# Tagged Questions

Design of cryptographic primitives (algorithms), like block ciphers, stream ciphers, random-number generators, hash functions, MACs, key exchanges, public-key encryption or signature schemes. Also tag with the relevant type of primitive. If you ask about a known existing algorithm, also tag with its ...

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### How to design a balanced LFSR with period 1023 and linear span 55?

I am trying various combination of LFSR's to try to design sequence of period 1023. The linear span should be 55 and the output 1023 bits has to be balanced. I have tried with five LFSR's with degree ...
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### Security Strength of Double Substitution Ciphers

I am wondering why people are using RSA keys when some types of double substitution ciphers seem to be just as secure if not better off. By some types of double substitution ciphers I mean one Which ...
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### Perfectly secure shift cipher

Prove that if only one character is encrypted using a shift cipher, then the shift cipher is perfectly secure. I want to show that $P(P=p | C=c)=P(P=p)$. But I don't know how to relate. Can anyone ...
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### Can you decrypt with 1 round key?

If one has the last round key in an SP or Feistel network (the 16th round key in DES for example), would that someone be able to decrypt the rest of the encryption?
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Good evening, I'm about to write my own quadratic sieve implementation in C using GMP library for large numbers. I'm facing an issue while attempting to do the last factorization step for the number: ...
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### Collisions in the cyclotomic knapsack function

I've been working my way through the paper “Efficient Collision-Resistant Hashing from Worst-Case Assumptions on Cyclic Lattices” by Peikert and Rosen, and I've come across something that doesn't seem ...
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### How to multiply a matrix of bits with another?

For example, assume I have two 4x4 matrices of bits: 1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 1 1 0 1 1 0 1 1 1 1 0 1 0 I want to apply ...
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### Expire signature

I am looking for a way to have a 'did it expired' key. There's a similar question here, but I have to prevent reusing of the 'did it expired' signature . A possible solution would be a two one-way ...
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### Difference between stream cipher and block cipher

A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher ...
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### Secret construction

Given set of random strings $w =\{s_1, s_2, s_3 ... s_n\}$ are there any algorithms to construct single master secret $S$ from them? Subsequently we should be able to deconstruct the $S$ back to ...
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### Security of randomized data dependant key schedules?

An example to demonstrate the point with two Feistel networks: Cipher A: \begin{align} roundkey = hash(key || counter)\\ roundkey = hash(key || roundkey)\\ left = left\oplus hash(roundkey || right)\\...
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### Verilog simulation & synthesis of Diffie-Hellman key exchange

Is there any freely available verilog implementaion of Diffie-Hellman key exchange? I couldn't find anything using google. So, assuming its not there I started implementing on my own. The code is ...
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### Validating subset of a password

Is anyone aware of an algorithm/method that allows me to validate just a few letters (or other characters) from a password. For example, the application asks for the 2nd, 3rd, 6th and 9th letters from ...
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### Is it safe to encrypt data using XOR along with CSPRNG seeded with a truly random number? [closed]

I was wondering a couple of days ago whether I could reverse the normal encryption order to produce a good AES based cipher variant. The method is as follows: Two users, Alice and Bob, each have a ...
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### How are boolean functions used in cryptography?

I recently started becoming interested in Boolean functions. Because they are defined as $f: \{0, 1\}^n \rightarrow \{0, 1\}$, or in other words only over $\{0, 1\}$, I guessed they can somehow be ...
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### Why should I make my cipher public?

As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However Kerckhoffs's principle states that A cryptosystem should be secure ...
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### What is the difference between SHA-3(Keccak) and previous generation SHA algorithms?

SHA-1 and SHA-2 share the same structure and mathematical operation as their predecessors - SHA-0 and MD5. Both SHA-0 and MD5 have been broken. This is one of the main reasons why SHA-1 is considered ...
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### How is a per round key generated in DES algorithm?

I am reading over a slide that I found online regarding the DES algorithm for encryption and I am a little confused about the per round key generation. From the slide below, I understand that each per ...
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### Why does DES implement so much Cross Wiring?

I've been going through DES, and I find that most of the blocks simply Reshuffle the input, which may be termed as "Cross Wiring". This include IP, IP-1, PC-1, PC-2 and even Expansion (although this ...
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### help with cryptanalysis of a sponge permutation

I have been studying and researching hash functions. So far my research has led me to the sponge construction. It appears that the permutation used in the sponge to stir the state is more or less ...
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### Modular Multiplicative Inverse and RSA?

In trying to understand this specific part of the RSA algorithm, I found this online: $$e \cdot d = 1 \pmod{(p-1)\cdot(q-1)}$$ Therefore: e \cdot d \cdot d^{-1}= d^{-1} \cdot 1 \pmod{(p-1)\...
I'm new to cryptography and am currently working on the following question: Say you are given a public-key encryption scheme $\Pi_e=(\mathcal{K}_e,\mathcal{E},\mathcal{D})$ and a signature scheme \$...