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

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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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Can you help me understand the Common Modulus Attack in a Lucas Group?

I'm trying to decrypt a message that is encrypted using a LUC encryption scheme and running into roadblocks. I know that with RSA if Alice and Bob use the same public modulus but different encryption ...
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How has the “power” of cryptography evolved from its beginnings till today? [closed]

I'm looking for a historical overview for the change in cryptographic power, which I hereby define as How far state-of-the-art cryptography is ahead of state-of-the-art cryptanalysis. In other ...
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285 views

If Bob steals Alice's private key, how exactly would he read her encrypted documents?

So Bob grabs Alice's secret key when she isn't looking and her encrypted files, doesn't he need to know her passphrase to read her files? What I am reading is that no he does not need it but as far ...
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Is it possible to work out the hash algorithm from a list of known message-hash pairs?

For example, in my situation I know hash(20) = 486e9638177faf1f34e49910491b77af. I also know the hashes for all values from 0 to 20. Is it possible to work out the ...
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breaking fully homomorphic encryption schemes

Fully homomorphic encryption schemes allow one to evaluate any arbitrary computation over encrypted data. Intuitively this seems to be too weak, irrespective of how we achieve this. An adversary who ...
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How can I do a brute force (ciphertext only) attack on an CBC-encrypted message?

Given a CBC ciphertext and IV, how can I find the encryption key? We are limited with an 8 chars key, each char in the range of [a..h], so I can generate every possible key (these are only $8^8 = ...
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How can I break REDSHIRT / REDSHIRT2 encryption?

Recently, a user on Gaming.SE asked a question about whether the user password in the video game Uplink could be modified after being initially set. The game does not contain an option to change the ...
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Can you help with that definition for a CCA?

The following is a definition taken from Introduction to Modern Cryptography by Katz and Lindell. I'm having a hard time understanding some basic concepts! Can you please help me? ...
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224 views

How feasible is word-level frequency analysis over English (or any language)?

Say I have some black box which, given any English word, deterministically outputs a token for that word. Assume our black box is implemented using strong cryptography, i.e. the hardness of reversing ...
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236 views

Regarding Key Strength with DES and Blowfish

I had a small discussion with my friend on combining two encryption algorithms based on 64-bit keys. Say I have a plain text of 64 bits which I want to encrypt. My Key size is 128 bits. I will use ...
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796 views

Theoretical attack on RSA

The high security of RSA is granted, because it is very hard to factorize $$ N = p * q $$ Nevertheless, there is actually no need of factorizing $N$, in order to generate the $Private$ $Key$, but the ...
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DES — Can I recover the key when I have both ciphertext and the plaintext?

Given a message and DES encrypted form of said message, is it possible to efficiently compute the key used to encrypt the data?
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How to attack a fixed LCG with partial output?

To show some colleague programmers exactly how broken C's rand() is (at least on Windows) I decided to break it. So everyone knows the exact parameters, MSVC's ...
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290 views

How to encrypt data and know it will be secure for at least a few decades?

I want to encrypt a small amount (few kilobytes) of data and be fairly confident it will be secure against brute force decryption and advances in cryptanalysis for at least a a few decades (or more). ...
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Decrypting DES with decrypted and encrypted data

I got two 8-byte strings. One which is decrypted is: FF FF FF FF FF FF FF FF and one which is encrypted is: ...
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Can I build a secure tweakable block cipher from a normal one by adding key and tweak?

Let (E,D) be a secure block cipher. Consider the following tweakable block cipher: ...
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Getting the encryption method and key from the encrypted data and the raw data

I have some pairs of plaintext and ciphertext data, from which I need to be able to decrypt the other passwords stored in the database. The password field typically contains something like ...
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Where can I find useful data for cryptography/coding theory?

When implementing cryptographic/coding theory algorithms one need to use data like big prime numbers, numbers in $Z_n$ and their inverses, irreducible polynomials in $Z_n[x]$ and so on... While ...
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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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Difficulty of breaking RSA for a given key size

Is it true that breaking a 1024-bit RSA key is as difficult as breaking a 128 bit symmetric key (e.g. AES)? I know that breaking a RSA key is equivalent to factoring the modulus $N$. To factor it, you ...
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What's the alternative should PKI collapse?

Premises - please check it and critique if necessary: If factoring the products of large numbers suddenly becomes simple, PKI goes up in smoke. Is this correct? If it is correct, it seems to me ...
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What are the standard procedures in cryptanalysis to analyze unknown ciphertext?

What are the "standard procedures in cryptanalysis" to analyze unknown ciphertext? In other words: Are there any protocols, officially acknowledged checklists or something like that which represent a ...
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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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Is there a name for this 'enhanced' caeser shift and if so, is it trivial to break?

