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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.EOFException;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import org.bouncycastle.bcpg.InputStreamPacket;
import org.bouncycastle.bcpg.SymmetricEncIntegrityPacket;
import org.bouncycastle.bcpg.SymmetricKeyAlgorithmTags;
import org.bouncycastle.openpgp.operator.PGPDataDecryptor;
import org.bouncycastle.openpgp.operator.PGPDataDecryptorFactory;
import org.bouncycastle.openpgp.operator.PGPDigestCalculator;
import org.bouncycastle.util.Arrays;

/**
 * A PGP encrypted data object.
 * 
 * Encrypted data packets are decrypted using a {@link PGPDataDecryptor} obtained from a
 * {@link PGPDataDecryptorFactory}.
 * 
 */
public abstract class PGPEncryptedData
    implements SymmetricKeyAlgorithmTags
{
    protected class TruncatedStream extends InputStream
    {
        int[]         lookAhead = new int[22];
        int           bufPtr;
        InputStream   in;

        TruncatedStream(
            InputStream    in)
            throws IOException
        {
            for (int i = 0; i != lookAhead.length; i++)
            {
                if ((lookAhead[i] = in.read()) < 0)
                {
                    throw new EOFException();
                }
            }

            bufPtr = 0;
            this.in = in;
        }

        public int read()
            throws IOException
        {
            int    ch = in.read();

            if (ch >= 0)
            {
                int    c = lookAhead[bufPtr];

                lookAhead[bufPtr] = ch;
                bufPtr = (bufPtr + 1) % lookAhead.length;

                return c;
            }

            return -1;
        }

        int[] getLookAhead()
        {
            int[]    tmp = new int[lookAhead.length];
            int    count = 0;

            for (int i = bufPtr; i != lookAhead.length; i++)
            {
                tmp[count++] = lookAhead[i];
            }
            for (int i = 0; i != bufPtr; i++)
            {
                tmp[count++] = lookAhead[i];
            }

            return tmp;
        }
    }

    InputStreamPacket        encData;
    InputStream              encStream;
    TruncatedStream          truncStream;
    PGPDigestCalculator      integrityCalculator;

    PGPEncryptedData(
        InputStreamPacket    encData)
    {
        this.encData = encData;
    }

    /**
     * Return the raw input stream for the data stream.
     * 
     * Note this stream is shared with all other encryption methods in the same
     * {@link PGPEncryptedDataList} and with any decryption methods in sub-classes, so consuming
     * this stream will affect decryption.
     * 
     * @return the encrypted data in this packet.
     */
    public InputStream getInputStream()
    {
        return encData.getInputStream();
    }

    /**
     * Checks whether the packet is integrity protected.
     *
     * @return true if there is a modification detection code package associated with
     *         this stream
     */
    public boolean isIntegrityProtected()
    {
        return (encData instanceof SymmetricEncIntegrityPacket);
    }

    /**
     * Verifies the integrity of the packet against the modification detection code associated with
     * it in the stream.
     * 
     * Note: This can only be called after the message has been read.
     * 
     * @return true if the message verifies, false otherwise.
     * @throws PGPException if the message is not {@link #isIntegrityProtected() integrity
     *             protected}.
     */
    public boolean verify()
        throws PGPException, IOException
    {
        if (!this.isIntegrityProtected())
        {
            throw new PGPException("data not integrity protected.");
        }

        //
        // make sure we are at the end.
        //
        while (encStream.read() >= 0)
        {
            // do nothing
        }

        //
        // process the MDC packet
        //
        int[] lookAhead = truncStream.getLookAhead();

        OutputStream dOut = integrityCalculator.getOutputStream();

        dOut.write((byte)lookAhead[0]);
        dOut.write((byte)lookAhead[1]);

        byte[] digest = integrityCalculator.getDigest();
        byte[] streamDigest = new byte[digest.length];

        for (int i = 0; i != streamDigest.length; i++)
        {
            streamDigest[i] = (byte)lookAhead[i + 2];
        }

        return Arrays.constantTimeAreEqual(digest, streamDigest);
    }
}