1
vote
1answer
68 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 ...
3
votes
4answers
511 views

Which tamper-protection algorithm provides the shortest output?

Background When making web applications you sometimes need to pass data along with the form which isn't visible to the user. Database IDs are the most common, but texts and IDs are sometimes ...
1
vote
1answer
84 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 ...
1
vote
1answer
65 views

Prefix property for variable length pseudo-random generators

I'm currently working through Katz & Lindell's Introduction to Modern Cryptography. In section 3.4.2, they introduce the definition of a variable output-length pseudorandom generator $G$. The ...
1
vote
0answers
45 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 ...
4
votes
2answers
141 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 ...
4
votes
1answer
193 views

how to use common modulus attack?

I am struck with the following problem: Let Alice, Bob, Chris and Eve communicate over a public network. They encrypt all messages they send using RSA system. Bob and Chris have the RSA modulus ...
5
votes
1answer
131 views

Perfect Forward Secrecy with NaCl

I would like to use the NaCl cryptography library (or TweetNaCl), but I would very much like to have perfect forward secrecy for my application. From what I understand, all Bob needs to to decrypt a ...
2
votes
2answers
113 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 ...
0
votes
1answer
143 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?
0
votes
1answer
63 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 $p$? $\gamma = \alpha^k \mod p$ $\delta = (x-a\gamma)k^{-1} \mod p-1$
1
vote
1answer
52 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 ...
1
vote
1answer
39 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 ...
1
vote
1answer
85 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 $h=H(RAM)$. ...
1
vote
3answers
141 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 ...
2
votes
1answer
128 views

Building a stream-cipher out of a hash function?

I've already read this: Is it feasible to build a stream cipher from a cryptographic hash function? However, my proposed construction differs… Suppose the hash generates N bits. These bits are split ...
5
votes
2answers
190 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 ...
0
votes
1answer
33 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 ...
0
votes
1answer
103 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 ...
4
votes
1answer
115 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.
2
votes
0answers
104 views

SIMONs Cryptoanalysis

I'm reading Cryptanalysis of the SIMON Family of Block Ciphers. In Section 3.1, it says: For SIMON, consider an n-bit input difference $\alpha= x\oplus x'$ to $F$ of Hamming weight one. As the ...
1
vote
1answer
41 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 ...
1
vote
1answer
86 views

Value of “counter” when decrypting AES in CTR mode?

If – when using AES in CTR mode – you decide to use the setup where you combine the IV and the block count value into a single value, as suggested here, the first 12 bytes are your IV and the last 4 ...
-1
votes
1answer
45 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: ...
3
votes
1answer
195 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
0answers
29 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: ...
2
votes
0answers
82 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 ...
1
vote
0answers
32 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 ...
3
votes
0answers
153 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 ...
2
votes
1answer
90 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 ...
6
votes
3answers
88 views

Why do “nothing up my sleeve numbers” have low entropy?

As a preface, forgive me for some of the links being from Wikipedia. I realize that academia frowns upon this. I came across this article about "nothing up my sleeve numbers". In it, it says: ...
1
vote
1answer
55 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
86 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 ...
3
votes
1answer
205 views

What is a generator?

I am trying to read up on the math behind DH and ran into the word generator used a lot. I am not sure what it means. Can someone explain? I see this question where the OP talks about g as the ...
2
votes
0answers
40 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 ...
3
votes
2answers
86 views

Associative standard cryptographic hash function

I am looking for a standard hash function which satisfies the following property: A hash function $H(a,b) = F(h(a),h(b))$ with $h$ (within $F$) any standard cryptographic hash function and $F$ an ...
2
votes
2answers
338 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
81 views

Low level explanation of how a message is sent via RSA

I understand that RSA encryption uses this formula: C = M^e (mod N) public key is e and N N is pq - p and q are private key. mod N makes above function one-way ...
7
votes
1answer
258 views

Is TEA considered secure?

Wikipedia claims that the best attack on the surprisingly simple TEA block cipher, that isn't a related-key attack, has a time complexity of $2^{121.5}$. So despite how unsophisticated the cipher ...
0
votes
1answer
207 views

modulo operations in crypto algorithms

Am not a mathematician. Every crypto specification I see uses the modulo operation. For example RSA - If $e$ is the public key and $m$ is the plaintext with a modulus $n$ - the cipher text is $c = ...
4
votes
2answers
91 views

Is it possible to construct a secure block cipher of size $2n$ given a secure block cipher of size $n$?

Given, say, the Blowfish block cipher, which is considered secure but only has a 64-bit block size, can we construct a secure block cipher of 128-bit block size? Say we run the key through two KDFs, ...
2
votes
1answer
88 views

RSA private exponent primality

I know that the public exponent is always a prime, but what about the private exponent? Is it always a prime too?
2
votes
1answer
113 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
1answer
241 views

Is AES key length a size of the key or entropy?

While educating my self I'm having hard time to understand what an 128 bit AES key actually means? Is it a key length or entropy? Please explain to me trough following example (assuming AES): ...
2
votes
3answers
216 views

Problems with using AES Key as IV in CBC-Mode

I'm a pentester and currently analysing a web application which are using some strange encryption scheme. The point is: They encrypt using AES-128, generate a (not cryptographic secure) key and use ...
-1
votes
1answer
55 views

How to benchmark custom crypto with OpenSSL benchmark suite? [closed]

OpenSSL benchmark suite currently tests the speed of algorithms supported by the library like md5 sha1 sha256 sha512. What if we have a custom crypto algorithm "abc" . How do we benchmark its ...
1
vote
1answer
145 views

Breaking RSA when some bits of one prime are known

RSA primes are 100 bit. You know first 80 bits of one of the primes. In this system, come up with an efficient way to decrypt the cipher-text. This was the question on my quiz. I'm not sure how ...
4
votes
2answers
169 views

What are Cryptographic Multi-linear Maps?

I've encountered this term many times in the fields of Fully-Homomorphic Encryption and Obfuscation. I want to learn those subject and Cryptographic Linear Maps seems to be an obstacle in the way. ...
3
votes
1answer
270 views

ECDSA vs RSA: Performance on Android platform and surprising results

For our privacy-preserving protocol, an encrypted channel is established. In order to protect our system from man-in-the-middle attacks, signature-based approach is used. After we've implemented it ...
1
vote
0answers
33 views

What constitutes a “description of B” for probabilistic encryption as defined in Cryptology 6.3.4?

On page 21 of the Rivest's Cryptology chapter, he defines a trapdoor predicate as a boolean function for which it is easy to choose an x such that ...

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