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108 lines
2.5 KiB
Go
108 lines
2.5 KiB
Go
//
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// Copyright Coinbase, Inc. All Rights Reserved.
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//
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// SPDX-License-Identifier: Apache-2.0
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//
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package v1
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import (
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"crypto/elliptic"
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"encoding/binary"
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"fmt"
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"source.quilibrium.com/quilibrium/monorepo/nekryptology/pkg/core/curves"
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)
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// Feldman Verifiable Secret Sharing Scheme
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type Feldman struct {
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threshold, limit uint32
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curve elliptic.Curve
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}
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// FeldmanResult contains all the data from calling Split
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type FeldmanResult struct {
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SecretShares []*ShamirShare
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Verifiers []*ShareVerifier
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}
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func NewFeldman(threshold, limit uint32, curve elliptic.Curve) (*Feldman, error) {
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if limit < threshold {
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return nil, fmt.Errorf("limit cannot be less than threshold")
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}
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if threshold < 2 {
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return nil, fmt.Errorf("threshold must be at least 2")
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}
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return &Feldman{
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threshold, limit, curve,
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}, nil
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}
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func (f Feldman) Split(secret []byte) ([]*ShareVerifier, []*ShamirShare, error) {
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field := curves.NewField(f.curve.Params().N)
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shamir := Shamir{f.threshold, f.limit, field}
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shares, poly, err := shamir.GetSharesAndPolynomial(secret)
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if err != nil {
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return nil, nil, err
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}
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// Generate the verifiable commitments to the polynomial for the shares
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verifiers := make([]*ShareVerifier, len(poly.Coefficients))
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for i, c := range poly.Coefficients {
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v, err := curves.NewScalarBaseMult(f.curve, c.Value)
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if err != nil {
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return nil, nil, err
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}
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verifiers[i] = v
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}
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return verifiers, shares, nil
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}
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func (f Feldman) Combine(shares ...*ShamirShare) ([]byte, error) {
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field := curves.NewField(f.curve.Params().N)
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shamir := Shamir{f.threshold, f.limit, field}
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return shamir.Combine(shares...)
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}
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// Verify checks a share for validity
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func (f Feldman) Verify(share *ShamirShare, verifiers []*ShareVerifier) (bool, error) {
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if len(verifiers) < int(f.threshold) {
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return false, fmt.Errorf("not enough verifiers to check")
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}
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field := curves.NewField(f.curve.Params().N)
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xBytes := make([]byte, 4)
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binary.BigEndian.PutUint32(xBytes, share.Identifier)
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x := field.ElementFromBytes(xBytes)
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i := share.Value.Modulus.One()
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// c_0
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rhs := verifiers[0]
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// Compute the sum of products
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// c_0 * c_1^i * c_2^{i^2} * c_3^{i^3} ... c_t^{i_t}
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for j := 1; j < len(verifiers); j++ {
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// i *= x
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i = i.Mul(x)
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c, err := verifiers[j].ScalarMult(i.Value)
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if err != nil {
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return false, err
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}
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// ... * c_j^{i^j}
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rhs, err = rhs.Add(c)
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if err != nil {
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return false, err
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}
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}
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lhs, err := curves.NewScalarBaseMult(f.curve, share.Value.Value)
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if err != nil {
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return false, err
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}
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// Check if lhs == rhs
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return lhs.Equals(rhs), nil
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}
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