What does modular inversion mean? I'm trying to implement an e-voting algorithm, which is described at the paper "Internet Voting Protocol Based on Improved Implicit Security" by Abhishek Parakh &amp; Subhash Kak.

At the Example 1 described in the paper where we have m1=85, at the step 2 it says

"2. Choose randomly and uniformly a number r1 = 101
and compute r2 = m1*(r1)^-1 = 85*(101)^-1=85*28=67mod257"

How is this possible? I mean how can 101^-1 be 28?
Notice that the result says 67 mod 257.
All calculations here are being done modulo 257. So, $101^{-1}$ is actually the modular inverse of $101 \bmod 257$, which is 28. Similarly, $85 \cdot 28$ is also done modulo 257.