## Basic Group Theory

Multiplicative inverses modulo $m$ The multiplicative group of integers modulo $m$ is defined as: \begin{align} \Z_m^* = \{a\ |\ \gcd(a,m) = 1\} \end{align} But why? This is because Euler’s theorem says that: \begin{align} \gcd(a,m) = 1\Rightarrow a^{\phi(m)} = 1 \end{align} This in turn, implies that every element in $\Z_m^*$ has an invers...

## What is a Merkle tree?

tl;dr: In this post, we demystify Merkle trees for beginners. We give simple, illustrated explanations, we detail applications they are used in and reason about their security formally. For more details, see this full post on Decentralized Thoughts.

## RSA Accumulators

An RSA accumulator is an authenticated set built from cryptographic assumptions in hidden-order groups such as $\mathbb{Z}_N^*$. RSA accumulators enable a prover, who stores the full set, to convince any verifier, who only stores a succinct digest of the set, of various set relations such as (non)membership, subset or disjointness. For example, ...

A vector commitment (VC) scheme allows a prover with access to a vector $\mathbf{v} = [ v_1, \dots, v_n ]$ to convince any verifier that position $i$ in $\mathbf{v}$ stores $v_i$ for any index $i\in[n]$. Importantly, verifers only store a succinct digest of the vector (e.g., a 32-byte hash) rather than the full vector $\mathbf{v}$.
Equations of the form $\sum_i a_i x_i = 0$ where the $x_i$’s are integer unknowns are called linear Diophantine equations. Their integer solutions can be computed using greatest common denominator (GCD) tricks. In this post, we go over a few basic types of such equations and their integer solutions.