tangle-network / cggmp-threshold-ecdsa

MPC protocols for threshold ECDSA
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[SPEC] Converting Shamir Shares to Additive Shares #4

Closed akileshtangella closed 1 year ago

akileshtangella commented 2 years ago

Say you have $t$ Shamir secret shares of a $t-1$ degree polynomial $(x_{i1},...,x{i_{t}})$ and a corresponding secret $x$.

Let the corresponding Lagrange basis polynomials be $(l_{i1},...,l{i_t})$.

Then: $$x = (x_{i1},...,x{i{t}}) \cdot (l{i1}(0),...,l{i_t}(0))$$

So to convert the $xi$ to additive shares simply multiply by $l{i}(0)$. Where do we get the Lagrange basis polynomials evaluated at 0...Zengo has already done it for us here. So we don't have to reimplement.