Inversion

This functionality takes a shared value $[x]$ and returns an uniform random sharing of $x^{-1}$. We will denote it as $[x^{-1}]$. It is implemented as follows:

1. $[r \in_R \mathbb{F_p}]$.
2. Open $[rx]$.
3. $[x^{-1}] = (rx)^{-1} [r]$.