0
votes
0answers
71 views

Can you determine an unknown value when it is combined with a known value and you are given the resulting hash?

Say I have a string that I want to keep hidden. You can then enter a known string which is added to the end of the hidden string and you are given the sha256 hash of the combined string. If you were ...
0
votes
1answer
231 views

Is C25519/Ed25519 “twist secure”?

This recent new curve mentions something that's new to me: twist security. http://safecurves.cr.yp.to/bada55.html Are the existing C25519/Ed25519 curves secure against this form of attack?
1
vote
1answer
110 views

Common password derivation function for different encryption methods

I'm writing a simple encryption/decryption library which has the following requirements: For key derivation functions, support: $SHA256 \rightarrow bcrypt$ (since $bcrypt$ limits input passphrase ...
2
votes
1answer
187 views

Simple way to extend AES to 256-bit block size

What is the best way to use standard AES with a 128-bit block size to act as a 256-bit block cipher? I am aware of CMC and EME which seem to serve this purpose, but they seem to be more complicated ...
4
votes
3answers
232 views

What is the purpose of update() in popular hash APIs?

E.g., in Python's hashlib, update() basically merges in string buffer functionality into the object. Several other digest APIs I've seen have this pattern. Considering this adds functionality that ...
0
votes
1answer
342 views

One-time pad, zero key and Shannon

I'm supposed to prove that OTP without the zero key $k=0^n$ is not perfectly secret anymore. I understand that it's not because an attacker learns something by looking at the plaintext and ciphertext. ...
1
vote
1answer
71 views

Security assessment between $g^{a_ix_i+r_i}$ and $g^{x_i+r_i}$ [closed]

Imagine a tagging system whose security requirements imply to learn nothing from the tag about the encoded value. We consider a plaintext space $X \in \mathbb{Z}_p$ and a group $\mathbb{G}$ where ...
2
votes
1answer
175 views

Is AES-256 over AES-128 weakening the original encryption?

When transfering data using TLS the browser and server agree the cipher suite to be used - so for example this could be chosen as AES-128 and is (probably) outside of my control. If I separately ...
1
vote
2answers
204 views

Pseudorandom generator and AE-secure encryption

How would you answer the following question (I have to translate it from German): The existence of a pseudorandom generator implies the existence of a AE-Secure encryption scheme (AE = ...
4
votes
2answers
176 views

How can one encrypt with RSA (or ElGamal) without revealing whom the ciphertext is intended for?

Imagine Alice wants to encrypt for Bob and post this encryption publicly, so that only Bob can decrypt but no one can other than Alice or Bob tell that the message was encrypted for Bob. The naive ...
1
vote
0answers
58 views

Berlekamp-Massey algorithm, correct stepping

I'm trying to use the Berlekamp-Massey algorithm on the following bit sequence: 0 1 0 0 1 0 0 1 0 1 I have the correct answer and most of the approach to get there, but I'm unable to fill in what I ...
-2
votes
0answers
1k views

encryption and decryption time for RSA-crt in java? [duplicate]

I try compute execution time for encrypt and decrypt data has 1024 bit = 128 byte in RSA-crt algorithm. I need this results. But I believe this result wrong. because big diffrent between execution ...
2
votes
2answers
145 views

Advantages to knowing $p$ and $q$ in Blum Blum Shub?

Do you gain any advantage by knowing the factorization of $M$ (over just knowing $M$ itself) in the Blum Blum Shub generator? The only advantage I see is being able to calculate the $i$-th number ...
1
vote
3answers
147 views

Is it possible to find data which makes sense when decrypted with different keys?

In theory, does a piece of data exist which, when encrypted (using any method) with a given key, makes some sense when decrypted with the wrong key? Here, a definition of "some sense" might be ...
6
votes
1answer
421 views

How random is the shared secret in the Diffie Hellman key agreement

How random is the value $ZZ$ in the DH protocol? This question was triggered by this somewhat naïve implementation in I2P shown by Sergei at Stackoverflow. Obviously $ZZ$ is distinguishable from a ...
1
vote
1answer
107 views

Ed25519 with R replaced by H(RAM)?

