# Tagged Questions

Questions about the construction and analysis of protocols and algorithms for secure computation and communication (including authentication, integrity, and privacy aspects).

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### dining metaphysicians problem

Here is a problem vaguely related to the dining philosophers problem. Three metaphysicians go to a restaurant for dinner. When it is time to pay the bill, they decide that they want one of them to pay ...
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### Can a public key be used to decrypt a message encrypted by the corresponding private key?

From what I have seen about usage of a pair of public and private keys, the public key is used for encrypting a message, and the private key is used for decrypting the encrypted message. If a message ...
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### Zero-knowledge proof: Abstract example

So I was reading about ZKP on wikipedia, the abstract example in summary goes like this: Peggy wants to prove to Victor that she knows the secret to a door inside a cave that connect A and B ...
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### Is it possible to derive a deterministic CSPRNG given two functions, at least one of which is a CSPRNG?

Let f and g be two functions with integer range 0..m-1. They may keep state and interact ...
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### If P=NP, are there cryptosystems that would require n^2 time to break?

If P does equal NP, will it still be possible do design a cryptosystem where the optimal cryptanalysis algorithm takes, say, the square of the time taken up by the legitimate encryption and decryption ...
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### Hash function which is invariant under small changes

I am looking for a hash function which is invariant under small changes. E.g., if I have two strings MyString and MySttring ...
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### Monoalphabetic Cipher Key

I am not sure how to get the key for the following Monoalphabetic Cipher question. This is a textbook question and I know the answer, but I juts dont know how they got the key. Question: ...
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### Bijective algorithm(s) that can shuffle a range of numbers back and forth

I've spent the last hours trying to come up with an algorithm that shuffles an ordered set of numbers (i.e a simple range of numbers) in a way that I can retrieve from a shuffled value the original ...
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### Why did RSA encryption become popular for key exchange?

This is a soft question. I don't know a lot about cryptography or its history, but it seems like a common use for RSA is to do key exchange by encrypting a symmetric key to send a longer message (e.g....
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### How do you rank the strings represented by a regular expression?

This question is about the use of 'ranking' of regular languages in applications such as format preserving encryption. I understand format preserving encryption but I don't understand how regular ...
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### What are some data mining algoritms used in finance?

I want to test the applicability of homomorphic encryption in the financial domain, as suggested in Can homomorphic encryption be practical ?. Now, I want to know what algorithms are used for ...
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### Show that a function is not a PRFs

I am studying cryptography and having some trouble with the PRFs How do I show that $f_{k}(x) =x+k^2$ mod $2^n$ is not a Psuedo Random function? I know that I need to show that the distignuisher can ...
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### Given RSA, why do we not know if public-key cryptography is possible?

I was on wikipedia on list of unsolved computer science problems and found this: Is public-key cryptography possible? I thought RSA encryption was a form of public-key cryptography? Why is this a ...
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### Can systems that prevent double-spending (e.g. crypto-currencies) be used to attach other unique data?

The Bitcoin-solution can be described as "[...] a solution to the double-spending problem using a peer-to-peer network. (official Bitcoin paper, PDF, abstract, first page). Now I wonder if a similar ...
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### “Most Similar Vector Problem” on an Integer Lattice

I am currently working on problem that I think could be expressed as an integer lattice problem, and hoping to find some guidance on this forum. Given $u \in \mathbb{R}^n$ and a bounded integer ...
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### About the definition of “differential privacy” in communication complexity

In the context of communication complexity I see a definition of differential privacy which isn't totally clear to me as to why it makes sense. So the two parties $A$ and $B$ draw two strings $X$ ...
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### Formula for sufficiently lengthy encryption key?

As you add length to an encryption key, at some point the message becomes impossible to brute-force decrypt. This is because at that point, if you go through all the possible keys, you'll get many ...
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### If one-way functions exist are we definitely using them?

I know that if one-way functions exist then there are certain universal one-way functions that exist, but to my knowledge they are too impractical to implement (which is the main reason why they are ...
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### What is the relation between differential-privacy mechanism and entropy?

Why do differential-privacy people care whether or not the noise function saturates the lower bound of Shannon entropy? For example : Laplace distribution that is used to model the noise function ...
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### How is Chinese Remainder Theorem used in the proof of correctness for RSA

Question At the very end of (most) proofs of RSA's correctness we have something like $$m^{ed}\equiv m\pmod p$$ $$m^{ed}\equiv m\pmod q$$ Therefore by the Chinese Remainder Theorem (CRT) m^{ed}\...
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### Hash values that are impossible to reach

I was just curious as to if there are (or could by) any hash values that are impossible to compute due to the implementation of the algorithm. For example, SHA-256 produces a value that is 256-bits ...
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### What is the complexity of finding the two prime numbers a composite number (used in RSA Protocol) is made of?

I am aware that as the number increases in Digits the process of locating the two prime numbers that when multiplied produce the given number is increased as well. I also know that is it somewhat ...
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### Is a very long plain text password harder to crack than a short complicated password? [closed]

Is it true that a password consisting of the alphabet, even of common known names is much harder to find for a computer program than a short password, even though it uses numbers and other characters?