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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 FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.
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package org.bouncycastle.openpgp;

import java.io.ByteArrayInputStream;
import java.io.EOFException;
import java.io.IOException;
import java.io.InputStream;
import java.util.zip.Inflater;
import java.util.zip.InflaterInputStream;

import org.bouncycastle.apache.bzip2.CBZip2InputStream;
import org.bouncycastle.bcpg.BCPGInputStream;
import org.bouncycastle.bcpg.CompressedDataPacket;
import org.bouncycastle.bcpg.CompressionAlgorithmTags;
import org.bouncycastle.bcpg.Packet;
import org.bouncycastle.bcpg.PacketTags;

/**
 * A PGP compressed data object.
 */
public class PGPCompressedData
    implements CompressionAlgorithmTags
{
    CompressedDataPacket    data;

    /**
     * Construct a PGP compressed data object from the passed in byte array.
     *
     * @param encData an encoding of PGP compressed data.
     * @throws IOException if an error occurs reading from the PGP input.
     */
    public PGPCompressedData(
        byte[]    encData)
        throws IOException
    {
        this(Util.createBCPGInputStream(new ByteArrayInputStream(encData), PacketTags.COMPRESSED_DATA));
    }

    /**
     * Construct a PGP compressed data object from the passed in input stream.
     *
     * @param inStream an input stream containing an encoding of PGP compressed data.
     * @throws IOException if an error occurs reading from the PGP input.
     */
    public PGPCompressedData(
        InputStream    inStream)
        throws IOException
    {
        this(Util.createBCPGInputStream(inStream, PacketTags.COMPRESSED_DATA));
    }

    /**
     * Construct a compressed data object, reading a single {@link PacketTags#COMPRESSED_DATA}
     * packet from the stream.
     *
     * @param pIn a PGP input stream, with a compressed data packet as the current packet.
     * @throws IOException if an error occurs reading the packet from the stream.
     */
    public PGPCompressedData(
        BCPGInputStream    pIn)
        throws IOException
    {

        Packet packet = pIn.readPacket();
        if (!(packet instanceof CompressedDataPacket))
        {
            throw new IOException("unexpected packet in stream: " + packet);
        }
        data = (CompressedDataPacket)packet;
    }

    /**
     * Return the {@link CompressionAlgorithmTags compression algorithm} used for this packet.
     *
     * @return the compression algorithm code
     */
    public int getAlgorithm()
    {
        return data.getAlgorithm();
    }

    /**
     * Return the raw input stream contained in the object.
     * 
     * Note that this stream is shared with the decompression stream, so consuming the returned
     * stream will affect decompression.
     * 
     * @return the raw data in the compressed data packet.
     */
    public InputStream getInputStream()
    {
        return data.getInputStream();
    }

    /**
     * Return an input stream that decompresses and returns data in the compressed packet.
     *
     * @return a stream over the uncompressed data.
     * @throws PGPException if an error occurs constructing the decompression stream.
     */
    public InputStream getDataStream()
        throws PGPException
    {
      if (this.getAlgorithm() == UNCOMPRESSED)
      {
          return this.getInputStream();
      }
      if (this.getAlgorithm() == ZIP)
      {
          return new InflaterInputStream(this.getInputStream(), new Inflater(true))
          {
              // If the "nowrap" inflater option is used the stream can
              // apparently overread - we override fill() and provide
              // an extra byte for the end of the input stream to get
              // around this.
              //
              // Totally weird...
              //
              protected void fill() throws IOException
              {
                  if (eof)
                  {
                      throw new EOFException("Unexpected end of ZIP input stream");
                  }

                  len = this.in.read(buf, 0, buf.length);

                  if (len == -1)
                  {
                      buf[0] = 0;
                      len = 1;
                      eof = true;
                  }

                  inf.setInput(buf, 0, len);
              }

              private boolean eof = false;
          };
      }
      if (this.getAlgorithm() == ZLIB)
      {
          return new InflaterInputStream(this.getInputStream())
          {
              // If the "nowrap" inflater option is used the stream can
              // apparently overread - we override fill() and provide
              // an extra byte for the end of the input stream to get
              // around this.
              //
              // Totally weird...
              //
              protected void fill() throws IOException
              {
                  if (eof)
                  {
                      throw new EOFException("Unexpected end of ZIP input stream");
                  }

                  len = this.in.read(buf, 0, buf.length);

                  if (len == -1)
                  {
                      buf[0] = 0;
                      len = 1;
                      eof = true;
                  }

                  inf.setInput(buf, 0, len);
              }

              private boolean eof = false;
          };
      }
      if (this.getAlgorithm() == BZIP2)
      {
          try
          {
              return new CBZip2InputStream(this.getInputStream());
          }
          catch (IOException e)
          {
              throw new PGPException("I/O problem with stream: " + e, e);
          }
      }

      throw new PGPException("can't recognise compression algorithm: " + this.getAlgorithm());
    }
}