Ed25519 signatures consist of $(R, s)$, where $s = r + H(RAM)a$ $mod$ $l$, and $R = rB$. This differs from the Schnorr signature where instead of $R$, the signature contains the hash $h=H(RAM)$. ...
6
votes
2answers
356 views

Why is XOR preferred over XNOR?

The XOR operator seems to be a frequently used building block inside many cryptographic primitives. As far as I can see, its most desirable properties seem be that for the XOR of two bits $a\oplus ...
4
votes
1answer
133 views

Hide message length in symmetric key cryptography

Is there any way to hide message length from adversary in symmetric key cryptography? Suppose we want to hide message length from active and efficient adversaries.
1
vote
1answer
111 views

ElGamal digital signature. why mod p-1 in delta?

Im trying to figure out why $\delta$ in the signature part of ElGamal has mod $p-1$ when $\gamma$ has mod $p$? $\gamma = \alpha^k \mod p$ $\delta = (x-a\gamma)k^{-1} \mod p-1$
0
votes
1answer
200 views

Is this EdDSA modification secure?

I am hoping to employ a signed set membership system which is valid iff each signer's contribution to the set is present. The system should allow for two or more mutually exclusive signed sets to be ...
1
vote
1answer
56 views

Knapsack Public to Private (Superincreasing knapsack)

I need to convert a hard knapsack to a superincreasing knapsack. I have these superincreasing values (3,6,11,22,43,87). I also have w = 7 and n = 173. I converted the superincreasing values to hard ...
3
votes
2answers
422 views

Shamir's secret sharing scheme - type of channels

For information theoretic security in Shamir's [m,m] secret sharing scheme, do i need both authentic and confidential channels? Another related query is ;is it true that if the channels are only ...
1
vote
1answer
65 views

How exactly are PRNG's used to encrypt material, and what makes one any more secure than the other?

I hope my question isn't as lazy as the title indicates. What I want to ask is how people literally integrate a PRNG with encrypting material. I wrote a silly little script in Python a while back ...
3
votes
1answer
467 views

Cryptographic hash function for 32-bit length input keys

Since I'm unfamiliar with cryptography area I haven't been able to find cryptographic hash functions for 32 bit integer keys. It seems all such functions (as I saw here) are designed to digest a ...
1
vote
1answer
58 views

Send a symmetric key generated by one user to other users (more than two) using a server

I am developing a software in which I am using AES for encrypting messages shared in a group. The key is generated by one user, the admin of the group and needs to be shared with other members. How ...
0
votes
1answer
44 views

Is the private key static or variable key in the elgamal algorithm?

I read about elgamal algorithm it work like the follow steps Bashir chooses a large random prime p and a primitive root α modulo p. Bashir then chooses a random integer a with 2 ≤ a < p - 1 and ...
5
votes
1answer
483 views

How to solve cipher encrypted with Vigenère + Columnar Transposition?

Vigenère's weakness is Kasiski's test and index of coincidence. However, if you put columnar transposition on top of Vigenère, that weakness is gone. The text is now shuffled and you can't search for ...
3
votes
2answers
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?
14
votes
1answer
3k views

Security strength of RSA in relation with the modulus size

NIST SP 800-57 §5.6.1 p.62–64 specifies a correspondence between RSA modulus size $n$ and expected security strength $s$ in bits: ...
2
votes
3answers
547 views

additive ElGamal encryption algorithm

I'm performing ElGamal encryption algorithm and using the additive homomorphic property so the product of two ciphertexts is the encryption of the sum of the plaintexts. The problem is that I need to ...
-1
votes
1answer
118 views

How is a Yubikey static password derived from the AES key?

When I configure a Yubikey slot as static, it's not documented or obvious how the static password yielded by a press relates to the AES key I set. For example: ...
2
votes
1answer
142 views

Berlekamp-Massey to construct minimal LFSR

Given the sequence 0010001111 (or any other, not homework, but exam practice), how do you use the Berlekamp-Massey algorithm to construct a minimal LFSR? I have read several definitions of how ...
3
votes
0answers
192 views

Pseudo-Hadamard transform, most significant bit

For clarification: The Pseudo-Hadamard transformation is a reversible transformation of a bit string that provides cryptographic diffusion. Splitting a bit string (with bit length of $2n$) into two ...
5
votes
1answer
278 views

Low complexity implementation of a small blocksize cipher (< 64 bit)

