# Questions tagged [group-theory]

Groups are an abstract algebraic concept based on a set and a group law (a binary function which closes the set).

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### Generation of the order $\lambda$ (which is lcm((p-1),(q-1))) element g in modified paillier, why $-a^{2n}$?

As the question states, in variants of paillier cryptosystem, such as CS01 and DT-PKC, when they want an element $g$ of order $\lambda$, they choose a random number $a$ from group $Z^*_{n^2}$ and ...
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### How to find integer point of a ec curve in a given range?

I was looking inside the basics of ecc and found the examples from Internet either uses continuous domain curve or use a very small prime number p like 17 in a ...
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### Setting up the discrete logarithm framework

The discrete logarithm problem over prime cyclic groups consist of finding $x$ satisfying $g^x\equiv h\bmod p$ where $g$ is generator of multiplicative group $\mathbb Z/p\mathbb Z$ at a large prime $p$...
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### Does Poly1305 have weak keys like GCM/GHASH?

Some block cipher keys are weak when used with GCM; see this question. This happens when the multiplier $H$ decided by the key ends up in a small-order subgroup of $\mathbb{F}_{2^{128}}$. Poly1305 ...
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### Why does Index Calculus work?

I understand how the Index Calculus algorithm works - I know & understand the steps. I understand how the steps are derived. However, I am not able to figure out why it works. I can understand why ...
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### what does "product of two cyclic groups" mean

I am reading "Elliptic curve cryptosystems" and the link is here（https://www.ams.org/journals/mcom/1987-48-177/S0025-5718-1987-0866109-5/S0025-5718-1987-0866109-5.pdf）. I don't understand ...
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### A finite group with a threshold functionality

I am trying to find a generator of a finite group that its powers devides the group into two parts. For example look at the last row of this table that shows the powers of 10 in the group Z_19. You ...
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### Using random invertible matrices over finite fields to define the hash of a list

This is a follow-up to a prior question Does matrix multiplication of hash digests admit manipulation of the result?; this formulation failed because it admitted singular matrices and therefore ...
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### Checking whether a particular group has an efficient, faithful representation as a matrix group

There are cryptographic protocols being developed for non-abelian groups. For some protocols it is necessary to know whether the group has an efficient representation as a matrix group (say, a matrix ...
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### Representation theory in cryptography/coding theory

How can representation theory be used in cryptography and/or coding theory? I am studying a MSc in pure mathematics and I am currently working on things related to biset functors, but cryptography and ...
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### How is a generator found for a group, both in case of DH & ECDH?

First step in DH & ECDH is to choose a random prime $p$. Then you choose a generator $g$ for the group $\mathbb Z_p^*$. How do you find a generator? Likewise in ECDH, you would need to find a ...
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### Practical use of semi direct product of group in Cryptography

Is there any example of semi-direct product used in any of cryptographic application? Does it help in choosing integers p, q in RSA?
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### How to get the order of a group generator in DH?

For a DH parameter prime, if the generator $g$ is 2, how do I get the order $q$?
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### Equality with Bilinear Maps

Let e be a bilinear pairing function and $g_1$ and $g_2$ be the generators of $G_1$ and $G_2$ Given $e(a,g_2^x), e(b,g_2^y), g_2^x,g_2^y$ is there a way to find out if $a = b$?
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### Is it hard to determine an automorphism when the mapped value by several compositions of the automorphism is given

Generalization of the Discrete logarithm problem to non-abelian groups is discussed by many authors. One of the generalizations is shown in MOR cryptosystem as in the below link, by considering the ...
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### Why Abstract Algebra in Cryptography?

I come from an engineering background, but not from computer science or anything to do with pure math. I studied applied math in college--never abstract algebra, number theory, or discrete math, ...
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### Solving Diffie-Hellman vs DLP

I'm wondering what is the current knowledge regarding the difficulty of solving the Diffie-Hellman problem (DHP). Obvisously solving the DLP (discrete log) is at least as hard as solving the DH ...
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### Why use prime number $q$ such $q$| $(p-1)$ in discrete logarithm based schemes?

