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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 1.5 and up.

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package org.bouncycastle.crypto.signers;

import java.math.BigInteger;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DSA;
import org.bouncycastle.crypto.DSAExt;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.Signer;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.params.ParametersWithRandom;

public class DSADigestSigner
    implements Signer
{
    private final DSA dsa;
    private final Digest digest;
    private final DSAEncoding encoding;
    private boolean forSigning;

    public DSADigestSigner(
        DSA     dsa,
        Digest  digest)
    {
        this.dsa = dsa;
        this.digest = digest;
        this.encoding = StandardDSAEncoding.INSTANCE;
    }

    public DSADigestSigner(
        DSAExt      dsa,
        Digest      digest,
        DSAEncoding encoding)
    {
        this.dsa = dsa;
        this.digest = digest;
        this.encoding = encoding;
    }

    public void init(
        boolean           forSigning,
        CipherParameters   parameters)
    {
        this.forSigning = forSigning;

        AsymmetricKeyParameter k;

        if (parameters instanceof ParametersWithRandom)
        {
            k = (AsymmetricKeyParameter)((ParametersWithRandom)parameters).getParameters();
        }
        else
        {
            k = (AsymmetricKeyParameter)parameters;
        }

        if (forSigning && !k.isPrivate())
        {
            throw new IllegalArgumentException("Signing Requires Private Key.");
        }

        if (!forSigning && k.isPrivate())
        {
            throw new IllegalArgumentException("Verification Requires Public Key.");
        }

        reset();

        dsa.init(forSigning, parameters);
    }

    /**
     * update the internal digest with the byte b
     */
    public void update(
        byte input)
    {
        digest.update(input);
    }

    /**
     * update the internal digest with the byte array in
     */
    public void update(
        byte[]  input,
        int     inOff,
        int     length)
    {
        digest.update(input, inOff, length);
    }

    /**
     * Generate a signature for the message we've been loaded with using
     * the key we were initialised with.
     */
    public byte[] generateSignature()
    {
        if (!forSigning)
        {
            throw new IllegalStateException("DSADigestSigner not initialised for signature generation.");
        }

        byte[] hash = new byte[digest.getDigestSize()];
        digest.doFinal(hash, 0);

        BigInteger[] sig = dsa.generateSignature(hash);

        try
        {
            return encoding.encode(getOrder(), sig[0], sig[1]);
        }
        catch (Exception e)
        {
            throw new IllegalStateException("unable to encode signature");
        }
    }

    public boolean verifySignature(
        byte[] signature)
    {
        if (forSigning)
        {
            throw new IllegalStateException("DSADigestSigner not initialised for verification");
        }

        byte[] hash = new byte[digest.getDigestSize()];
        digest.doFinal(hash, 0);

        try
        {
            BigInteger[] sig = encoding.decode(getOrder(), signature);

            return dsa.verifySignature(hash, sig[0], sig[1]);
        }
        catch (Exception e)
        {
            return false;
        }
    }

    public void reset()
    {
        digest.reset();
    }

    protected BigInteger getOrder()
    {
        return dsa instanceof DSAExt ? ((DSAExt)dsa).getOrder() : null;
    }
}