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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.4.
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package org.bouncycastle.jcajce.provider.asymmetric.ec;

import java.io.IOException;
import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;

import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1Encoding;
import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1Primitive;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DERSequence;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DSA;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.digests.SM3Digest;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.crypto.signers.SM2Signer;
import org.bouncycastle.jcajce.provider.asymmetric.util.DSABase;
import org.bouncycastle.jcajce.provider.asymmetric.util.DSAEncoder;
import org.bouncycastle.jcajce.provider.asymmetric.util.ECUtil;
import org.bouncycastle.util.Arrays;

public class GMSignatureSpi
    extends DSABase
{
    GMSignatureSpi(Digest digest, DSA signer, DSAEncoder encoder)
    {
        super("SM2", digest, signer, encoder);
    }

    protected void engineInitVerify(PublicKey publicKey)
        throws InvalidKeyException
    {
        CipherParameters param = ECUtils.generatePublicKeyParameter(publicKey);

        digest.reset();
        signer.init(false, param);
    }

    protected void doEngineInitSign(
        PrivateKey privateKey,
        SecureRandom random)
        throws InvalidKeyException
    {
        CipherParameters param;

        param = ECUtil.generatePrivateKeyParameter(privateKey);

        digest.reset();

        if (random != null)
        {
            signer.init(true, new ParametersWithRandom(param, random));
        }
        else
        {
            signer.init(true, param);
        }
    }

    static public class sm3WithSM2
        extends GMSignatureSpi
    {
        public sm3WithSM2()
        {
            super(new SM3Digest(), new SM2Signer(), new StdDSAEncoder());
        }
    }
    
    private static class StdDSAEncoder
        implements DSAEncoder
    {
        public byte[] encode(
            BigInteger r,
            BigInteger s)
            throws IOException
        {
            ASN1EncodableVector v = new ASN1EncodableVector();

            v.add(new ASN1Integer(r));
            v.add(new ASN1Integer(s));

            return new DERSequence(v).getEncoded(ASN1Encoding.DER);
        }

        public BigInteger[] decode(
            byte[] encoding)
            throws IOException
        {
            ASN1Sequence s = (ASN1Sequence)ASN1Primitive.fromByteArray(encoding);
            if (s.size() != 2)
            {
                throw new IOException("malformed signature");
            }
            if (!Arrays.areEqual(encoding, s.getEncoded(ASN1Encoding.DER)))
            {
                throw new IOException("malformed signature");
            }

            BigInteger[] sig = new BigInteger[2];


            sig[0] = ASN1Integer.getInstance(s.getObjectAt(0)).getValue();
            sig[1] = ASN1Integer.getInstance(s.getObjectAt(1)).getValue();

            return sig;
        }
    }

    private static class PlainDSAEncoder
        implements DSAEncoder
    {
        public byte[] encode(
            BigInteger r,
            BigInteger s)
            throws IOException
        {
            byte[] first = makeUnsigned(r);
            byte[] second = makeUnsigned(s);
            byte[] res;

            if (first.length > second.length)
            {
                res = new byte[first.length * 2];
            }
            else
            {
                res = new byte[second.length * 2];
            }

            System.arraycopy(first, 0, res, res.length / 2 - first.length, first.length);
            System.arraycopy(second, 0, res, res.length - second.length, second.length);

            return res;
        }


        private byte[] makeUnsigned(BigInteger val)
        {
            byte[] res = val.toByteArray();

            if (res[0] == 0)
            {
                byte[] tmp = new byte[res.length - 1];

                System.arraycopy(res, 1, tmp, 0, tmp.length);

                return tmp;
            }

            return res;
        }

        public BigInteger[] decode(
            byte[] encoding)
            throws IOException
        {
            BigInteger[] sig = new BigInteger[2];

            byte[] first = new byte[encoding.length / 2];
            byte[] second = new byte[encoding.length / 2];

            System.arraycopy(encoding, 0, first, 0, first.length);
            System.arraycopy(encoding, first.length, second, 0, second.length);

            sig[0] = new BigInteger(1, first);
            sig[1] = new BigInteger(1, second);

            return sig;
        }
    }
}