct a secure (IND-CPA) encryption scheme for fixed length messages. I was wondering if there is a natural way to extend this construction for messages of variable length such that ...
1
vote
2answers
186 views

CPA distinguisher for matrix multiplication in GF(256) with randomized padding

What is the best CPA distinguisher for the function $F_k:\{0,1\}^{8n}\to\{0,1\}^{16n}$ described below? Let $E_k$ be a $2n\times2n$ matrix with elements in $GF(2^8)$, selected by generating bit ...
1
vote
1answer
168 views

Encrypting the same message using different schemes

$E_1$ and $E_2$ are IND-CPA secure encryption schemes. $E$ is defined as: $k_1,k_2 \leftarrow K_1 \times K_2$ . $E_{k_1,k_2}(m) \leftarrow E_{1,k_1}(m)||E_{2,k_2}(m)$. Hope the notations are in an ...
1
vote
2answers
491 views

Chosen Plaintext Attacks against an Affine Cipher

Assuming the ability to launch Chosen Plaintext Attacks (CPA), how many oracle calls an attacker needs to break the affine cipher? and how
1
vote
1answer
337 views

Definition of cryptographic advantage vs. probability of success

In game-based security definitions, like for example the one defining IND-CPA security, a given cryptosystem is said to be secure if any probabilistic polynomial time adversary has only a negligible ...
1
vote
2answers
168 views

Is One Time Pad considered Chosen-Plaintext Attack Secure?

If we're considering Chosen-Plaintext Attack setting, then the adversary has access to the Encryption Oracle right, and we know that OTP is only considered secure if we use the key only once. How ...
1
vote
1answer
33 views

Does failure of indistinguishability of encryptions imply lack of CPA-security?

Suppose we have an encryption scheme that fails the indistinguishability of encryptions test. That is, given messages $m_0, m_1$ and a ciphertext $c_b = Enc_k(m_b)$, where $b \in \{0, 1\}$, the ...
1
vote
1answer
59 views

Chosen-plaintext attack

Let F be a block cipher with 128-bit block length. Consider the following encryption scheme for 256-bit messages: to encrypt message $M=m_1∥m_2$ using key $k$ (where $|m_1|=|m_2|=128$), choose random ...
1
vote
1answer
175 views

What is difference among eavesdropper attack, multiple message attack and CPA attack?

What is/are difference(s) among eavesdropper attack, multiple message attack and CPA attack in context of symmetric key encryption? I only know that in eavesdropper attack there is an adversary ...
1
vote
1answer
375 views

Is my understanding of CPA indistinguishability experiment correct?

Following are steps to conduct Chosen Plaintext Attack (CPA) indistinguishibility experiment $PrivK_\mathcal{A,E}^{eav}(n)$. $\mathcal{A}$ is the adversary $\mathcal{E}$ is the encryption scheme ...
1
vote
1answer
47 views

Determine if encryption is safe against CPA

The following encryption scheme encrypts each block of length $n$ of the plaintext separately: $c_i = k_1 \oplus F(k_2 \oplus p_i)$ Where $F$ is a strong pseudo-random permutation (i.e. it is easy ...