# Tagged Questions

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

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### What are possible caveats when generating a group for use as parameters for Diffie-Hellman key exchange?

As reusing a widely used group for Diffie-Hellman key exchanges might lead to far easier third-party key discovery through precomputation for that specific group, I would like to know what can ...
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### How much information is revealed?

Let Alice pick $v_{1}$,$m_{1}$$\in \mathbb{P} and Bob v_{2},m_{2}$$\in \mathbb{P}$. Now Alice computes $v_{1}$$^{m_{1}} and so Bob does v_{2}$$^{m_{2}}$ Then Alice exchanges $v_{1}$$^{m_{1}}$ ...
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### Best attack on double DES followed by XOR with third key

What is the best attack here? $E_k(m)=DES_{k1}(DES_{k2}(m)) \oplus k3$
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### How does the Flickr length extension attack work?

I am currently reading about the length extension attack on Flickr (pdf) but not understanding what's going on there. ...
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### Cycling hashing in PBKDF's and their limitations in strength?

I hope this question isn't too similar to one that's already been asked. I also want to point out I know part the answer already, AFAIK: I just want some expert input on this. Let's model a hash that ...
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### Semaev summation polynomials

I am little confused how this attack works. We have the points $P, Q$ such that $Q = nP$. We let $u_{1}$and $u_{2}$ such that $R(x,y)=u_{1}P+u_{2}Q$. Then if we find the solution $x_1,...,x_n$ of the ...
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### Practical attack on an impractical implementation of Rijndael (AES?)

My understanding of the cryptanalysis of AES, is based on what I've read on Wikipedia and my intuition, I have nothing but self-taught education on the subject. So I want you to imagine: I have only ...
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### Does frequent key change weaken encryption?

Following scenario: We are using OTR for communication between Alice and Bob which means after each successfull message exchange a re-keying happens for both parties leading to new AES-keys for the ...
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### Do we have anyway of knowing if avoiding self-permutation in RC4 makes it any stronger?

OK, so lots of questions get asked about RC4 but I want to make mine specific and hopefully answerable. RC4 Design as I'm sure anyone reading this knows, uses self permutation in stage 1 of the KSA. ...
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### Theoretical attacks on Block ciphers like Biclique attack on AES [closed]

Does anyone know theoretical attacks on block ciphers like AES, DES, etc? No matter what the ciphers are; I just want to get familiar with some theoretical attacks on block ciphers; Attacks like ...
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### RSA Attack, known public key and ciphertext. Can decrypt the message m?

I'm stuck with this problem: Assume Alice and Bob are using RSA to communicate each other. Given Bob's public key $(217,7)$, Alice send the ciphertext $c=53$ to Bob through RSA encryption. Now, Eve ...
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### HMQV and MQV group representation attack

Currently reading Krawczyk's HMQV paper https://eprint.iacr.org/2005/176 and trying to follow what he says about the group respetnation attack on MQV: It says that for any group of prime order $q$, ...
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### Cost of attack on DSA with attack on DLP

Are there any (recent) estimates of cost of attack on DSA by solving the discrete logarithm? I'm especially interested in attacks that use Pollard's rho algorithm. Are there any optimized ...
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### Attack on textbook RSA with small messages

We learned in class that if Alice wants to send a small message $m$, such that $m < N^{\frac{1}{e}}$, so we can break the system using $e$-th square of $m^e$. But what if Alice knows this flaw and ...
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### What happens when domain parameter $q$ in DSA is not prime number

Suppose that attacker creates domain parameters $(p,q,g)$ where $q$ is not prime number (but $q$ still divides $p-1$) and that signer generates new key pair using these domain parameters. How exactly ...
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### Are there any long term RC4 bias based exploits?

The RC4 cipher possibly exhibits low level bias in it's long run PRNG keystream. I'm specifically excluding short term bias attacks which I'm defining as outputs < 1024 bytes. Are there any real ...
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### How long does it take to decrypt an RSA/ECB encrypted message?

I am working on assignment where I have specific scenarios and I am little bit struggling with this one. Alice sends and email to Bob.For this e-mail, she uses the following method of encryption: ...
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### One Time Pads and reuse of the decrypted message?

