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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 1.8 and up.
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package org.bouncycastle.jcajce.provider.asymmetric.ecgost12;

import java.math.BigInteger;
import java.security.AlgorithmParameters;
import java.security.InvalidKeyException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SignatureException;
import java.security.spec.AlgorithmParameterSpec;

import org.bouncycastle.asn1.pkcs.PKCSObjectIdentifiers;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.asn1.x509.X509ObjectIdentifiers;
import org.bouncycastle.crypto.DSAExt;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.digests.GOST3411_2012_256Digest;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.params.ECKeyParameters;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.crypto.signers.ECGOST3410Signer;
import org.bouncycastle.jcajce.provider.asymmetric.util.ECUtil;
import org.bouncycastle.jce.interfaces.ECKey;
import org.bouncycastle.jce.interfaces.ECPublicKey;
import org.bouncycastle.jce.provider.BouncyCastleProvider;

/**
 * Signature for GOST34.10 2012 256. Algorithm is the same as for GOST34.10 2001
 */
public class ECGOST2012SignatureSpi256
    extends java.security.SignatureSpi
    implements PKCSObjectIdentifiers, X509ObjectIdentifiers
{
    private Digest                  digest;
    private DSAExt                  signer;
    private int size = 64;
    private int halfSize = size/2;

    public ECGOST2012SignatureSpi256()
    {
        this.digest = new GOST3411_2012_256Digest();
        this.signer = new ECGOST3410Signer();
    }

    protected void engineInitVerify(
        PublicKey   publicKey)
        throws InvalidKeyException
    {
        ECKeyParameters    param;

        if (publicKey instanceof ECPublicKey)
        {
            param = (ECKeyParameters)generatePublicKeyParameter(publicKey);
        }
        else
        {
            try
            {
                byte[]  bytes = publicKey.getEncoded();

                publicKey = BouncyCastleProvider.getPublicKey(SubjectPublicKeyInfo.getInstance(bytes));

                param = (ECKeyParameters)ECUtil.generatePublicKeyParameter(publicKey);
            }
            catch (Exception e)
            {
                throw new InvalidKeyException("cannot recognise key type in ECGOST-2012-256 signer");
            }
        }

        if (param.getParameters().getN().bitLength() > 256)
        {
            throw new InvalidKeyException("key out of range for ECGOST-2012-256");
        }

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

    protected void engineInitSign(
        PrivateKey  privateKey)
        throws InvalidKeyException
    {
        ECKeyParameters param;

        if (privateKey instanceof ECKey)
        {
            param = (ECKeyParameters)ECUtil.generatePrivateKeyParameter(privateKey);
        }
        else
        {
            throw new InvalidKeyException("cannot recognise key type in ECGOST-2012-256 signer");
        }

        if (param.getParameters().getN().bitLength() > 256)
        {
            throw new InvalidKeyException("key out of range for ECGOST-2012-256");
        }
        
        digest.reset();

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

    protected void engineUpdate(
        byte    b)
        throws SignatureException
    {
        digest.update(b);
    }

    protected void engineUpdate(
        byte[]  b,
        int     off,
        int     len) 
        throws SignatureException
    {
        digest.update(b, off, len);
    }

    protected byte[] engineSign()
        throws SignatureException
    {
        byte[]  hash = new byte[digest.getDigestSize()];

        digest.doFinal(hash, 0);

        try
        {
            byte[]          sigBytes = new byte[size];
            BigInteger[]    sig = signer.generateSignature(hash);
            byte[]          r = sig[0].toByteArray();
            byte[]          s = sig[1].toByteArray();

            if (s[0] != 0)
            {
                System.arraycopy(s, 0, sigBytes, halfSize - s.length, s.length);
            }
            else
            {
                System.arraycopy(s, 1, sigBytes, halfSize - (s.length - 1), s.length - 1);
            }
            
            if (r[0] != 0)
            {
                System.arraycopy(r, 0, sigBytes, size - r.length, r.length);
            }
            else
            {
                System.arraycopy(r, 1, sigBytes, size - (r.length - 1), r.length - 1);
            }

            return sigBytes;
        }
        catch (Exception e)
        {
            throw new SignatureException(e.toString());
        }
    }
    
    protected boolean engineVerify(
        byte[]  sigBytes) 
        throws SignatureException
    {
        byte[]  hash = new byte[digest.getDigestSize()];

        digest.doFinal(hash, 0);

        BigInteger[]    sig;

        try
        {
            byte[] r = new byte[halfSize];
            byte[] s = new byte[halfSize];

            System.arraycopy(sigBytes, 0, s, 0, halfSize);

            System.arraycopy(sigBytes, halfSize, r, 0, halfSize);
            
            sig = new BigInteger[2];
            sig[0] = new BigInteger(1, r);
            sig[1] = new BigInteger(1, s);
        }
        catch (Exception e)
        {
            throw new SignatureException("error decoding signature bytes.");
        }

        return signer.verifySignature(hash, sig[0], sig[1]);
    }

    protected void engineSetParameter(
        AlgorithmParameterSpec params)
    {
        throw new UnsupportedOperationException("engineSetParameter unsupported");
    }

    protected AlgorithmParameters engineGetParameters()
    {
        return null;
    }

    /**
     * @deprecated replaced with "#engineSetParameter(java.security.spec.AlgorithmParameterSpec)"
     */
    protected void engineSetParameter(
        String  param,
        Object  value)
    {
        throw new UnsupportedOperationException("engineSetParameter unsupported");
    }

    /**
     * @deprecated
     */
    protected Object engineGetParameter(
        String      param)
    {
        throw new UnsupportedOperationException("engineSetParameter unsupported");
    }

    static AsymmetricKeyParameter generatePublicKeyParameter(
            PublicKey key)
    throws InvalidKeyException
    {
        return (key instanceof BCECGOST3410_2012PublicKey) ? ((BCECGOST3410_2012PublicKey)key).engineGetKeyParameters() : ECUtil.generatePublicKeyParameter(key);
    }
}