Questions tagged [attack]

A cryptographic attack tries to theoretically and/or practically attack the security properties of a cipher and/or algorithm.

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If I allow arbitrary initial hash values for SHA-256, what would be the weaknesses?

The initial hash values (IV) of SHA256 are fixed. However, let's say I modify the SHA256 hash function to allow the attacker to use arbitrary IVs rather than fixed IVs. What kind of weaknesses would ...
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How is asymmetric cryptography safe if one of the keys is public?

Let's suppose $A$ is sending a message $m$ to $B$ using asymmetric cryptography. To guarantee the authenticity of a message, $A$ encrypts $m$ with $A$'s private key: $E(m, k_A) = m_A$. Then, $A$ ...
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averaging attack at Google's RAPPOR

I am reading the paper - "RAPPOR: Randomized Aggregatable Privacy-Preserving Ordinal Response". I don't understand how the averaging attack works. I searched for "average attack" but couldn't find a ...
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Is there a term for cryptographic attacks where you can verify decryption without assuming any known structure on the plaintext?

The questions https://security.stackexchange.com/questions/1696/if-someone-breaks-encryption-how-do-they-know-theyre-successful and https://security.stackexchange.com/questions/119887/how-to-know-if-a-...
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Primal and dual attack against NTRU

I am looking at the primal attack against schemes in the second round of the NIST Post-Quantum Standardization Project. The cost of primal attack usually comes from an estimate described in NewHope ...
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How to implement a Bit-Flipping attack on AES-CBC? [migrated]

Performing a task with root-me.org, as I understand from the attack statement, it is necessary to perform a byte shift to receive is_member = true After reading some instructions, I supplement the ...
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Another cut-and-paste attack on CBC mode

I am trying to see specific cases of attacks in the CBC mode, in particular, I am investigating some attacks such as Example of a cut-and-paste attack on CBC. Here I have posted something similar and ...
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How can an adversary use Mac(.) with a new key to compute a tag although this is not a query to Mac oracle?

I am having some troubles understanding the power of an adversary in attack against a MAC. In chapter 4 in Introduction of Modern Cryptography by Jonathan Katz and Yehuda Lindell, specifically in the ...
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How to find/determine the Polybius Square Key and Additive Key, if only the Cipher Text is known, in Nihilist Cipher?

I was solving a Crypto Challenge in a CTF, and I got a Nihilist Cipher Encoded String. Luckily the additive key was given as a hint, and for the polybius Square key, I kinda bruteforced or I just got ...
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Example of a cut-and-paste attack on CBC

I am reading the cryptography book by Stamp and there is a cut-and-paste attack on $ECB$ and this is easy to follow because $ECB$ is relatively simple, the problem is that I would like to see an ...
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Is it possible to perfom quadratic residues attack on elgamal cryptosystem?

I understand how this attack work on mental poker, but i am unable to see how can i apply it in Elgamal.
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How large a product out of 3 close-by factors need to be to avoid factorization?

For encryption a prime $P = 2 \cdot Q \cdot R \cdot S +1$ was used. An adversary want to solve the discrete log problem $m \equiv g^i \bmod P$. For this he want to use the Pholig-Hellmann algorithm. ...
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Given generator $g$ with prime order $k$ in $\bmod P$. Does increasing $P = 2 \cdot c \cdot k +1$ decrease security? Increasing $g$ increase security?

An adversary wants to find $a$ in $$m \equiv g^a \bmod P$$ He knows prime $P = 2 \cdot c \cdot k +1$ with it's primes $c,k$, value $m$ and $g$. And he also knows that $g$ only has an order of $k$, ...
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Attack on envelop mac

As I am reading the Paper: MDx-MAC and building fast MACs from hash functions I am having trouble understanding the Attack on the Envelope Method (4.3). In particular, I am not sure what chaining ...
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Is direct RSA encryption of AES keys secure?

I am wondering: If we take this scheme/procedure and each of it seems very secure (to me at least), is it truly secure or is there a vulnerability hidden in the process? This is the scheme: Bob has ...
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attacks and defenses on multi party Diffie-Hellman protocol

I tried to create a 4 partied Diffie-Hellman implementation for 4 people using its cyclic group property. However, I am not sure regarding those 2 issues, and would appreciate if you could elaborate ...
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Is double encryption really a bad idea? Are meet-in-the-middle attacks practical at all?

