analysing cryptographic algorithms, potentially uncovering weaknesses in them (e.g. "breaking" them or casting doubts on their actual security)

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Applications of 3-collisions

I recently read Improved Generic Algorithms for 3-Collisions by Joux and Lucks (Asiacrypt 2009), available as http://eprint.iacr.org/2009/305.pdf. I was wondering about applications of this technique ...
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2answers
432 views

Given a message and signature, find a public key that makes the signature valid

Given a message $M$ and a signature $S$, is it feasible to find a RSA public key $(n,e)$ such that $S$ verifies as a valid signature on $M$ (using this public key)? What if we're given one public key ...
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6k views

Definition of Textbook RSA

What is the definition of Textbook RSA? What are some of the properties of textbook RSA? How does it differ from other RSAs?
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22k views

How does one attack a two-time pad (i.e. one time pad with key reuse)?

My question might appear the same as the question Taking advantage of one-time pad key reuse?, but actually I did read all the answers and none of them helped me with the details I need. I am new to ...
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2answers
362 views

Is it generally possible to employ brute force methods when the encryption scheme is not known? Why or why not?

Lets say you are presented with an encrypted string of bits or text, and no other information. What would be necessary before you could apply brute force methods to decrypting the string? How would a ...
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815 views

Finding CRC collisions for specific divisor

My current textbook (Information Security: Principles and Practice by Mark Stamp) discusses how to determine the CRC of data via long-division, using XOR instead of subtraction to determine the ...
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325 views

What do recent announcements about solving the DLP in $GF(2^{6120})$ mean for RSA

After just reading the post Do recent announcements about solving the DLP in $GF(2^{6120})$ apply to schemes proposed for cryptographic use? I was a bit confused. DSA, ElGamal and others are based on ...
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2answers
874 views

Why are bitwise rotations used in cryptography?

Any understanding I have of cryptography stops right around the cipher level. As such, I'm just curious as to why bit shifts and moreover circular bit shift are so prevalent in cryptography.
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276 views

How insecure in practice?

I am in attempt to understand relative insecurity of certain encryption schemes. Particularly of interest is DES and RC2. I know AES is better and should be used to encrypt. But practically, if ...
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1answer
505 views

Hill-cipher, disordered alphabet

I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model? In other words, instead of the ...
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1answer
905 views

Using an MD5 hash as a password

Suppose Alice is using a password prompt that only accepts up to 32 characters for any particular password. Memorization of long strings of random characters is not one of Alice's strengths, so she ...
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1answer
3k views

repeating-key xor and hamming distance

I read that to break repeating-key xor you can do the following: try a keysize $n$ and compute the hamming distance between the first $n$ bits of the encrypted string and the bits $n+1$ to $2n$ of the ...
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303 views

Security of tokenization of plain text conversations - cryptanalysis

I came across a marketing video here. They claim to perform AES encryption and tokenization of sensitive data, at the corporate gateway, before it leaves the company firewall destined for the public ...
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1answer
2k views

decryption many time pad [duplicate]

I have eleven ciphertexts that were encrypted with the same key (which I don't know). I want to decrypt the last ciphertext. I read similar question like Many time pad attack but I can't solve my ...
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1answer
65 views

Comprehension question on a signature protocol based on the RSA assumption

We have the following two-party protocol between Alice and Bob. Alice sends messages $m_1, m_2, \ldots \in_R \mathbb{Z}_n^*$ to Bob and Bob signs these values by calculating $v_1, v_2, \ldots \in_R ...
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1answer
98 views

What does “adaptively secure” mean?

In a paper it says "In the generic group model, the PRF is adaptively secure for inputs of $\mathbb{Z}_q^n$". Maybe a stupid question, but what does "adaptively secure" mean exactly?
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1answer
261 views

Understanding one-way hash functions construction

I understand the needs that lead to the development of cryptography and I am quite familiar with the uses we make of the cryptographic tools. But, as a programmer, I am conditioned to see them as ...
2
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1answer
101 views

Why constrain the message lengths in indistinguishability in the presence of an eavesdropper?

