ceremonyclient/nekryptology/pkg/sharing/v1/feldman.go

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2023-07-05 05:32:28 +00:00
//
// Copyright Coinbase, Inc. All Rights Reserved.
//
// SPDX-License-Identifier: Apache-2.0
//
package v1
import (
"crypto/elliptic"
"encoding/binary"
"fmt"
"source.quilibrium.com/quilibrium/monorepo/nekryptology/pkg/core/curves"
)
// Feldman Verifiable Secret Sharing Scheme
type Feldman struct {
threshold, limit uint32
curve elliptic.Curve
}
// FeldmanResult contains all the data from calling Split
type FeldmanResult struct {
SecretShares []*ShamirShare
Verifiers []*ShareVerifier
}
func NewFeldman(threshold, limit uint32, curve elliptic.Curve) (*Feldman, error) {
if limit < threshold {
return nil, fmt.Errorf("limit cannot be less than threshold")
}
if threshold < 2 {
return nil, fmt.Errorf("threshold must be at least 2")
}
return &Feldman{
threshold, limit, curve,
}, nil
}
func (f Feldman) Split(secret []byte) ([]*ShareVerifier, []*ShamirShare, error) {
field := curves.NewField(f.curve.Params().N)
shamir := Shamir{f.threshold, f.limit, field}
shares, poly, err := shamir.GetSharesAndPolynomial(secret)
if err != nil {
return nil, nil, err
}
// Generate the verifiable commitments to the polynomial for the shares
verifiers := make([]*ShareVerifier, len(poly.Coefficients))
for i, c := range poly.Coefficients {
v, err := curves.NewScalarBaseMult(f.curve, c.Value)
if err != nil {
return nil, nil, err
}
verifiers[i] = v
}
return verifiers, shares, nil
}
func (f Feldman) Combine(shares ...*ShamirShare) ([]byte, error) {
field := curves.NewField(f.curve.Params().N)
shamir := Shamir{f.threshold, f.limit, field}
return shamir.Combine(shares...)
}
// Verify checks a share for validity
func (f Feldman) Verify(share *ShamirShare, verifiers []*ShareVerifier) (bool, error) {
if len(verifiers) < int(f.threshold) {
return false, fmt.Errorf("not enough verifiers to check")
}
field := curves.NewField(f.curve.Params().N)
xBytes := make([]byte, 4)
binary.BigEndian.PutUint32(xBytes, share.Identifier)
x := field.ElementFromBytes(xBytes)
i := share.Value.Modulus.One()
// c_0
rhs := verifiers[0]
// Compute the sum of products
// c_0 * c_1^i * c_2^{i^2} * c_3^{i^3} ... c_t^{i_t}
for j := 1; j < len(verifiers); j++ {
// i *= x
i = i.Mul(x)
c, err := verifiers[j].ScalarMult(i.Value)
if err != nil {
return false, err
}
// ... * c_j^{i^j}
rhs, err = rhs.Add(c)
if err != nil {
return false, err
}
}
lhs, err := curves.NewScalarBaseMult(f.curve, share.Value.Value)
if err != nil {
return false, err
}
// Check if lhs == rhs
return lhs.Equals(rhs), nil
}