Meet-in-the-middle attacks are used to justify that attacks on ECC and double encryption will have complexity of $O(\sqrt{n})$ for ECC and $O(2^{n+1})$ for double encryption complexity instead of $O(n)...
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Security implications of not concatenating a 1 to the IV when using GCM

When using GCM, The padding string $0^{31}||1$ is appended to the IV. I know that this is done to avoid the repetition of $E_K(0^n)$ which is used as key for the GHash. But what would be the security ...
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Why is this attack complexity equal that exact number of bit operations?

in the this paper,section 3,autors attack hamsi-256. Im trying to make a parametrized version, so i need to understand how do they estimate the complexity of attack in bit operations,that reads as ...
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What is a multi-target attack?

What exactly is a multi-target attack? How does the attack work on different cryptographic schemes (block ciphers, hash functions, elliptic curves)? How can it be avoided?
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Any speed up methods for finding the index of a random value produced by the Inversive congruential generator?

The Inversive congruential generator produces random values with: $$x_{n+1} = a\cdot x_{n}^{-1} + b \mod P$$ (special case if $x_n=0$ -> $x_{n+1}=b$) starting with an initial value $x_0$ With well ...
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Is the 51% attack the only technical reason no country will ever adopt Bitcoin as a state currency? [closed]

Is the 51% attack the only technical reason no country will adopt Bitcoin as a state currency? Let's suppose a country with a GDP size less than 300 Billion USD adopt the Bitcoin as an official ...
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Attacking any one in many public keys

The problem of finding private key from public key is typically studied in the one-key setup: what's the expected cost of breaking one key (e.g. by factoring a public modulus, or solving a discrete ...
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What if an AES Whitebox 1024-bit (or larger key) is created? Does it increase complexity consistently?

Following the Chow et al paper and Muir's tutorial, I was able to implement the AES algorithm using tables embedding keys of 128, 192 and 256-bit sizes, later extended to 1024, 2048 and 4096-bit sizes....
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Why is the Domingo-Ferrer cryptosystem not used in practice?

The Domingo-Ferrer cryptosystem is a fully homomorphic cryptosystem. It works fast enough. I have only seen known-plaintext attacks. Is this a reason not to use it in practice? Or are there more ...
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Attacking Double DES (OFB Mode) given the initialization vector and not much else? [closed]

I'm given a Plaintext/Ciphertext pair, a Ciphertext to break, and the IV. One thing of note is that the same IV is used for both DES Encryptions, so $$C = E_{K_2}(E_{K_1}(P, IV), IV)$$ I have limited ...
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Complexity of AES key reconstruction from state 1 output and plaintext

Assuming that I am able to read the plaintext as well as the output of round 1 of an AES-128 encryption: Is it possible (if yes: how?) to obtain the RoundKey with a lower complexity than brute-force ...
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How to solve a simple case of a RLWE problem

I've been reading up on the Ring Learning with Errors problem and the proposed attacks, in relation to homomorphic encryption. Some of the literature has been quite difficult to understand - what I ...
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Does knowing that the exponent is in a certain range help solving discrete log?

given: $c=g^i \bmod P$ $g$ generator for group with group size $\varphi(P)$ $g,P,\varphi(P)$,c is known by the attacker He wants to know $i$. Now the attacker also knows $j,k$ with $j<i<k$ $k-j$...
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How do discrete logarithm with modulo a prime and a non-prime compare?

Let $c_N = g^i \mod N$ and $N=p \cdot q$ and $c_P = f^j \mod P$ and $P$ a prime We assume $N,P$ has the same bit-length. $P$ is the best type of prime you can choose (e.g. safe prime). $N$ is a ...
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On the security definitions [duplicate]

What are the differences between: perfect security information-theoretic security semantic security indistinguishability Are there any other definitions which are closely related? Do these security ...
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Why, in general, are proofs of possession (POPs) “clearly” not sufficient for substantiating the knowledge of secret key (KOSK) assumption?

In Ristenpart, Yilek - The Power of Proofs-of-Possession: Securing Multiparty Signatures against Rogue-Key Attacks they have the statement: In general, such proofs of possession (POPs) are clearly ...
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The opponent possesses a string of ciphertext and has access to the encryption machinery as black box. What is the name of this attack?

a quick example: The encryption function: y = E(x) = (13x + 9)(mod 27), when the letters A–Z are taken to be the numbers 0–25 and the Space (punctuation) is the number 26. The opponent has the ...
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1answer
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Randomization attack in voting protocols

I'm looking into coercion-resistant voting schemes and one of the key elements of coercion-resistance is resistance to randomization attacks. Juels et al. define a randomization attack as one where ...
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How “Resending Message Attack” works?

