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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.asn1;

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

/**
 * Base class for generators for indefinite-length structures.
 */
public abstract class BERGenerator
    extends ASN1Generator
{
    private boolean _tagged = false;
    private boolean _isExplicit;
    private int _tagNo;

    protected BERGenerator(OutputStream out)
    {
        super(out);
    }

    protected BERGenerator(OutputStream out, int tagNo, boolean isExplicit)
    {
        super(out);

        // TODO Check proper handling of implicit tagging
        _tagged = true;
        _isExplicit = isExplicit;
        _tagNo = tagNo;
    }

    public OutputStream getRawOutputStream()
    {
        return _out;
    }

    private void writeHdr(int tag) throws IOException
    {
        _out.write(tag);
        _out.write(0x80);
    }

    protected void writeBERHeader(int tag) throws IOException
    {
        if (_tagged)
        {
            int tagNum = _tagNo | BERTags.CONTEXT_SPECIFIC;

            if (_isExplicit)
            {
                writeHdr(tagNum | BERTags.CONSTRUCTED);
                writeHdr(tag);
            }
            else
            {
                if ((tag & BERTags.CONSTRUCTED) != 0)
                {
                    writeHdr(tagNum | BERTags.CONSTRUCTED);
                }
                else
                {
                    writeHdr(tagNum);
                }
            }
        }
        else
        {
            writeHdr(tag);
        }
    }

    protected void writeBEREnd() throws IOException
    {
        _out.write(0x00);
        _out.write(0x00);

        if (_tagged && _isExplicit)  // write extra end for tag header
        {
            _out.write(0x00);
            _out.write(0x00);
        }
    }
}