I need your help with a very basic concept in cryptography which I can't understand/prove on my own. I'm trying to prove and understand why, under "indistinguishability in the presence of an ...
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3answers
2k views

How practical are side-channel attacks and how much of a concern are they?

I see a lot of research in very sophisticated side-channel attacks on crypto systems. Most (but definitely not all) seem to follow a trend, namely, the crypto system does something very dumb like ...
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1answer
181 views

How to design a practical and secure MAC scheme?

I am sorry, but I need to introduce some concepts which are not directly related to cryptography to make myself clear, I hope I won't stun you with this ... (I'd rather explain it here than redirect ...
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1answer
1k views

What is the probability of breaking the AES algorithm?

I am doing a project which requires the encryption to be done using AES. Is it really possible (technically) to crack AES? If yes, please tell me: What is the probability of breaking AES? How ...
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3answers
321 views

Comparison: complexity measures vs. security

Assume that you want to compare several cryptographic primitives (say, encryption schemes), and choose one. You need to consider several complexity measures, such as the key length, encryption time, ...
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1answer
522 views

How would one crack a weak but unknown encryption protocol?

I asked a question on security.stackexchange, but was told it would be a better fit here: http://security.stackexchange.com/questions/32779/how-would-one-crack-a-weak-but-unknown-encryption-protocol ...
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1answer
325 views

Understanding a Blowfish cryptanalysis

I'm reading a cryptanalysis on Blowfish, and I've come across something that I don't quite get. Let's denote $$\delta = a \oplus a'$$ where a and a' are bytes that cause a collision in some S-box ...
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0answers
64 views

Strength of Combining Hash functions [duplicate]

If I combine two hash functions, what will the impact on the strength of the resulting function. If I combine in following way: H1*H2 (multiply) H1 + H2 (concat) H1 Xor H2 H1 (H2) EDIT: Lets say H1 ...
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1answer
433 views

Possibility of factorisation of rsa modulus due to vulnerability in java implementation code

Below is my implementation of the RSA algorithm. Actually I'm choosing the private key (d) instead of public key (e) and computing the public key.It is working fine but I want to know if this is a ...
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1answer
152 views

How to solve the reverse of an equation that uses MOD?

I've been tasked with reverse engineering an unknown crypto function. The function uses the following constants: $a=380951$: I noticed that this is a prime number $b=3182$: I noted that this is a ...
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1answer
310 views

Question about why RSA is hard to attack

I think I understand why RSA is hard to attack but I'd like to get clarification if I actually do. Assume there are two people, Alice and Bob, who are attempting to communicate privately but that we ...
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2answers
1k views

Cryptanalysis of Linear Feedback Shift Registers

It is well known that simple m-sequence linear feedback shift registers have a linear algebraic structure and therefore the generator seed can easily be deduced using the Berlekamp-Massey algorithm. ...
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1answer
136 views

ASCII Message in TV show? [closed]

So I was watching a popular series tonight (Person Of Interest) when suddenly during a frame change I noticed a very quick blue screen with writing. At the time I never thought anything of it as it ...
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1answer
296 views

Measuring Shannon's diffusion

Shannon's idea of diffusion is fundamental to cryptography. Besides being a descriptive idea, is there any work on measuring or expressing it? Saying something like "System A has more diffusion than ...
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1answer
227 views

What is 'security margin'?

I have been academic papers about Rijndael, Serpent, and Twofish, and there is this term that is vague to me. I cannot find a tangible definition in google. Can someone briefly define, describe, ...
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2answers
1k views

AES-CTR vulnerability to cryptographic oracle

I am new to cryptographic issues and from what I googled so far I could not retrieve the information I need. Consider the use of AES-128 in CTR mode. Let M be the set of possible plaintexts, for ...
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299 views

Source for examples with broken cryptography

I've heard again and again that many crypto systems have been broken in the past for one reason or another and that it is best to use one that has been peer reviewed, etc etc. However, I've yet to see ...
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1answer
149 views

Do practical key-less cryptosystems exist?

