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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How to find the secret of a hashing algorithm? [closed]

I'm trying to do a CTF challenge where you have to find the secret of a hashing algorithm used to hash strings. You are given two strings and their corresponding hashes. There is also a hint to use a ...
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154 views

Is it possible to acquire the next keystream of Salsa20? (If you know part of the keystream)

When you know the Salsa20's keystream, how do you know the next keystream? In other words, can I infer the next keystream when I don't know the Key and IV of Salsa20, but the keystream is ...
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67 views

Which RSA keys are actually attacked In practice?

I understand that it is important to check how hard it is to attack RSA keys, but I assume at some key size attackers would not even try. Real attacks are carried out by exploiting mistakes in the ...
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Security of a custom AES-based attack model

Provided that: $AES(plaintext, key)$ is the standard AES block cipher for a 128 bits block. $\oplus$ is the standard XOR operation. $K$ is a constant 128 bits pseudorandom value, which is used as the ...
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Breaking vulnerable ElGamal encryption

In a ctf, I encountered this problem: ...
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May an adversary be fooled by a random-looking input that in reality is fixed?

I want to prove the EUF-CMA-security of a signature scheme. It is a variation on an established scheme, therefore I would ideally like to reduce the new-scheme-security to the old-scheme-security. the ...
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55 views

When attacking keyed algorithms, how is it known when the output is correct?

I've been reading a little bit about cryptanalysis, and I'm wondering how attacking algorithms that work on keys is performed. It's obvious how algorithms like MD5 are attacked, in pseudocode: ...
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97 views

Cracking RSA with small plaintext

I read a CTF writeup about cracking 4 primes RSA numbers from here: Given $p, q, r,$ and $p+q+r$ are prime numbers. The challenge encrypts the flag with a modulus $N=(p∗q∗r)∗(p+q+r)$ and gives the ...
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Code implementation of NTRU lattice attack

As mentioned here, BKZ is one way to break the NTRU lattice What is the most efficient attack on NTRU? I was wondering if there are any source codes which actually implemented this on the weaker NTRU ...
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The aftermath and considerations of the new record of 30750-Bit Binary Field Discrete Logarithm - 2020

Granger et al. recently published a paper about breaking a record for discrete logarithm on the Binary field Computation of a 30 750-Bit Binary Field Discrete Logarithm, Robert Granger and Thorsten ...
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207 views

What is the ChainOfFools/CurveBall Attack on ECDSA on Windows 10 CryptoAPI

What is the ChainOfFools/CurveBall Attack on ECDSA on Windows 10 CryptoAPI (Crypt32.dll) Could someone provide details?
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Understanding birthday attacks on 256 bit hashing and 512 bit hashing [duplicate]

Does it make a difference against birthday attacks if the algorithm that I am using is 512 bit hashing?
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Check linear and differential cryptanalysis of a new block cipher [duplicate]

I am developing a new block cipher primitive. I would like to test its robustness against attacks of linear and differential cryptanalysis. Is there a tool to evaluate this using only plaintext and ...
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Iterations of pollards kangaroo attack on elliptic curves

I want to understand the Pollard kangaroo attack on elliptic curves. I found this Pollard's kangaroo attack on Elliptic Curve Groups Q/A pretty helpful, but not complete. The posts provides a pretty ...
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MOV-attack on ecc: Time complexity and example

There already is this pretty big post about the MOV-attack. It states, that the discrete logarithm problem on elliptic curves can be transformed to a discrete logarithm problem over a finite field. ...
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Pollard Rho Optimization

One of the most important attacks on Elliptic Curve cryptography is Pollard's Rho method. The effect on security can be seen on SafeCurves. This attack is pretty old and there has been a bunch of ...
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Curve25519 Attacks and Security

Curve25519 is a pretty secure way to exchange a key. In the original Paper and on SafeCurves a lot of attacks and security aspects are mentioned: Attacks: Brute force: This one is theoretically ...
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Pohlig Hellman and small subgroup attacks

While studying Curve25519 I read about the small subgroup attack in chapter 3. So far i know, that you need a point with a small subgroup to do such an attack. Curve25519 has a basepoint with prime ...
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56 views

Proof by reduction for for digital signature scheme $S_2((sk_0,sk_1), m):=(S(sk_0,m), S(sk_1,m))$

Let $(Gen,S,V)$ be a secure signature scheme (existentially unforgeable under a chosen message attack) with message space${\{0,1\}}^n$. Generate two signing/verification key ...
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Adversary for attack on one variant of ElGamal

I came by the following question: Consider the following variant of ElGamal encryption. Let $p= 2q+ 1$, let $G$ be the group of squares modulo $p$ so $G$ is a subgroup of $Z_p^*$ of order $q$, and ...
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Which cryptographic attacks can AES 128-bit withstand?

Most of the articles and forums are stating that the Brute Force attack can test the strength of an encryption algorithm. However, I am looking for those attacks that AES can withstand for the testing ...
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How to solve high exponent attack for same message in RSA

I know Håstad's broadcast attack when e = 3, but what if e = getPrime(randint(350))?
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Can we defends against RSA Timing attack when we use java.math.BigInteger.modPow?