Searching for "small blocksize cipher" finds a number of discussions on the topic, mostly refering to FPE. This one in particular suggests using AES as the round function of a Feistel network. The ...
4
votes
3answers
1k views

Hash function in PBKDF2

From this excellent answer I learned (correct me if I am wrong) that when writing a block cipher with say key size 128 bit, one has to pad the password given (variable size) so that it becomes exactly ...
1
vote
0answers
57 views

Constructing of 16x16 Involutory Binary Matrices of Branch Number 7

In the PDF “Algebraic Construction of 16×16 Binary Matrices of Branch Number 7 with One Fixed Point”, it was given that: ...
1
vote
0answers
98 views

P10 to P8 in S-DES

I am new to cryptography. During my learning I am facing a problem with S-DES. I am confused where P10 to P8 come from, and I’m ...
2
votes
0answers
96 views

Can I decrypt the message having it encrypted with different keys?

I have some encrypted binary data I want to decrypt. I know nothing about it, but I can get it encrypted with as many different (random, unknown) symmetric keys as I want. The keys are long enough to ...
2
votes
1answer
103 views

Is it possible to combine two operations rotation and xor?

I can see many xor and rotation operations in cryptography. I just want to make some algorithms fast. So, I'm looking for a way to combine between xor rotation operations. I don't know if it is ...
1
vote
1answer
151 views

Is Bitmessage a Mix network?

I've read the Bitmessage whitepaper and I found it rather lacking on the details, but it promises anonymous routing. Can it be considered a variation on mix-networks, where each peer acts as a ...
15
votes
1answer
542 views

What exactly is the base for the KECCAK (SHA3) claim that a security strength of 256 bits is “post-quantum sufficient”?

On page 14 of "Keccak and the SHA-3 Standardization" (February 6, 2013) it says: Instantiation of a sponge function the permutation KECCAK-f 7 permutations: b → ...
4
votes
1answer
124 views

Why has the sponge construction's generic collision finding attack a complexity of O(min(2^(-n/2) , 2^(-c/2)))?

I'm trying to learn about sponge function for creating hash functions and generic attacks on it. I'm looking for the collision finding attack scenario which leads to $O(\min(2^{-n/2} , 2^{-c/2}))$ ...
2
votes
2answers
360 views

Seed / reseed DRBG too often?

There are limits to how much you can use a the DRBG before you have to reseed it, but are there any limits to how much you must use it before reseeding? Take the following scenarios as an example… ...
8
votes
1answer
442 views

Cryptographically secure keyed rolling hash function

I could not find any mention on the Internet of a proven/known cryptographically secure keyed rolling hash function (ie rolling MAC). Has the question been studied, is it possible to build one ? By ...
1
vote
1answer
81 views

Key exchange if forward secrecy is not an issue

Let's say that we have a cryptosystem where Forward Secrecy and Man-In-The-Middle attacks are not an issue, purely hypothetically. Would it be correct for a key exchange with any given public-key ...
2
votes
1answer
76 views

Is concatenated data hashed with scrypt vulunarable to a length extension attack?

scrypt takes a salt, and a password plus some cost paramters to generate a key. So say we define it as this: key = scrypt(password, salt, cost) I am interested ...
2
votes
1answer
118 views

Is there a number of tries after which Hashcash is guaranted to be solved?

Hashcash is a proof-of-work system in which the Sender needs to find $Y$ such as the first (let's say 20) bits of $H(X+Y)$ are zeros where $H$ is a one-way hash function, $X$ is a fixed value and $+$ ...
2
votes
0answers
47 views

space complexity of quantum collision search

Is there a known way to reduce the space complexity of quantum collision search beyond what is offered by the built-in time-space tradeoff, while keeping the time complexity significantly below what ...
-1
votes
1answer
259 views

How can I convert numbers into prime numbers?

I'm working with one-way accumulators, but I'm not knowledgable in cryptography. Is there an easy peasy way to hash numbers (or whatever) into prime numbers? Obviously I'd like it to be collision ...
2
votes
1answer
107 views

Changing a bit Changes Hash

I am very curious on the fact that SHA, MD5 and all related Hash Functions change the output even by changing one bit in input Message. Why is that?

15 30 50 per page