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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 and up. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.crypto.signers;

import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.CryptoServicesRegistrar;
import org.bouncycastle.crypto.DSAExt;
import org.bouncycastle.crypto.params.ECDomainParameters;
import org.bouncycastle.crypto.params.ECKeyParameters;
import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
import org.bouncycastle.crypto.params.ECPublicKeyParameters;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.math.ec.ECAlgorithms;
import org.bouncycastle.math.ec.ECConstants;
import org.bouncycastle.math.ec.ECMultiplier;
import org.bouncycastle.math.ec.ECPoint;
import org.bouncycastle.math.ec.FixedPointCombMultiplier;
import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.BigIntegers;

/**
 * GOST R 34.10-2001 Signature Algorithm
 */
public class ECGOST3410Signer
    implements DSAExt
{
    ECKeyParameters key;

    SecureRandom    random;

    public void init(
        boolean                 forSigning,
        CipherParameters        param)
    {
        if (forSigning)
        {
            if (param instanceof ParametersWithRandom)
            {
                ParametersWithRandom    rParam = (ParametersWithRandom)param;

                this.random = rParam.getRandom();
                this.key = (ECPrivateKeyParameters)rParam.getParameters();
            }
            else
            {
                this.random = CryptoServicesRegistrar.getSecureRandom();
                this.key = (ECPrivateKeyParameters)param;
            }
        }
        else
        {
            this.key = (ECPublicKeyParameters)param;
        }
    }

    public BigInteger getOrder()
    {
        return key.getParameters().getN();
    }

    /**
     * generate a signature for the given message using the key we were
     * initialised with. For conventional GOST3410 the message should be a GOST3411
     * hash of the message of interest.
     *
     * @param message the message that will be verified later.
     */
    public BigInteger[] generateSignature(
        byte[] message)
    {
        byte[] mRev = Arrays.reverse(message); // conversion is little-endian
        BigInteger e = new BigInteger(1, mRev);

        ECDomainParameters ec = key.getParameters();
        BigInteger n = ec.getN();
        BigInteger d = ((ECPrivateKeyParameters)key).getD();

        BigInteger r, s;

        ECMultiplier basePointMultiplier = createBasePointMultiplier();

        do // generate s
        {
            BigInteger k;
            do // generate r
            {
                do
                {
                    k = BigIntegers.createRandomBigInteger(n.bitLength(), random);
                }
                while (k.equals(ECConstants.ZERO));

                ECPoint p = basePointMultiplier.multiply(ec.getG(), k).normalize();

                r = p.getAffineXCoord().toBigInteger().mod(n);
            }
            while (r.equals(ECConstants.ZERO));

            s = (k.multiply(e)).add(d.multiply(r)).mod(n);
        }
        while (s.equals(ECConstants.ZERO));

        return new BigInteger[]{ r, s };
    }

    /**
     * return true if the value r and s represent a GOST3410 signature for
     * the passed in message (for standard GOST3410 the message should be
     * a GOST3411 hash of the real message to be verified).
     */
    public boolean verifySignature(
        byte[]      message,
        BigInteger  r,
        BigInteger  s)
    {
        byte[] mRev = Arrays.reverse(message); // conversion is little-endian
        BigInteger e = new BigInteger(1, mRev);
        BigInteger n = key.getParameters().getN();

        // r in the range [1,n-1]
        if (r.compareTo(ECConstants.ONE) < 0 || r.compareTo(n) >= 0)
        {
            return false;
        }

        // s in the range [1,n-1]
        if (s.compareTo(ECConstants.ONE) < 0 || s.compareTo(n) >= 0)
        {
            return false;
        }

        BigInteger v = BigIntegers.modOddInverseVar(n, e);

        BigInteger z1 = s.multiply(v).mod(n);
        BigInteger z2 = (n.subtract(r)).multiply(v).mod(n);

        ECPoint G = key.getParameters().getG(); // P
        ECPoint Q = ((ECPublicKeyParameters)key).getQ();

        ECPoint point = ECAlgorithms.sumOfTwoMultiplies(G, z1, Q, z2).normalize();

        // components must be bogus.
        if (point.isInfinity())
        {
            return false;
        }

        BigInteger R = point.getAffineXCoord().toBigInteger().mod(n);

        return R.equals(r);
    }

    protected ECMultiplier createBasePointMultiplier()
    {
        return new FixedPointCombMultiplier();
    }
}