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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A question about "attacks on MAC key space"
At page 336 in "Handbook of Applied Cryptography - Menezes", I see the sentence
For $n$-bit MAC with $t$-bit key space this requires $2^t$ MAC operations, after which one expects $1+2^{(t-n)...
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A RSA crypto CTF chanllenge with known private key ,cipher text and modules factor n
Here is a CTF chanllege about RSA(The competition has ended for serval hours),and here is the critical encryption code:
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How by changing ciphertext can the attacker know the length of the padding in POODLE attack
I try to understand the POODLE attack. I read that the attacker try to change one byte of the last block of the ciphertext. Becasue the last block is only have padding and the last byte is the padding ...
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Can an attacker impersonate any party in a modified Needham-Schroeder protocol?
We are doing symmetric Needham-Schroeder protocol in university. We were provided a modified protocol and needed to answer the following questions.
Original protocol:
Modified protocol:
Task
a) An ...
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CTF - AES CBC Challenge
I am currently facing a CTF training challenge which is based on AES CBC and exploiting the fact that the IV is known.
This is the challenge:
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how 0/n split prevent BEAST attack against TLS?
I read that to mitigrate BEAST attack openSSL tried to inject empty TLS record before each real TLS record. and by doing that there is no opportunity to execute an attack but i dont understand way?
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RSA Oracle - CTF
I am trying to solve a challenge regarding a RSA oracle which allows me to encrypt/decrypt any plaintext/ciphertext I want, but there are a few checks that I have to bypass, and my goal is to decrypt ...
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CTF - AES Padding Oracle
I am trying to solve another CTF challenge. The challenge consists in trying to exploit an oracle that decrypts any hex text we send (see code below). I am kind of stuck on this one as this is not a ...
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CTF - DES Challenge
I am trying to solve a CTF challenge based on DES. I attached the code of the challenge to the question. So far I have noticed that the otp used for the encryption is the same for the entire session, ...
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AES CBC - Find IV (CTF)
I am currently trying to solve a training challenge based on AES with CBC.
This is the infos I'm given:
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The specific nature of a 51% attack - in relation to a multiple choice question that I was marked as incorrect on
In a course I'm taking the following question came up:
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Use of PRNG's vulnerability for attacking a cryptographic system
I am investigating a PRNG which fails some of NIST tests (FFT, ApproximateEntropy and Serial). If such a generator is used in cryptography for any purpose, what consequences can it has? How this ...
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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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Leaking key when adding small order point
I was (trying to) read the following paper: https://eprint.iacr.org/2015/673.pdf
Page 2 says:
A related attack is to replace a point P with P + T, where T lies in a small
subgroup. If the user ...
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Solve congruent equation likes N = p*q c1 = (2*p + 3*q)**e1 mod N c2 = (5*p + 7*q)**e2 mod N
Here is a CTF crypto challenge likes(its write up is public on https://ctftime.org/writeup/15438):
$$N = p*q\\
c1 = (2*p + 3*q)^{e_{1}} mod N\\
c2 = (5*p + 7*q)^{e_{2}} mod N$$
After i transform these:...
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Predicting compromised OpenSSL 3.0 DRBG
The OpenSSL 3.0 rand function's DRBG uses the getrandom() system call to get 48 bytes of secure entropy from the kernel. It also ...
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Best practices on implementing a password manager
I'm a dev new to security and cryptography.
I'm writing a password manager and Time-based OTP combo in dart/flutter to use in multiple devices and platform for fun and use it personally for real. I ...
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The second moment and fourth moment of $\mathcal{P}(V)$?
Backgroud: I am reading the paper "Learning a Parallelepiped: Cryptanalysis of GGH and NTRU Signatures". (here is the link). And I got stuck in understanding the computation of moment.
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Double Ratchet Algorithm: Active Man in the Middle Attack without Root-Key or Ratchet-Key
I am currently studying the Double Ratchet Algorithm from Signal (link to the documentation). I stepped over a security issue which I was not able to find that it was mentioned anywhere.
