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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;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.bouncycastle.bcpg.SignaturePacket;
import org.bouncycastle.bcpg.UserAttributeSubpacket;

abstract class PGPDefaultSignatureGenerator
{
    protected byte lastb;
    protected OutputStream sigOut;
    protected int sigType;
    protected final int version;

    public PGPDefaultSignatureGenerator(int version)
    {
        this.version = version;
    }
    
    public void update(
        byte b)
    {
        if (sigType == PGPSignature.CANONICAL_TEXT_DOCUMENT)
        {
            if (b == '\r')
            {
                byteUpdate((byte)'\r');
                byteUpdate((byte)'\n');
            }
            else if (b == '\n')
            {
                if (lastb != '\r')
                {
                    byteUpdate((byte)'\r');
                    byteUpdate((byte)'\n');
                }
            }
            else
            {
                byteUpdate(b);
            }

            lastb = b;
        }
        else
        {
            byteUpdate(b);
        }
    }

    public void update(
        byte[] b)
    {
        this.update(b, 0, b.length);
    }

    public void update(
        byte[]  b,
        int     off,
        int     len)
    {
        if (sigType == PGPSignature.CANONICAL_TEXT_DOCUMENT)
        {
            int finish = off + len;

            for (int i = off; i != finish; i++)
            {
                this.update(b[i]);
            }
        }
        else
        {
            blockUpdate(b, off, len);
        }
    }

    private void byteUpdate(byte b)
    {
        try
        {
            sigOut.write(b);
        }
        catch (IOException e)
        {
            throw new PGPRuntimeOperationException(e.getMessage(), e);
        }
    }

    protected void blockUpdate(byte[] block, int off, int len)
    {
        try
        {
            sigOut.write(block, off, len);
        }
        catch (IOException e)
        {
            throw new PGPRuntimeOperationException("unable to update signature: " + e.getMessage(), e);
        }
    }

    protected void updateWithIdData(int header, byte[] idBytes)
    {
        this.update((byte)header);
        this.update((byte)(idBytes.length >> 24));
        this.update((byte)(idBytes.length >> 16));
        this.update((byte)(idBytes.length >> 8));
        this.update((byte)(idBytes.length));
        this.update(idBytes);
    }

    protected void updateWithPublicKey(PGPPublicKey key)
        throws PGPException
    {
        byte[] keyBytes = getEncodedPublicKey(key);

        if (version == SignaturePacket.VERSION_4)
        {
            this.update((byte) 0x99);
            this.update((byte) (keyBytes.length >> 8));
            this.update((byte) (keyBytes.length));
        }
        else if (version == SignaturePacket.VERSION_5)
        {
            this.update((byte) 0x9A);
            this.update((byte) (keyBytes.length >> 24));
            this.update((byte) (keyBytes.length >> 16));
            this.update((byte) (keyBytes.length >> 8));
            this.update((byte) (keyBytes.length));
        }
        else if (version == SignaturePacket.VERSION_6)
        {
            this.update((byte) 0x9B);
            this.update((byte) (keyBytes.length >> 24));
            this.update((byte) (keyBytes.length >> 16));
            this.update((byte) (keyBytes.length >> 8));
            this.update((byte) (keyBytes.length));
        }
        this.update(keyBytes);
    }

    private byte[] getEncodedPublicKey(
        PGPPublicKey pubKey)
        throws PGPException
    {
        byte[] keyBytes;

        try
        {
            keyBytes = pubKey.publicPk.getEncodedContents();
        }
        catch (IOException e)
        {
            throw new PGPException("exception preparing key.", e);
        }

        return keyBytes;
    }

    protected void getAttributesHash(PGPUserAttributeSubpacketVector userAttributes)
        throws PGPException
    {
        //
        // hash in the attributes
        //
        try
        {
            ByteArrayOutputStream bOut = new ByteArrayOutputStream();
            UserAttributeSubpacket[] packets = userAttributes.toSubpacketArray();
            for (int i = 0; i != packets.length; i++)
            {
                packets[i].encode(bOut);
            }
            updateWithIdData(0xd1, bOut.toByteArray());
        }
        catch (IOException e)
        {
            throw new PGPException("cannot encode subpacket array", e);
        }
    }
}