Hopefully the question isn't too broad or off. Basically, are there any practical, and by practical I mean secure enough that a ciphertext only attack is infeasible, key-less cryptosystems? For ...
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1answer
128 views

How does this Cesar Cipher work?

Suppose I have this cryptogram: http://pastebin.com/QpfRMBg1 Clearly it's a Ceasar Cipher which decrypts to: http://pastebin.com/r50heZuU with a simple shift of 10... OKay, no big deal. Let's take a ...
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1answer
165 views

How can we determine if two discrete logarithms are equal?

Let $p$ be a prime number, and let $g_{1},g_{2},...,g_{n}$ be $n$ generator of $Z^{*}_{p}$. We have a list $y_{1},y_{2},\dotsc,y_{n}$ of elements in $Z^{*}_{p}$ such that for every $i\in ...
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142 views

High Power Computing for Crypto Research

Obviously, much of cryptography requires the assumption that adversaries have bounded computing power. So, I was wondering, are there any fields in cryptographic research for which high power ...
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2k views

Why do block ciphers need a non-linear component (like an S-box)?

Why is there a requirement of "Non-Linear functions" as a component of many popular block ciphers (e.g. the S-box in DES or 3DES)? How does it make the cipher more secure? The only intuition I have ...
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1answer
175 views

Is this attack for RSA possible?

$N=p·q$ ($p$ and $q$ are prime numbers), $m_1, ..., m_x$ are the messages, $e$ and $d$ are RSA encryption and decryption exponents, respectively. I am given $e, m_1, m_1^e, m_1^d, ..., m_x, m_x^e, ...
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1answer
229 views

Why doesn't this replay attack work on ECDSA?

I've just started working with elliptic curves and ECSDA in particular, so my understanding of the underlying math isn't great. The thing I'm currently stuck on is trying to understand why replay ...
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2answers
1k views

How can I break a Vigenère cipher with partial plain text?

I have an exercise to do regarding Vigenère encryption. I've got the cipher text: WNZTNVIEEGTJYKRRWYUELWNZTNV and a partial plain text: ...
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1answer
1k views

Get RSA PlainText without Knowing Private Key

If I have an algorithm,RSA-Crack(), that, for a given RSA public key (n,e), is able to decrypt 1% of the messages encrypted with that key (without knowledge of the corresponding private key).Can there ...
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3answers
380 views

Safety of DSA key parameters sharing

I'm looking for a solution to use in a context where I need to be able to generate new asymmetric key pairs quickly (using a widely recognized algorithm, and EC-DSA is not applicable). It sounds like ...
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1answer
532 views

detecting ROT13/base64 encryption

Is there anyway to detect that a cipher is encrypted with ROT13 Or Base64 without decrypting it using the algorithm and check that the result means (works)? I mean does an algorithm exists that test ...
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2answers
196 views

Breaking RSA, given a special kind of oracle that decrypts related ciphertexts for us

Let $c=E^{RSA}_{e}(w)$ be the ciphertext belonging to the plaintext $w$ if an $RSA$ system is used. Assume that the public exponent $e$ satisfies $e \le 10$. Furthermore, assume there is an oracle ...
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Can two different pairs of RSA key have the same modulus?

Can $n=pq$ be part of two different pairs of RSA keys? If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related? What will be the security concerns for the two users?
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257 views

What is the complexity of the Square attack against the reduced 4-rounds 128-bit Rijndael variant?

I'm looking at a square attack against a reduced version of AES-128 with only 4 rounds (with block and key size of each 128 bit). I have a set of 256 plaintext-ciphertext block pairs. What is the ...
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1answer
368 views

How was the complexity of the Biclique Attack calculated?

The abstract of the Biclique Attack paper claims: The first key recovery attack on the full AES-128 with computational complexity $2^{126.1}$. What does $2^{126.1}$ mean in this context? And ...
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AES encryption for images [closed]

Why AES based encryption is not recommended for encrypting images with high redundancy in their content? for example, the encryption of an image which shows a bird in the blue sky. Most of the pixels ...