# Shamir's Secret Sharing of modular inverse

Given a secret $K \bmod q$ which is shared among entities $E_1,…,E_n$ using polynomial Shamir's Secret Sharing, how can the inverse of $k$ be shared without revealing $k$ and $k^{-1}$?

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I'm assuming $q$ is prime.

Note that $k^{-1}\equiv k^{q-2}\bmod{q}$.

We can compute $k^{q-2}\bmod{q}$ via a multiparty computation protocol such as the one outlined in this paper ($\S3.1)$. Or, since $q$ is likely a public parameter, you can even do the simple square-and-multiply algorithm (for example, VIFF provides this capability).

You might also be able to compute the modular inverse via the extended euclidean algorithm implemented using general MPC techniques. The reason I say might is that the extended euclidean algorithm does have a conditional branch in it. While this is technically not a problem for MPC, it could leak information in the real world that you wouldn't want to leak.

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