In discrete logarithm based schemes on finite field we have a prime number $q$ that divides $p-1$ and $q$ is to specify a subgroup with the order $q$. But why do we do that? Why do not we work on the ...
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### Why is the point at infinity on Edwards Curve different to Weierstrass curves?

If I understand correctly, the identity point on all elliptic curves is the point at infinity. But on the Edwards curve, this can be written in Affine form? Does this have something to do with the ...
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### Schoofs Algorithm

I studied Schoofs Algorithm described by Washington. On page 125 he says that we could write $y'/y$ as a function of $x$, which makes sense since earlier on the page he denotes $y'= r_{2,j}(x)y$. But ...
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### Message mapping to elliptic curve in BLS signature

In the BLS signature the subgroup $G$ of elliptic curve constructed with point $P$ with prime order $q$ by $G=\langle P\rangle$. The $h(x)$ is a hash function. The point $S$ is map (image) of $h(m)$ ...
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### Safe primes and subgroups

I've been reading about safe primes and their use in: Cryptography Engineering by Niels Ferguson, Bruce Schneier, and Tadayoshi Kohno. Having a safe prime $q$ with $q=2p+1$ where $p$ is a Sophie ...
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### How do I generate a number from a Z* order q set

I would like to generate a random number based on this set, firstly how do I generate the numbers that belong to the Z* order q set. I have the q value and the prime p value. Also if there's a ...
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### RSA assumption and relationship given by generating elemts of a Cayley graph

I have read a very interesting description of computation related to the RSA group as follows. "By the Chinese remainder theorem, we have that: (\mathbb{Z}/pq\mathbb{Z})^* \cong (\mathbb{Z}/p\...
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### Collusion resistance in proxy re-encryption scheme depending on bilinear map rules

A proxy re-encryption scheme is collusion resistant, if the proxy and a delegatee are not able to recover the secret key of the delegator. For example, when we have a message that was originally ...
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### Computing discrete logarithms in the subgroup generated by 1 + N

When I read about DLP, I found that there are groups where the computation is easy. I found that it is known that computing discrete logarithms in the subgroup generated by 1 + N is easy. For example :...
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### Mapping a value $g^x \bmod p$ to a small interval $[1...H]$

My question is in $\mathbb{Z}_p^{*}$ context, where $p=q\cdot k+1$ for two primes $p,q$ and $k \in \mathbb{Z}$; $g$ is the generator of the subgroup $G_q$ of $\mathbb{Z}_p^{*}$, of order $q$. Let's ...
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### Are there any zero knowledge protocols which do not rely on a Group?

To me (new), it seems that a lot of cryptography relies on group theory. Are there any zero knowledge protocols which do not rely on a group?
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### RSA and the strong RSA assumptions

The RSA assumption: Given a randomly generated RSA modulus $n$, exponent $r$ and a random $z \in \mathbb{Z}_n^{*}$, find $y$ such that $y^r=z$. The strong RSA assumption: Given a randomly chosen ...
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### Prime numbers of the form $(2^k)p+1$, for a given prime $p$

Let $p$ be a prime. (say 256 bit) Does there a exist a prime $q$ such that $q = (2^k)p + 1$, for a large $k$ (something like 256), if it does exist, is there a way to find out for which all $k$ such ...
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### Relationship between generating elements given by cycles in Cayley graph

The strong RSA assumption is that the following problem is hard to solve. "Given a randomly chosen RSA modulus $n$ and a random $z \in \mathbb{Z}_n^*$, find $r>1$ and $y \in \mathbb{Z}_n^*$ such ...
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### Understanding the groups used in bilinear Ate-pairing

The bilinear ate pairing $e:G_1\times G_2 \rightarrow G_T$ is defined over the following groups: \begin{equation} \begin{aligned} & G_1 = E(\mathbb{F}_p)[r] \cap Ker(\pi_p-), \\ & G_2 = E(...
Decaf is a point compression method that builds a prime-order group for (twisted) Edwards curves and Montgomery curves with cofactor $h = 4$ based on the Jacobi quartic [H2015]. The promise is to ...