Function RecoverCommit

share/poly.go:394–428

RecoverCommit reconstructs the secret commitment p(0) from a list of public shares using Lagrange interpolation.

(g kyber.Group, shares []*PubShare, t, n int)

// RecoverCommit reconstructs the secret commitment p(0) from a list of public
// shares using Lagrange interpolation.
func RecoverCommit(g kyber.Group, shares []*PubShare, t, n int) (kyber.Point, error) {
	x := make(map[int]kyber.Scalar)
	for i, s := range shares {
		if s == nil || s.V == nil || s.I < 0 || n <= s.I {
			continue
		}
		x[i] = g.Scalar().SetInt64(1 + int64(s.I))
	}

	if len(x) < t {
		return nil, errors.New("share: not enough good public shares to reconstruct secret commitment")
	}

	num := g.Scalar()
	den := g.Scalar()
	tmp := g.Scalar()
	Acc := g.Point().Null()
	Tmp := g.Point()

	for i, xi := range x {
		num.One()
		den.One()
		for j, xj := range x {
			if i == j {
				continue
			}
			num.Mul(num, xj)
			den.Mul(den, tmp.Sub(xj, xi))
		}
		Tmp.Mul(num.Div(num, den), shares[i].V)
		Acc.Add(Acc, Tmp)
	}

	return Acc, nil
}