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The Bouncy Castle Java API for handling the OpenPGP protocol. This jar contains the OpenPGP API for JDK 1.4. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.
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package org.bouncycastle.bcpg;

import java.io.ByteArrayInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.channels.FileChannel;

import org.bouncycastle.util.Arrays;

class StreamUtil
{
    private static final long MAX_MEMORY = Runtime.getRuntime().maxMemory();

    /**
     * Find out possible longest length, capped by available memory.
     *
     * @param in input stream of interest
     * @return length calculation or MAX_VALUE.
     */
    static int findLimit(InputStream in)
    {
        // TODO: this can obviously be improved.
        if (in instanceof ByteArrayInputStream)
        {
            return ((ByteArrayInputStream)in).available();
        }
        else if (in instanceof FileInputStream)
        {
            try
            {
                FileChannel channel = ((FileInputStream)in).getChannel();
                long size = (channel != null) ? channel.size() : Integer.MAX_VALUE;

                if (size < Integer.MAX_VALUE)
                {
                    return (int)size;
                }
            }
            catch (IOException e)
            {
                // ignore - they'll find out soon enough!
            }
        }

        if (MAX_MEMORY > Integer.MAX_VALUE)
        {
            return Integer.MAX_VALUE;
        }

        return (int)MAX_MEMORY;
    }

    static void writeNewPacketLength(OutputStream out, long bodyLen)
        throws IOException
    {
        if (bodyLen < 192)
        {
            out.write((byte)bodyLen);
        }
        else if (bodyLen <= 8383)
        {
            bodyLen -= 192;

            out.write((byte)(((bodyLen >> 8) & 0xff) + 192));
            out.write((byte)bodyLen);
        }
        else
        {
            out.write(0xff);
            writeBodyLen(out, bodyLen);
        }
    }

    static void writeBodyLen(OutputStream out, long bodyLen)
        throws IOException
    {
        out.write((byte)(bodyLen >> 24));
        out.write((byte)(bodyLen >> 16));
        out.write((byte)(bodyLen >> 8));
        out.write((byte)bodyLen);
    }

    static void writeKeyID(BCPGOutputStream pOut, long keyID)
        throws IOException
    {
        pOut.write((byte)(keyID >> 56));
        pOut.write((byte)(keyID >> 48));
        pOut.write((byte)(keyID >> 40));
        pOut.write((byte)(keyID >> 32));
        pOut.write((byte)(keyID >> 24));
        pOut.write((byte)(keyID >> 16));
        pOut.write((byte)(keyID >> 8));
        pOut.write((byte)(keyID));
    }

    static long readKeyID(BCPGInputStream in)
        throws IOException
    {
        long keyID = (long)in.read() << 56;
        keyID |= (long)in.read() << 48;
        keyID |= (long)in.read() << 40;
        keyID |= (long)in.read() << 32;
        keyID |= (long)in.read() << 24;
        keyID |= (long)in.read() << 16;
        keyID |= (long)in.read() << 8;
        return keyID | in.read();
    }

    static void writeTime(BCPGOutputStream pOut, long time)
        throws IOException
    {
        StreamUtil.writeSeconds(pOut, time / 1000);
    }

    static long readTime(BCPGInputStream in)
        throws IOException
    {
        return readSeconds(in) * 1000L;
    }

    static void writeSeconds(BCPGOutputStream pOut, long time)
        throws IOException
    {
        StreamUtil.write4OctetLength(pOut, (int)time);
    }

    static long readSeconds(BCPGInputStream in)
        throws IOException
    {
        return ((long)read4OctetLength(in)) & 0xFFFFFFFFL;
    }

    static void write2OctetLength(OutputStream pOut, int len)
        throws IOException
    {
        pOut.write(len >> 8);
        pOut.write(len);
    }

    static int read2OctetLength(InputStream in)
        throws IOException
    {
        return (in.read() << 8) | in.read();
    }

    static void write4OctetLength(OutputStream pOut, int len)
            throws IOException
    {
        pOut.write(len >> 24);
        pOut.write(len >> 16);
        pOut.write(len >> 8);
        pOut.write(len);
    }

    static int read4OctetLength(InputStream in)
        throws IOException
    {
        return (in.read() << 24) | (in.read() << 16) | (in.read() << 8) | in.read();
    }

    static int flag_eof = 0;
    static int flag_isLongLength = 1;
    static int flag_partial = 2;
    /**
     * Note: flags is an array of three boolean values:
     * flags[0] indicates l is negative, flag for eof
     * flags[1] indicates (is)longLength = true
     * flags[2] indicate partial = true
     */
    static int readBodyLen(InputStream in, boolean[] flags)
        throws IOException
    {
        Arrays.fill(flags, false);
        int l = in.read();
        int bodyLen = -1;
        if (l < 0)
        {
            flags[flag_eof] = true;
        }
        if (l < 192)
        {
            bodyLen = l;
        }
        else if (l <= 223)
        {
            bodyLen = ((l - 192) << 8) + (in.read()) + 192;
        }
        else if (l == 255)
        {
            flags[flag_isLongLength] = true;
            bodyLen = StreamUtil.read4OctetLength(in);
        }
        else
        {
            flags[flag_partial] = true;
            bodyLen = 1 << (l & 0x1f);
        }
        return bodyLen;
    }

    static void write8OctetLength(OutputStream pOut, long len)
        throws IOException
    {
        pOut.write((int) (len >> 56));
        pOut.write((int) (len >> 48));
        pOut.write((int) (len >> 40));
        pOut.write((int) (len >> 32));
        pOut.write((int) (len >> 24));
        pOut.write((int) (len >> 16));
        pOut.write((int) (len >> 8));
        pOut.write((int) len);
    }

    static long read8OctetLength(InputStream in)
        throws IOException
    {
        return ((long) in.read() << 56) |
            ((long) in.read() << 48) |
            ((long) in.read() << 40) |
            ((long) in.read() << 32) |
            ((long) in.read() << 24) |
            ((long) in.read() << 16) |
            ((long) in.read() << 8) |
            ((long) in.read());
    }

}