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package shamirsecretsharing
import (
"crypto/rand"
"errors"
"math/big"
)
const (
// bits = 1024
bits = 2048
)
// Create calculates the secrets to share from given parameters
// t: number of secrets needed
// n: number of shares
// p: random point
// k: secret to share
func Create(t, n, p, k *big.Int) (result [][]*big.Int, err error) {
if k.Cmp(p) > 0 {
return nil, errors.New("Error: need k<p. k: " + k.String() + ", p: " + p.String())
}
//generate the basePolynomial
var basePolynomial []*big.Int
basePolynomial = append(basePolynomial, k)
for i := 0; i < int(t.Int64())-1; i++ {
randPrime, err := rand.Prime(rand.Reader, bits/2)
if err != nil {
return result, err
}
basePolynomial = append(basePolynomial, randPrime)
}
//calculate shares, based on the basePolynomial
var shares []*big.Int
for i := 1; i < int(n.Int64())+1; i++ {
var pResultMod *big.Int
pResult := big.NewInt(int64(0))
for x, polElem := range basePolynomial {
if x == 0 {
pResult = pResult.Add(pResult, polElem)
} else {
iBigInt := big.NewInt(int64(i))
xBigInt := big.NewInt(int64(x))
iPowed := iBigInt.Exp(iBigInt, xBigInt, nil)
currElem := iPowed.Mul(iPowed, polElem)
pResult = pResult.Add(pResult, currElem)
pResultMod = pResult.Mod(pResult, p)
}
}
shares = append(shares, pResultMod)
}
//put the share together with his p value
result = packSharesAndI(shares)
return result, nil
}
func packSharesAndI(sharesString []*big.Int) (r [][]*big.Int) {
for i, share := range sharesString {
curr := []*big.Int{share, big.NewInt(int64(i + 1))}
r = append(r, curr)
}
return r
}
func unpackSharesAndI(sharesPacked [][]*big.Int) ([]*big.Int, []*big.Int) {
var shares []*big.Int
var i []*big.Int
for _, share := range sharesPacked {
shares = append(shares, share[0])
i = append(i, share[1])
}
return shares, i
}
// LagrangeInterpolation calculates the secret from given shares
func LagrangeInterpolation(p *big.Int, sharesGiven [][]*big.Int) *big.Int {
resultN := big.NewInt(int64(0))
resultD := big.NewInt(int64(0))
//unpack shares
sharesBigInt, sharesIBigInt := unpackSharesAndI(sharesGiven)
for i := 0; i < len(sharesBigInt); i++ {
lagrangeNumerator := big.NewInt(int64(1))
lagrangeDenominator := big.NewInt(int64(1))
for j := 0; j < len(sharesBigInt); j++ {
if sharesIBigInt[i] != sharesIBigInt[j] {
currLagrangeNumerator := sharesIBigInt[j]
currLagrangeDenominator := new(big.Int).Sub(sharesIBigInt[j], sharesIBigInt[i])
lagrangeNumerator = new(big.Int).Mul(lagrangeNumerator, currLagrangeNumerator)
lagrangeDenominator = new(big.Int).Mul(lagrangeDenominator, currLagrangeDenominator)
}
}
numerator := new(big.Int).Mul(sharesBigInt[i], lagrangeNumerator)
quo := new(big.Int).Quo(numerator, lagrangeDenominator)
if quo.Int64() != 0 {
resultN = resultN.Add(resultN, quo)
} else {
resultNMULlagrangeDenominator := new(big.Int).Mul(resultN, lagrangeDenominator)
resultN = new(big.Int).Add(resultNMULlagrangeDenominator, numerator)
resultD = resultD.Add(resultD, lagrangeDenominator)
}
}
var modinvMul *big.Int
if resultD.Int64() != 0 {
modinv := new(big.Int).ModInverse(resultD, p)
modinvMul = new(big.Int).Mul(resultN, modinv)
} else {
modinvMul = resultN
}
r := new(big.Int).Mod(modinvMul, p)
return r
}