In the second chapter of Bruce Schneier's book Applied Cryptography, 2nd Edition, 1996, we find this in section 2.7.1 Resending the Message as a Receipt: ...
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Cipher Block Chaining - How do you change the plaintext of all blocks?

I've just read about CBC encryption and decryption and was wondering how you can change every plaintext block? Let's say you have an initialization vector, 3 ciphertext blocks, know each block-cipher-...
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How does the CBC Padding Oracle Attack work in General?

Right now we don't have a single generalized question asking how the CBC padding oracle attack works. We have e.g. this one and this one but the former is distracted by the concrete values being ...
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Probabilistic padding to avoid Håstad’s Broadcast Attack

Can someone explain why probabilistic padding like OAEP is particularly useful for avoiding Håstad’s Broadcast Attack? I don't really get the reason why.
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How safe is a prime with $P=2 \cdot Q \cdot R \cdot S \cdot t+1$ for discrete logarithm? How to enhance/compare?

To get some certain properties for my use case I need a prime $P$ which has the form: $P=2\cdot Q \cdot R \cdot S \cdot t+1$ with $Q,R,S,t$ primes as well. Why that form - Use case Together with ...
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Does a different exponent and base but same key help to resolve discrete logarithm?

E.g.: $k = N^a \mod P$ The attacker knows the prime $P$ and $N$, which is also a prime and (1.) prime root of $P$ or (2.) has a cycle size of $s$, so $1 = N^s \mod P$, (and for $\forall s'<s$, $...
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Unique numbers with $P^aQ^bR^c \mod N$ for each combination of $a,b,c$ possible? Would it be safer than separate form (like $T^a \mod P$ for each)?

Main question: Is the computation of $a,b,c$ in $P^aQ^bR^c \mod N$ (much) harder than in $T_p^a \mod P$, $T_q^b \mod Q$, $T_r^c \mod R$ ? (assuming the first form exists) $P^aQ^bR^c \mod N$ With $P^...
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How much more secure is $c = mg_1^r + g_2(g_1(g_1^r-1)/(g_1-1)) \mod p$ compared to just $c = mg_1^r \mod p$ (dis. log), all known but $r$?

To encode a message $m$ to a cipher $c$ you can use the only hard solvable problem of computing the discrete logarithm with a generator $g$ in base over a prime $p$. $c = mg_1^r$ mod p If an ...
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Example of product cipher more secure than its components

Can anyone give me an example of a product cipher which more secure than its components. Here is how the explanation should proceed. There is cipher C1 which can be attacked with the technique A1, ...
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How you find the unpadded short plaintext using RSA?

I am currently trying to understand how you find the unpadded short plaintext when you are using RSA. Please help with explaining the process and so I can understand this topic more.
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RSA encryption extremely large public exponent

I'm trying to do learn a bit about RSA by doing CTF's and now I am doing one problem probably more than 7 hours so I would really appreciate a hint here from an expert. I have an encrypted message $c$...
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Weak/Vulnerable AES Implementations [closed]

I'm trying to complete a cryptography CTF challenge in which I have a cipher I'm supposed to attain the original plaintext from. I have reason to believe it's been encrypted using AES, but I'm unaware ...
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Prove that a given signature scheme is secure under random message attacks

This is a follow up to my previous question. Consider the following signature scheme: $\operatorname{KeyGen} (1^k$) : On input of a security parameter $k$, choose a symmetric bilinear group with $...
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Prove that a signature scheme is RUF-NMA and not EUF-CMA

I am working on the following exercise: Now, assume the following signature scheme: $\operatorname{KeyGen} (1^k$) : On input of a security parameter $k$, choose a symmetric bilinear group with ...
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LWE: does using only a small subspace of the plaintext space influence the security of the encryption scheme?

Regarding LWE schemes where the encryption is performed this way: for $m \in \mathbb{Z}_t$, compute $c = LWE_{\mathbf{s}}^{t/q}(m) = \{ \mathbf{a}, \mathbf{a \cdot s} + m\cdot q/t + e\} \in \mathbb{Z}...
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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 ...

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