I assume that ...
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Are Veracrypt, XTS and flash memory attacks feasible?
Some time ago I had a question here about WinRAR security and probably the most common recommendation I came across was to use Veracrypt.
Veracrypt uses XTS, but according to others, XTS encryption is ...
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How can I extend the RSA LSB Oracle attack, to n-LSBs?
Suppose we have an RSA-Oracle that can encrypt and decrypt our input. The the decryption output is equal to: $ (C^d \mod N) \mod 2^n $.
How can I extend the LSB oracle attack, using the using ...
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Cost of attacking textbook RSA signature of $b$-bit hash of the message
A signature system applies textbook RSA to a $b$-bit hash of the message. What's the cost (preferably, as CPU time on common hardware) of existential forgery assuming known signature of $r$ random ...
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Crack RSA with $e$ and $d$?
Is it possible to decipher a ciphertext, in RSA with small primes (two 128-bit factors) when we only have ciphertext $c$, private exponent $d$ and public exponent $e=65537$ to crack it? I try hard on ...
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Stuck on a cryptanalytical research project [closed]
This is not a technical question, but rather it seeks advice on what to do if cryptanalytical research goes wrong.
I've discovered a new attack that works great in theory, but in practice, it fails.
I ...
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Is it insecure to sign the value 0 with ElGamal?
Is it insecure to sign the plaintext 0 with ElGamal signature algorithm? Can this leak the private key, give the possibility to forge other signatures or does provide any other attack vector?
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Decentralized identity, how to prevent duplicate uses of the same identity
In a system that relies on decentralized identity, is it possible to prevent duplicate uses of the same real-life identity, i.e. same real-life person creates more than one user entity (each is ...
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Breaking Ed25519 Discrete Logarithm with Degenerate Curve Attack
Following this question ed25519 attacks and also this paper on degenerate curve attacks https://eprint.iacr.org/2015/1233.pdf, I tried to implement my own attack: Given a point (0, y) and a scalar (k),...
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Significance of theoretical weaknesses?
What is the significance of theoretical weaknesses?
Any real life incident where a theoretical weakness was ignored and later it compromised the system? Whats the dividing line between theoretical and ...
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How does the security of AES change if we allow multiple uses in a row? How does it change if we limit the key space? And introduce a filter function?
$$f_0 = A$$
$$f_{n+1}=AES(f_n,k_n)$$
$$f_i = B$$
For given 128-bit values $A, B$ we want to find a chain of suitable 128-bit keys $k_1$ to $k_i$.
The total length $i$ is undetermined. Every valid key ...
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Does privacy-preserving linear (logistic ) regression necessary?
The linear regression is simply $y=Wx+b$, where the server holds $W$ and $b$ and the client holds $x$. The private linear regression means the client sends encrypted $x$ to the server, and receives ...
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Non-uniform plaintext distribution -- AES attack
Are their any attacks on AES (any mode) in which before the attack is mounted, the adversary is aware that the input distribution is non-uniform?
Most of the work assume that the plaintext is ...
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Can the BB'06 attack to PKCS padding be generalized?
As Bleichenbacher states, a wrong implementation of the parsing function of the padding could lead to a signature forgery attack when using a low exponent.
In the provided example he assumes the ...
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Trying to understand the 2nd subgroup in the Weil Pairing used for the MOV attack
EDIT: The bounty is actually to draw more attention. I accidentally chose the wrong reason.
$E$ – Elliptic Curve over finite field $\mathbb F_p$.
Let $k$ be the embedding degree of the Curve with ...
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Generic attack on Hmac sha256
Is there any generic attack on HMAC SHA 256 ?
I am currently reading up on attacks on Hmac sha-256. However most of the papers I've found are about side channel attacks such as Differential power ...
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How to solve a system of modular equations with exponential difference
I`m solving one crypto problem on rsa.
p^e - q^e = C1 (mod n)
(p-q)^e = C2 (mod n)
n = p*q*r; p,q,r are prime numbers
e = 2 * 65537
We have e, n, C1, C2.