How is it insecure when Alice encrypts a message with a One-Time-Pad to Bob, and Bob then uses the decrypted message from Alice as the next One-Time-Pad? For example: Alice sends Bob a message ...
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### RSA 1024 bit forge a new matching signature from a chosen message

I have an RSA signature scheme with 1024-bit key where I know the following: Public modulus $N$ Public exponent $e=3$ A lot of signatures Summary: To compute a signature, an MD5 hash is ...
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### ephemeral-ephemeral ECDH and MITM attack [duplicate]

ephemeral-ephemeral ECDH is it susceptible to MITM attack ? if yes, can you please give me a detailed answer ( Workflow that can be followed by the attacker) and how we can prevent this attack ...
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### Shared modulus attack on RSA

Consider a scenario in which a group of people use a common modulus $n$ in the textbook RSA crypto system, where $n=pq$, $p$ and $q$ are large distinct prime numbers. Is there any chance for an ...
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### Is there any practical attack to create a printable chosen prefix MD5 collision?

I would like to create two ASCII text messages with the same MD5. Is this possible? If not, is there a similar but less strict attack that could work? Or to rephrase my last question: what are the ...
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### ElGamal scheme attack when one message is known and ciphertexts are intercepted

Let $G$ be a finite cyclic group of order $p$ and let $pk = (g, h=g^a)$ and $sk=(g,a)$ be Bob's ElGamal public/secret key pair in $G$. To encrypt a message $m$, a random number $r$ is selected and a ...
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### Voting protocol - How many dishonest tallying centres can the protocol cope with?

We have the voting protocol that is described here. I am looking at the following exercise: Determine how many dishonest tallying centres the protocol can cope with before the following properties ...
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### Correlation attack on Lagged Fibonacci Generator

Say that one uses a known LFG (2 known lags $a$ & $b$, known inner state length $N$, known modulo $m$) to hide some text with a basic à la Vigenère cipher: $C_i=(P_i + {PRNG}_i) \bmod 26$. Say ...
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### security in format preserving encryption

In format preserving encryption an attacker knows the format and data type of the plain text. The format of the plain text and cipher text are same. usually cipher text are unreadable form. then how ...
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### OTP - Reusing key IS SECURE! PROVED! [duplicate]

EDIT: This is not a duplicate, in this I have explained WHY this should be secure and not if it is secure, so please stop down-vote if you haven't read everything. I have questioned before about my ...
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### How multiplicative property of RSA can be exploited

It is known that RSA has the multiplicative property $E(m_1) \cdot E(m_2) = E(m_1 \cdot m_2)$ because $m_1^e \cdot m_2^e = (m_1 \cdot m_2)^e \pmod n$ But how can this property be used to break ...
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### Identification by RSA

Suppose we have identification scheme as such: Bob takes random $r$, and encrypts it with Alices public RSA key $(n,e)$, sends $r^e$ to Alice. Alice decrypts $r$ and sends it back to Bob, becomes ...
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### Key Confirmation Attack [Key Distribution Center (KDC)]

Okay this is how I understood it according to this: Alice $A$ establishes a connection to $KDC$ and prepares for session key Exchange $k_{ses}$ $A$ encrypts the request with her key $k_A(A, B)$ ...
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### Statistical saturation attack on block ciphers

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is A Statistical Saturation Attack against the Block Cipher ...
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Server A produces signed messages on request. You can tell server A what type of RSA padding to use in the signature (either RSASSA-PKCS1v1_5 or RSASSA-PSS). Either way, the message will be signed ...
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### Fault encryption attack on RSA [duplicate]

How to recover plaintext, if we know public key(RSA), correctly encrypted text and text encrypted with faulty public key, where one bit is changed?
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### When is a cipher considered broken?

We've all read how some people claim AES is broken because there was supposedly a way to get the plain text from a cipher text faster than brute-force. But is this the definition? Is a cipher broken ...
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### AES as Integer Program

I read that AES can be broken by solving a Linear Integer Program (IP). From a given encrypted text, how can one construct this IP? How big is the system?
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### What aspect of elliptic curve encryption paradigms makes them especially susceptible to quantum based attack algorithms?

This was a statement made during a talk at today's DFN-CERT conference but unfortunately it wasn't explained further. Can anyone shed light on why elliptic curves are susceptible to quantum based ...
For variable-length messages, there are three different ways to generate the tags. $Tag = MAC_k(\Sigma_i m_i)$. $t_i = MAC_k(m_i)$ and $Tag = (t_1, ..., t_l)$. $t_i = MAC_k(i||m_i)$ ...