# Tagged Questions

Questions on cryptography and cryptanalysis, encryption and decryption, and the making and breaking of codes and ciphers. Consider posting your question at Cryptography.SE.

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### Number of Points on an Elliptic Curve

If I have an elliptic curve $$E: y^2 = x^3 + bx + c$$, with $b, c$ integers mod some prime $p$. And $x^3 + bx + c$ has at least one root mod $p$. How can I show that the number of points on the ...
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### Need clarity on calculating the y coordinate in elliptic curve cryptography

I'm just new to elliptic curve cryptography. I have been working on RSA for quite some time. Moreover I'm not from a mathematical background. The whole concept looks very complex. So tell me my ...
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### Determine if an algorithm has solutions or not

I m actually studying Turing works concerning Enigma's machine. Actually, I am wondering if it does exist anything, an algorithm or something that allows people to tlel if an algorithm can be break ...
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### Find element with order 12 of multiplicative group using CRT

I have been stuck on this question for a long time and don't really understand how the Chinese remainder the is related to the order of a unit. Use the Chinese Remainder Theorem to find an element ...
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### What is the advantage or disadvantage of Pollards rho algorithm compare to Parallel Substitution or Quantum Algorithm?

I’m trying to combine the two algorithms but for the mean time I want to know your opinion or suggestion. I use my spare time to create a Pollards rho algorithm using basic formula in spreadsheet to ...
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### Question related to the security of RSA method

I learned about the RSA method, where if B wants to send a message $M$, say $0 \leq M <n = pq$ to A with public key $(n,e)$, then B sends $M'= M^e (mod \ n)$. Then A can decode this message using ...
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### Understanding Quadratic Residue Modulo n Structure

Quadratic Residue Modulo n: $a \in \mathbb Z_n^*$ is quadatic residue of modulo n if there exists an element $x \in \mathbb Z_n^*$ such that $$x^2 \equiv a \mod n$$ I'm not getting the intuition ...
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### Prove entropy theorem

Doing a course of cryptography I have been asked to prove the following: $H(X,Y) = H(Y) +H(X|Y)$. But I simply do not know where to start, so a hint in the right direction would be very much ...
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### How to expand Diffie-Hellman key exchange for multiple users?

To provide OTR (off the record) security for XMPP group chats we've discussed an idea for a Diffie-Hellman key exchange algorithm for multiple users. It should work as follows: Choose a cyclic group ...
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### Efficiency of McEliece Cryptography

Most of the sources say McEliece has never gained acceptance because of its large size of private and public keys. However I have never heard about the size (or length) of its ciphertext. For example, ...
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### Security of such cryptosystem design?

Is one able to reveal $m$ when $$С = (m + r)^e \bmod N$$ $C$ is known $r$ is known $e$ is known $N$ is known and not prime
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### Converting Walsh coefficients to values of a function

I assume I know the Walsh coefficients of a function f: $\mathbb{F}_{2^n}$ to $\mathbb{F}_{2}$. Is there any efficient possibility to get the values of the function f ?
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### Type(s) of Hashing function that keeps the ordering information

I am asking this question from a perspective that we need to store a set of hashed data that can be queried later for in an ordered fashion. My situation is that some data has to be encrypted, I am ...
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### Primality of $2^{255}-19$

I need a test for primality that I apply to $2^{255}-19$ (which is claimed to be prime) and certify to be correct with the ACL2 theorem prover. This means that I must be able to code the test in ...
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### How to find the period of a recurrence relation

Given the recurrence relation $s_{i+5}=s_{i+1} + s_i$ over $\mathbb{F}_2$ with initial states $s_0 = 1, s_1 = 1, s_2 = 1, s_3 = 0, s_4 = 1$ What is the best/quickest way to find the period of the ...
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### How do you find the smallest legitimate encryption exponent when you are only give a p and q value in a given range?

I have been given this as an assignment question but I'm not sure approach it. EDIT: Sorry I should have added more details. It is a cryptosystem using the RSA scheme. p and q are both old prime ...
How do I find $b$ and $d$ in the equation: $\begin{bmatrix}6 & 25\\12 & 15\end{bmatrix} \times \begin{bmatrix}2 & b\\5 & d\end{bmatrix}$ = \$\begin{bmatrix}22 & 17\\10 & 22\end{...