This cipher shifts the letter that it will shift to, after each shift... I used to play with this when I was a kid. I was thinking about it recently and realized that it wasn't as simple as I ...
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164 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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Reversing DJB2 Hashes

I have some spare time, and a few hundred DJB2-hashed values sitting around. I thought I'd try to do something "useful" and invert DJB2, such that I could calculate the plaintext of the hashes (which ...
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differential and linear cryptanalysis

I have been reading about differential and linear cryptanalysis. They were mainly introduced by Adi Shamir and Biham to show weakness of DES. However, many of articles state that they have been ...
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Where can i find relative software for differential cryptanalysis?

Is there any publicly known software for differential cryptanalysis over some cryptographic building blocks as blocks ciphers or cryptographic hash functions
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Is XOR in a CBC-like mode secure?

Assuming that $K_{n}$, $P_{n}$, and $C_{n}$ are individual bytes of the key, plaintext, and ciphertext respectively. The first byte of ciphertext is computed like this: $C_{1} = K_{1} \oplus P_{1}$ ...
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204 views

Attacks on AES-128 (ECB) based on some knowledge of plaintext

I'm working with a third party protocol which employs AES-128 in ECB mode of operation to encrypt a packet composed of 16-byte blocks (it encrypts each block independently). I'm trying to determine if ...
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130 views

Is it a good idea to use Lagrange/Newtonian interpolation for encryption?

I see that Lagrange interpolation is commonly used for secret sharing, but could it be used for encryption? The goal is to reduce database I/O and compute new values on the fly. Suppose the use case ...
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140 views

XOR with constant key in CBC mode

I have a hypothetical encryption scheme where somebody uses the one-time pad in CBC mode. That is, the block cipher is $E(k, m) :=k⊕m$, and that block cipher is used in CBC mode. Now, I am assuming ...
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Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
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170 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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349 views

ElGamal message signatures retrieving the secret value x

If the GCD(r, p-1) is small and the value k is used to sign a message using ElGamal is also small. Then the secret value of x can be determined. Why is this true? How would one retrieve x?
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PBKDF2 Salt and Password Ordering

I am currently reading about PBKDF2, and understand that the salt is used only once, while the password is used multiple times in the computation of the final key (see this question). How would the ...
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238 views

True Random Number Generator by milliseconds per keystroke (TRNG-Kms)

The simplest way to generate truly random numbers for OTP keys is to measure the time in milliseconds between each keystroke on a keyboard. The randomness depends on the user typing in various speeds. ...
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141 views

Twisted curves in protocol

I've come to understand that twisted curves, as for instance defined in the Brainpool specifications, are $F(p)$-isomorphic to their regular $F(p)$ equivalents. So brainpoolP256r1 is isomorphic to ...
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123 views

Is ISAAC Cipher Cryptographically Secure?

http://en.wikipedia.org/wiki/ISAAC_(cipher) This question was asked before but the answers seem vague, and I want to know about ISAAC specifically, not ISAAC+. It seems some cryptanalysis was ...
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119 views

How would using only one s-box affect security of Blowfish?

Schneier says: Fewer and smaller S-boxes. It may be possible to reduce the number of S-boxes from four to one. Additionally, it may be possible to overlap entries in a single S-box: entry 0 ...
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307 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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Are asymptotic lower bounds relevant to cryptography?

An asymptotic lower bound such as exponential-hardness is generally thought to imply that a problem is "inherently difficult". Encryption that is "inherently difficult" to break is thought to be ...
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KDF based on HMAC-SHA-256

Is KDF based on HMAC-SHA-256(Hashed Message Authentication Code, Secure Hash Algorithm) algorithm a suitable option to generate symmetric key from the secret key? What is the basic funda, behind ...
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118 views

Why is Lamport-Diffie secure?

Why is Lamport-Diffie secure? I note that there is a demonstration based on onewayness (in the book postquantum cryptography). But a one way function is not sufficient to ensure that it can not infer ...
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158 views

Derive a public EC key from two public EC keys

Alice has two EC key pairs: $a_1$, $a_2$ are private keys (integers), $A_1$, $A_2$ are the corresponding public keys (points). Alice and Bob want to create a new public key $C$. Alice must prove that ...
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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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Any historical accounts of cryptanalysis of Jefferson's wheel cipher?

David Kahn in his book "The Codebreakers" wrote about Jefferson's wheel cipher, saying that To this day the Navy uses it… (the book was first published in 1967) Are there any historical accounts ...
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260 views

Has there been any cryptanalysis of RC4-52?

Several websites ( such as Is there a secure cryptosystem that can be performed mentally? ) briefly mention RC4-52 as a modification of standard RC4. RC4-52 has only with 52 instead of 256 elements ...
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299 views

Security analysis of a matrix multiplication protocol

Suppose Alice would like to obtain the product of two mXm matrices i.e. A and B. Alice has A, whereas Bob has B. Since Alice does not want to reveal A to Bob, she chooses a mXm random invertable ...