2
votes
1answer
476 views

How is padding oracle attack mitigated by encrypt-then-MAC?

Let us suppose Alice sends a message to Bob. As far as I know, the most popular scheme of MAC-then-encrypt is as follows: Alice computes the HMAC of the plaintext using her private key, and then ...
1
vote
2answers
295 views

weaknesses in ElGamal with public key of small order

Suppose $p=29$, $\alpha = 2 \in F_p^*$ is a generator of $F_p^*$. Bob picks $d \in \{2,...,27\}$ such that $\beta = \alpha ^d=28 \pmod{29}$. He then sends his $(p,\alpha ,\beta)$ to Alice who herself ...
1
vote
2answers
156 views

Understanding math behind RSA key derivation [duplicate]

I was reading through the key derivation for RSA. Here are the steps per wiki - Select strong primes $p$ and $q$ such that $pq = n$ $\phi(n)$ = $(p-1)(q-1)$ select $e$ such that $e$ and $\phi(n)$ ...
2
votes
1answer
220 views

How do P2P clients like uTorrent maintain authenticity of files?

I am very new to concepts of cryptography so bear with me and feel free to fill in any information that may seem obvious but something you think someone may not know. At first I was under impression ...
0
votes
0answers
77 views

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

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 ...
4
votes
1answer
124 views

Cryptographic pseudo-random generation of address subsets

Suppose I have devices which broadcast, on an insecure channel, 32-bit addresses. I want to make these devices untraceable - one way to do so is to allocate for each device a distinct subset of ...
2
votes
1answer
1k views

Complement property of DES

I'm supposed to show the complement property of DES ($c=DES(p,k) => \bar c = DES(\bar p, \bar k)$). My idea was to just start the algorithm with $\bar p$ and see what happens (ignoring the initial ...
-2
votes
0answers
117 views

Symmetric-Key Based authentication [closed]

So i've managed to answer all the other cryptography questions, but I have not come across this exam style question before, and need help.
2
votes
1answer
275 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 ...
2
votes
1answer
400 views

Message lengths with AES CTR mode?

Im working on my own AES implementation as a personal educational exercise (not for production use), and a book im using as a resource states: In the case of CTR, we are merely XORing the output ...
-1
votes
2answers
426 views

Shamir Secret sharing - Can share generator keep x values secret?

I'm wondering, in Shamir secret sharing, can generator of the shares, keep the x values which are used in evaluating the polynomial to obtain y values (i.e., the shares) secret, and whenever the ...
0
votes
1answer
684 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. ...
3
votes
2answers
239 views

Which concrete applications benefit from Oblivious RAM constructions?

The main motivation for Oblivious RAM (ORAM) is that, for instance, in the "cloud" setting a client outsources his data to a server in the form of encrypted blocks. Later on, he wants to perform read ...
2
votes
1answer
264 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
1answer
74 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
778 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
131 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
110 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
67 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
211 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 ...
6
votes
1answer
2k 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 ...
12
votes
1answer
608 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 ...
3
votes
2answers
193 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
285 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
195 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
72 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
74 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
124 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 $h=H(RAM)$. ...
2
votes
3answers
165 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 ...
1
vote
1answer
220 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 ...
6
votes
2answers
571 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
69 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
237 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
151 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
1answer
168 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
66 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 ...
2
votes
1answer
811 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 ...
0
votes
1answer
202 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
689 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
92 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: ...
3
votes
1answer
142 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
2answers
385 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
238 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
212 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 ...
9
votes
3answers
410 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: ...
2
votes
1answer
90 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
137 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
304 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 ...
3
votes
0answers
59 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 ...
4
votes
2answers
245 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 ...

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