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

import org.bouncycastle.bcpg.AEADAlgorithmTags;
import org.bouncycastle.bcpg.AEADEncDataPacket;
import org.bouncycastle.bcpg.BCPGInputStream;
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;
import org.bouncycastle.util.io.TeeInputStream;

/**
 * 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, AEADAlgorithmTags
{
    protected static class TruncatedStream
        extends InputStream
    {
        int[] lookAhead = new int[22];
        int bufPtr;
        InputStream in;
        byte[] readBuffer = new byte[8192];

        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;
        }

        public int read(byte[] b)
            throws IOException
        {
            return read(b, 0, b.length);
        }

        public int read(byte[] b, int off, int len)
            throws IOException
        {
            // Efficient index check copied from BufferedInputStream
            if ((off | len | off + len | b.length - (off + len)) < 0)
            {
                throw new IndexOutOfBoundsException();
            }
            else if (len == 0)
            {
                return 0;
            }

            // read into our buffer
            int maxRead = Math.min(readBuffer.length, len);
            int bytesRead = in.read(readBuffer, 0, maxRead);
            if (bytesRead < 0)
            {
                return -1;
            }

            // Copy lookahead to output
            int bytesFromLookahead = Math.min(bytesRead, lookAhead.length);
            for (int i = 0; i < bytesFromLookahead; i++)
            {
                b[off + i] = (byte)lookAhead[(bufPtr + i) % lookAhead.length];
            }

            // write tail of readBuffer to lookahead
            int bufferTail = bytesRead - bytesFromLookahead;
            for (int i = bufferTail; i < bytesRead; i++)
            {
                lookAhead[bufPtr] = readBuffer[i] & 0xff; // we're not at end of file.
                bufPtr = (bufPtr + 1) % lookAhead.length;
            }

            // Copy head of readBuffer to output
            if (bufferTail != 0)
            {
                System.arraycopy(readBuffer, 0, b, off + bytesFromLookahead, bufferTail);
            }
            
            return bytesRead;
        }
        
        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 using a modification detection code package.
     *
     * @return true if there is a modification detection code package associated with
     * this stream
     */
    public boolean isIntegrityProtected()
    {
        return (encData instanceof SymmetricEncIntegrityPacket);
    }

    /**
     * Checks whether the packet is protected using an AEAD algorithm.
     *
     * @return true if there is a modification detection code package associated with
     * this stream
     */
    public boolean isAEAD()
    {
        if (encData instanceof AEADEncDataPacket)
        {
            return true;
        }
        if (encData instanceof SymmetricEncIntegrityPacket)
        {
            return ((SymmetricEncIntegrityPacket) encData).getVersion() == SymmetricEncIntegrityPacket.VERSION_2;
        }
        else
        {
            return false;
        }
    }

    /**
     * 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
        }

        if (isAEAD())
        {
            // AEAD data needs no manual verification, as decryption detects errors automatically
            return true;
        }

        //
        // 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);
    }

    /**
     * Return the version number of the Encrypted Session Key Packet.
     *
     * @return version
     */
    public int getVersion()
    {
        throw new UnsupportedOperationException("not supported - override required");
    }

    /**
     * Return the symmetric encryption algorithm that is used by the packet.
     *
     * @return algorithm
     */
    public int getAlgorithm()
    {
        throw new UnsupportedOperationException("not supported - override required");
    }

    boolean processSymmetricEncIntegrityPacketDataStream(boolean withIntegrityPacket, PGPDataDecryptor dataDecryptor, BCPGInputStream encIn)
        throws IOException
    {
        encStream = new BCPGInputStream(dataDecryptor.getInputStream(encIn));

        if (withIntegrityPacket)
        {
            truncStream = new TruncatedStream(encStream);

            integrityCalculator = dataDecryptor.getIntegrityCalculator();

            encStream = new TeeInputStream(truncStream, integrityCalculator.getOutputStream());
        }

        byte[] iv = new byte[dataDecryptor.getBlockSize()];

        for (int i = 0; i != iv.length; i++)
        {
            int ch = encStream.read();

            if (ch < 0)
            {
                throw new EOFException("unexpected end of stream.");
            }

            iv[i] = (byte)ch;
        }
        int v1 = encStream.read();
        int v2 = encStream.read();

        if (v1 < 0 || v2 < 0)
        {
            throw new EOFException("unexpected end of stream.");
        }

        // Note: the oracle attack on "quick check" bytes is not deemed
        // a security risk for PBE (see PGPPublicKeyEncryptedData)

        boolean repeatCheckPassed = iv[iv.length - 2] == (byte)v1
            && iv[iv.length - 1] == (byte)v2;

        // Note: some versions of PGP appear to produce 0 for the extra
        // bytes rather than repeating the two previous bytes
        boolean zeroesCheckPassed = v1 == 0 && v2 == 0;
        return !repeatCheckPassed && !zeroesCheckPassed;
    }
}