I was looking at an algorithm somebody came up with, and it invovled an inverse modulo function. I had not come across this before so it really grabbed by attention as I tried to understand what it does.
This page on Khans Academy seemed like a very useful introduction to the process.
In, short the multiplicative modular inverse iA of a number A for a modulo C is defined as
(A * iA) mod C = 1
Sometimes the modular inverse does not exist though, both A and C need to be coprime.
The interesting thing about the modular inverse is that it returns a value within the range C that will be unique.
This allows it to be used as a hashing or an obfuscation method that can be implemented fairly cheaply.