It's ...
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Do we always choose a generator of prime order for ECDH? If yes, then why?
I am looking at the description of Weil MOV Attack from the Vanstone, Menezes Book (Guide to Elliptic Curve Cryptograpy)
Suppose now that the prime order $n$ of $P \in E(F_q)$ satisfies
$gcd(n,q) = 1$...
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Various attacks on cipher-images & tools, especially stream-cipher?
What kind of attack of image encryption that exist out there, especially if the cipherimage was created using secure stream-cipher like Salsa20 (256 key) or ChaCha20 (256 key)?
From https://cr.yp.to/...
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CTF AES attack (ECB, CBC, CFB, OFB, CTR) [closed]
Hi I'm trying to understand the logic behind a CTF challenge basically we are given a program which encrypt some data, we have the following options:
Select encryption mode (EBC, CBC, CTR, OFB, CFB)
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Dividing an encrypted file is secure against classical or quantum
I'm very new to cryptography and this may sound so foolish. Often I read quantum computers will brute force keys. Let's assume this is true (does it depend on key length? or on an algorithm? I don't ...
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How to factorize RSA modulus while given two Public Exponents and the difference between two Private Exponents?
The RSA modulus is the product of two $2048$-bit primes.
And the two Public Exponents are both $16$-bit.
I also got the difference between two Private Exponents $\left | d_1-d_2 \right |.$
Is there ...
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Clarification on expected number of plaintext, ciphertext pairs needed to identify AES keys
I'm aware of the many questions on this topic, but I'm still not sure what went wrong with my reasoning here below.
I'm assuming use of AES with key size $2^{256}$ and messages of size $2^{128}$, ...
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Ed25519 - Is there a reason to not trust the signer - i.e. can there be a dishonest signer?
Ed25519 uses a composite order Elliptic Curve but works in the prime order subgroup of the main group. The signing operation should be using a generator/Base Point from the prime order subgroup.
This ...
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Is there any attack from a malicious adversary against semi-honest MPC protocol?
For semi honest MPC protocols, adding some errors can easily make the output wrong.
I wonder beyond that, can malicious adversaries make some attacks against semi-honest protocols to obtain private ...
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Trying out the small subgroup attack on a group of non-prime order using a simple additive group instead of an Elliptic Curve Group?
This is the attack I am talking about - Why are the lower 3 bits of curve25519/ed25519 secret keys cleared during creation?
An elliptic curve group of order $8p$ where $p$ is a prime.
Let $G$ be the ...
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Is it possible to break SHA-256 with hash tables?
If x is the value you're hashing, $t(x)$ is the hash of that value, and $t(x+s)$ is that value with salt, then hashed. You could create a hash table where the key is $t(x+s)$, the value is $x+s$, and ...
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How to break RSA when $q = e^{-1} \bmod p$?
This problem was posed during a recently ended contest.
I am given a value for n and e. e = 65537. n is a 2048-bit number.
p is a 1024-bit prime number.
q is the modular inverse of e mod p. That's ...
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Encrypting two messages with the same content with different keys/IVs still secure if attacker knows they are the same?
I want to store two ciphertexts inside the same file.
The data stored in each ciphertext is the same (except for padding), but the data was encrypted with different keys and IVs. Both ciphertexts were ...
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Key Committing AES-GCM
There is a handful of attacks against AEAD, and GCM in particular, which demonstrates that it is feasible for an attacker $\mathcal{A}$ to obtain a ciphertext $C$ which encrypts to multiple key/...
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An adversary does know the block cipher key and some bits of the message and the cipher (same length as key). Does this help him to find pairs? (ECB)
Given a $N$-bit AES or similar block cipher with his $N$-bit key $k$.
We can encrypt a $N$-bit message $m$ to a $N$-bit cipher $c$ and decrypt it again.
$$E(m,k) = c$$
$$D(c,k) = m$$
An adversary does ...
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