meerkat-java/destributed-key-generation/src/main/java/FeldmanVerifiableSecretSharing/VerifiableSecretSharing.java

package FeldmanVerifiableSecretSharing;

import ShamirSecretSharing.SecretSharing;
import org.bouncycastle.util.Arrays;
import org.factcenter.qilin.primitives.CyclicGroup;
import org.factcenter.qilin.primitives.concrete.Zpstar;

import java.math.BigInteger;
import java.util.Random;

/**
 * Created by Tzlil on 1/27/2016.
 */
public class VerifiableSecretSharing extends SecretSharing {

    private final CyclicGroup<BigInteger> group;
    private final BigInteger g; // public generator of group
    private final BigInteger[] commitments;


    /**
     * @param group a cyclic group of prime order p.
     *              it must be chosen such that computing discrete logarithms is hard in this group.
     */
    public VerifiableSecretSharing(CyclicGroup<BigInteger> group, int t, int n, BigInteger s, Random random) {
        super(group.orderUpperBound(), t, n, s, random);
        this.group = group;
        this.g = group.getGenerator();
        this.commitments = generateCommitments();
    }

    /**
     * @return commitments[i] = g ^ polynomial.coefficients[i]
     */
    private BigInteger[] generateCommitments() {
        BigInteger[] coefficients = polynomial.getCoefficients();
        BigInteger[] commitments = new BigInteger[coefficients.length];
        for (int i = 0 ; i < commitments.length;i++){
            commitments[i] = group.multiply(g,coefficients[i]);
        }
        return commitments;
    }

    /**
     * @param i share holder id
     * @param commitments
     * @param group
     *
     * @return product of commitments[j] ^ (i ^ j) == g ^ polynomial(i)
     */
    public static BigInteger verify(int i,BigInteger[] commitments,CyclicGroup<BigInteger> group) {
        BigInteger v = group.zero();
        int power = 1;
        for (int j = 0 ; j < commitments.length ; j ++){
            v = group.add(v,commitments[i].pow(power));
            power *=i;
        }
        return v;
    }


    /**
     * getter
     * @return generator of group
     */
    public BigInteger getGenerator() {
        return g;
    }

    /**
     * getter
     * @return copy of commitments
     */
    public BigInteger[] getCommitments() {
        return Arrays.clone(commitments);
    }
}