I think that Montgomery and sliding window techniques are applied within java.math.BigInteger.modPow (and more specifically ...
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Attack against $\operatorname{DESA}_{k,k_1} (x) \underset{\text{def}}= \operatorname{DES}_k(x)⊕k_1$

Problem 5.16 in this pdf, from the book: Christof Paar, Jan Pelzl, Understanding Cryptography 5.16. This is your chance to break a cryptosystem. As we know by now, cryptography is a tricky business. ...
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Why does the Bombe not consider the Ringstellung when determining stecker pairs?

I am doing research on the Bombe Machine, and I read that when the Bombe tries to find the stecker (plugboard) partners of letters, it only needs to use the rotor types and arrangement and also the "...
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Why are the indicator drums of a Bombe machine labelled from A to Z clockwise?

I am researching the Bombe Machine, having just completed a project on the Enigma Machine. I visited The Rutherford Journal - The Turing Bombe, and it claimed that The scales on the three ...
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723 views

Does having a hash of a password jeopardize the security of plaintext that was encrypted with that password?

A malicious actor wants to decrypt ciphertext that was encrypted with a password. He also possesses the hash (bcrypt/PBKDF2) of that password. Does he have any significant practical advantage in ...
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Is Joux's multicollision attack applicable for truncated SHA256?

Joux presents an elegant way to generate k-way multi-collisions with about as much effort as generating a single collision: https://www.iacr.org/archive/crypto2004/31520306/multicollisions.pdf ...
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XOR cipher with fixed key and known relation among plaintexts

I have three messages, each known to be XOR-encoded, with the same key used for each message of this XOR cipher. Encoded message 1: $e_1\,=\,00100111010$ Encoded message 2: $e_2\,=\,01001110110$ ...
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Secret sharing scheme WITHOUT Shamir Secrete Sharing

I am planning to use the following XOR scheme to divide a secret into only 2 shares (I do not want to use Shamir Secret Sharing for different reasons that are beyond the scope of this post). Here's ...
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Can MFA codes be predicted if you know what their values were at a previous time?

I use an app on my phone that, after scanning a QR code on a website, gives me 6-digit codes which change every 30 seconds. These codes can be used for Multi-Factor Authentication (MFA) on some ...
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Can someone explain the timing attack on RSA with Montgomery multiplication?

I have read several articles on the temporal attack against Montgomery multiplication to speedup RSA computations. However, I do not quite get the principle. I understood that there is a additional ...
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137 views

Breaking ciphertext using a quantum computer when the public key is not available

As we know about Shor's algorithm on quantum computers it is possible to crack RSA / ECC easily if we have enough qubits. Is it possible to crack RSA / ECC on a quantum computer if we only have ...
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157 views

Find prime $p$ such that $a^x\equiv b\pmod p$ for many $x\in[1,p)$

Given haphazard large integers $a$ and $b$ (like few thousand bits), can we efficiently find (and how) some integer triplet $(p,x,k)$ with $p$ a large prime (like a thousand bits) $a^x\equiv b\pmod p$...
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is there an effective attack against rsa by knowing the coefficient $u$ of the private key? [duplicate]

consider that an attacker has the public key $(n, e)$, where $n$ is $1024$ bits long, the $32$ largest bits of $p$ (where $n = p \cdot q$), as well as $u = p^{-1} \pmod q$. Is there an attack that ...
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Things already known to adversary before planning an attack

This is a very basic and practical question. Since I started reading cryptography recently, these things keep bugging me. What are the things that an adversary is usually aware of before planning an ...
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An insecure signature with message recovery, Dan Boneh

Someone can help me to resolve this questions from the book of Dan Boneh (University of Standford)
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Can Trivium ciphertext be decrypted by an adversary if the key is known, but the IV is not?

Suppose that the adversary is able to recover the key of Trivium cipher. But the associated IV is unknown to him. Will he be able to decrypt the ciphertexts without any complexity?
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RSA CTF Guidance

There's a great cryptography based ctf site that I've been going through. There's this one question around RSA that has been puzzling me for over a week now. Please don't tell me the answer, I'm only ...
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136 views

Short padding + known prefix RSA attack

Suppose I have a message in the form of: $m = \text{prefix} \mathbin\| \text{secret} \mathbin\| \text{padding}$ We have: $e=3$ $n$ $n_\text{padding}$ (length of the random padding, <$1/4$ of $m$)...
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AES 5-round Square attack, how to choose plaintexts?

I know that we need to generate a delta-set (with 256 plaintexts). During the attack, we guess 4 bytes of round 5 key and 1 bytes of round 4 key, which allows us the byte at one position of the state ...
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RSA calculate $d$ using Chinese Remainder Theorem with $d_p$, $d_q$ and $e$

Suppose for a RSA system I have the following variables given: modulus $n$, expononent $e$, $d_p$ and $d_q$Where, $d_p = d\bmod(p-1)$ and $d_q = d\bmod(q-1)$, Is it possible to find the private ...
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KRACK - Is all-zero key installed by the acces point?

I don't really understand (and I think it's not described in the official paper) how the encryption can work when the encryption key is set to 0 by a KRACK attack on a device using wpa_supplicant 2.4 ...
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65 views

Key at start or end of HMAC

In the book Information Security Principles and Practice, the author explains why HMAC should be used for integrity with a symmetric key with the message, to prevent a MiTM to replace both the message ...
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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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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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