# All Questions

250 views

Passwords hashed under Unix/Linux systems can use several hashing algorithms. ...
91 views

### Possible to check if $a \in \mathrm{QR}_n$?

It is possible to check $a \in \mathrm{QR}_p \text{ iff } a^{(p-1)/2} \equiv 1\ (\bmod\ p)$ if $p$ is a prime. $n$ is a large RSA modulus. Is it also possible to check if $a \in \mathrm{QR}_n$ if the ...
138 views

### Logical OR operation in a homomorphic additive cryptosystem

Suppose we have a cryptosystem homomorphic for addition (say Paillier's). Is there a way to perform a logical OR operation between two binary values (with a binary result). We can, of course, obtain ...
205 views

### How can Shamir's method for secret sharing work in the GF(256)?

I have been reading about Shamir's secret sharing, and maybe i completely misunderstand something, but the field is meant to be a prime. However, when i look at the implementations of this algorithm, ...
154 views

### Merkle Tree High Tree

I'm reading Merkle tree signatures and I like know if the security of the that scheme depends, also, of the high of the tree or only depends that hash function is resistant to collision?
229 views

### Public-key cryptosystems without poly-time quantum attacks

It's well-known that Shor's algorithm can solve integer factorization, discrete logarithm and discrete log over elliptic curves in cubic time. This implies that cryptosystems like RSA, ElGamal, and ...
146 views

### 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 ...
302 views

### How to calculate entropy of a combined key

I know how to calculate the entropy of a key that relates to its selection process. For example if the key space is $1000$, entropy of a randomly chosen key is $1000$. Suppose now you have two keys ...
320 views

### How does the IV or initial counter increase internally for each block in AES CTR mode?

I am working on Intel AES-NI library and since the AES function is coded using low-level assembly instructions. I could not see how the IV or initial counter increases in CTR mode. In general, how ...
352 views

### Definition of a CSPRNG

I am interested in what conditions are necessary and sufficient to define a cryptographically secure pseudo-random number generator (CSPRNG). Wikipedia lists two defining characteristics: It ...
234 views

### Is there a companion algorithm for OTP to ensure integrity and/or authentication?

One-Time Padding is (theoretically) perfect encryption algorithm. Let's assume that these problems are solved: Fast generation of independent and identically distributed random variables Perfect key ...
2k views

### How much is slower GCM AES-128 than one of the stream chipher eSTREAM?

Evaluate the speed of the algorithm GCM AES-128 and compare it whith speed of one of the stream chipher in eSTREAM. Can someone give such an assessment?
251 views

### How does redundancy in a file effects performance and security of encryption?

Does redundancy in a file or data effect performance and security of encryption? (for example for AES, if not for general case) Is it easier to encrypt something that has high redundancy or something ...
129 views

### Why are huge prime numbers important in cryptography?

I read an article the other day about the search for prime numbers. According to the article and several online sources the biggest prime number is over 17 million digits! This made me wonder why ...
1k views

### Difference between salted hash and keyed hashing?

A cryptographic salt is additional input other than message itself for a hash function so that it prevents attacker from launching dictionary attacks . Usually the salt is stored along with the hash ...
581 views

### Where is the S-Box generated in Rijandel/AES?

It's rather kind of lame questions, and I can't find good and clear explanation: In which step of Rijandel is S-box generated? Is the S-box reused in every round of cipher or is generated in every ...
196 views

### Can GCM be used in incremental mode?

I want to sync blobs over the network and the most frequent change will be appending to blob. To minimize data amount sent I would prefer to encrypt appended data, send it over the network, append to ...
288 views

### Is it possible that two distinct RSA moduli share both of their prime factors?

I am using an algorithm (can be found here1) that can compute efficiently the GCD of multiple RSA keys. It intended for RSA keys that were generated with low entropy and may have one of their primes ...
148 views

### Could this be a valid variation of the Schnorr protocol?

The Schnorr protocol is a 3-steps proof of knowledge of a discrete logarithm, whose interactive version works as follows. Let $p$ and $q$ be two public primes, such that $q \mid (p-1)$, and let $G$ ...
245 views

### Allowing the user to choose the hashing formula at the registration

I was thinking of a potential new way to increase someone's account security. The idea was allowing the user to choose one of the hash formulas available in the software (example: "MD5", "SHA512" and ...
221 views

### Efficiency of finding sub group order vs factorization

Suppose you got a prime $p = 2\mathbb\Pi_{i=0}^{n-1}q_i+1$, where $2^{k-1} \lt q_i \lt 2^k$ for some $k$ and all $0 \le i \lt n$, and that you also got a generator $g$ of one of the prime order sub ...
108 views

### Secure Sketch Construction

I would like a user to choose a point on an image which is represented by (x,y). This is meant to be a security token. When a user uses this location to login they are unlikely to to choose the exact ...
229 views

### Quadratic residuosity problem reduction to integer factorization

How can one show how to reduce the quadratic residuosity problem to an integer factorization?
594 views

### Encrypted counter as IV in CBC mode encryption using same key

I'm currently working with a secure transport protocol that defines the IV to be a counter (incremental nonce) to be encrypted with the same key. This is a followup to a protocol that did not provide ...
163 views

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### Why can't I break ElGamal encryption by brute-forcing the secret exponent?

I am doing a course on cryptography on coursera and one of the topics covered was the ElGamal Encryption system. I am using the terms as defined in Wikipedia. Alice publishes $g$ and $g^x$. ...
759 views

### How to generate a public key from a private ECDSA key?

Having some specific ECDSA curve and a private key, how does one calculate the public key? I am having a hard time finding the algorithm and equations for it.
582 views

### Public key cryptography - public key encrypts and cannot decrypt?

I understand the basics behind public key cryptography, in that each party has two keys - the public one encrypts, and the private one decrypts. What I cannot figure out is, How does the public key ...
250 views

I've been looking at the weakness with plain/textbook RSA, where the same message is encrypted and sent to multiple destinations. In this case, it is possible to recover the message. Given that an ...
306 views

### Two step encryption

Is there any asymmetric cryptography algorithm which will allow recursive encryption. ...
160 views

### Is there an advantage to storing keys split between several hashes?

I have a question about the way to store a key or password that was used for encryption, so that the application can check if the user put in the right key for decryption. If I make a mistake, please ...
522 views

### Looking for examples for “proof by reduction”

Im looking for examples for a proof by reduction. For example: Let $A=(Gen, H)$ be a hash function. We define a new Hash function $A'=(Gen',H')$ with Gen=Gen' $H'_s(x)=H_s(H_s(x))$ It should be ...
1k views

### Cracking the Beaufort cipher

Is there any easy way to crack a Beaufort cipher? We have a Vigenère table, and are trying to guess the keyword. Any easier way?
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### Impact of algorithms for factoring using elliptic curves over $\mathbb{Q}$

Recently a few papers have appeared that describe a new approach to factoring, using elliptic curves over $\mathbb{Q}$. See, e.g., Factoring integers and computing elliptic curve rational points, ...