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The Bouncy Castle Java APIs for the OpenPGP Protocol. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.

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package org.bouncycastle.openpgp.operator.bc;

import java.io.OutputStream;
import java.security.SecureRandom;

import org.bouncycastle.crypto.CryptoException;
import org.bouncycastle.crypto.Signer;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.openpgp.PGPException;
import org.bouncycastle.openpgp.PGPPrivateKey;
import org.bouncycastle.openpgp.operator.PGPContentSigner;
import org.bouncycastle.openpgp.operator.PGPContentSignerBuilder;
import org.bouncycastle.openpgp.operator.PGPDigestCalculator;
import org.bouncycastle.util.io.TeeOutputStream;

public class BcPGPContentSignerBuilder
    implements PGPContentSignerBuilder
{
    private BcPGPDigestCalculatorProvider digestCalculatorProvider = new BcPGPDigestCalculatorProvider();
    private BcPGPKeyConverter           keyConverter = new BcPGPKeyConverter();
    private int                         hashAlgorithm;
    private SecureRandom                random;
    private int keyAlgorithm;

    public BcPGPContentSignerBuilder(int keyAlgorithm, int hashAlgorithm)
    {
        this.keyAlgorithm = keyAlgorithm;
        this.hashAlgorithm = hashAlgorithm;
    }

    public BcPGPContentSignerBuilder setSecureRandom(SecureRandom random)
    {
        this.random = random;

        return this;
    }

    public PGPContentSigner build(final int signatureType, final PGPPrivateKey privateKey)
        throws PGPException
    {
        AsymmetricKeyParameter privKeyParam = keyConverter.getPrivateKey(privateKey);
        final PGPDigestCalculator digestCalculator = digestCalculatorProvider.get(hashAlgorithm);
        final Signer signer = BcImplProvider.createSigner(keyAlgorithm, hashAlgorithm, privKeyParam);

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

        return new PGPContentSigner()
        {
            public int getType()
            {
                return signatureType;
            }

            public int getHashAlgorithm()
            {
                return hashAlgorithm;
            }

            public int getKeyAlgorithm()
            {
                return keyAlgorithm;
            }

            public long getKeyID()
            {
                return privateKey.getKeyID();
            }

            public OutputStream getOutputStream()
            {
                return new TeeOutputStream(new SignerOutputStream(signer), digestCalculator.getOutputStream());
            }

            public byte[] getSignature()
            {
                try
                {
                    return signer.generateSignature();
                }
                catch (CryptoException e)
                {    // TODO: need a specific runtime exception for PGP operators.
                    throw new IllegalStateException("unable to create signature");
                }
            }

            public byte[] getDigest()
            {
                return digestCalculator.getDigest();
            }
        };
    }
}