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The Bouncy Castle Java API for handling the OpenPGP protocol. This jar contains the OpenPGP API for JDK 1.5 to JDK 1.7. 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.openpgp.examples;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.security.NoSuchProviderException;
import java.util.Iterator;

import org.bouncycastle.openpgp.PGPCompressedDataGenerator;
import org.bouncycastle.openpgp.PGPException;
import org.bouncycastle.openpgp.PGPLiteralData;
import org.bouncycastle.openpgp.PGPPrivateKey;
import org.bouncycastle.openpgp.PGPPublicKey;
import org.bouncycastle.openpgp.PGPPublicKeyRing;
import org.bouncycastle.openpgp.PGPPublicKeyRingCollection;
import org.bouncycastle.openpgp.PGPSecretKey;
import org.bouncycastle.openpgp.PGPSecretKeyRing;
import org.bouncycastle.openpgp.PGPSecretKeyRingCollection;
import org.bouncycastle.openpgp.PGPUtil;
import org.bouncycastle.openpgp.operator.jcajce.JcaKeyFingerprintCalculator;
import org.bouncycastle.openpgp.operator.jcajce.JcePBESecretKeyDecryptorBuilder;

class PGPExampleUtil
{
    static byte[] compressFile(String fileName, int algorithm) throws IOException
    {
        ByteArrayOutputStream bOut = new ByteArrayOutputStream();
        PGPCompressedDataGenerator comData = new PGPCompressedDataGenerator(algorithm);
        PGPUtil.writeFileToLiteralData(comData.open(bOut), PGPLiteralData.BINARY,
            new File(fileName));
        comData.close();
        return bOut.toByteArray();
    }

    /**
     * Search a secret key ring collection for a secret key corresponding to keyID if it
     * exists.
     * 
     * @param pgpSec a secret key ring collection.
     * @param keyID keyID we want.
     * @param pass passphrase to decrypt secret key with.
     * @return the private key.
     * @throws PGPException
     * @throws NoSuchProviderException
     */
    static PGPPrivateKey findSecretKey(PGPSecretKeyRingCollection pgpSec, long keyID, char[] pass)
        throws PGPException, NoSuchProviderException
    {
        PGPSecretKey pgpSecKey = pgpSec.getSecretKey(keyID);

        if (pgpSecKey == null)
        {
            return null;
        }

        return pgpSecKey.extractPrivateKey(new JcePBESecretKeyDecryptorBuilder().setProvider("BC").build(pass));
    }

    static PGPPublicKey readPublicKey(String fileName) throws IOException, PGPException
    {
        InputStream keyIn = new BufferedInputStream(new FileInputStream(fileName));
        PGPPublicKey pubKey = readPublicKey(keyIn);
        keyIn.close();
        return pubKey;
    }

    /**
     * A simple routine that opens a key ring file and loads the first available key
     * suitable for encryption.
     * 
     * @param input data stream containing the public key data
     * @return the first public key found.
     * @throws IOException
     * @throws PGPException
     */
    static PGPPublicKey readPublicKey(InputStream input) throws IOException, PGPException
    {
        PGPPublicKeyRingCollection pgpPub = new PGPPublicKeyRingCollection(
            PGPUtil.getDecoderStream(input), new JcaKeyFingerprintCalculator());

        //
        // we just loop through the collection till we find a key suitable for encryption, in the real
        // world you would probably want to be a bit smarter about this.
        //

        Iterator keyRingIter = pgpPub.getKeyRings();
        while (keyRingIter.hasNext())
        {
            PGPPublicKeyRing keyRing = (PGPPublicKeyRing)keyRingIter.next();

            Iterator keyIter = keyRing.getPublicKeys();
            while (keyIter.hasNext())
            {
                PGPPublicKey key = (PGPPublicKey)keyIter.next();

                if (key.isEncryptionKey())
                {
                    return key;
                }
            }
        }

        throw new IllegalArgumentException("Can't find encryption key in key ring.");
    }

    static PGPSecretKey readSecretKey(String fileName) throws IOException, PGPException
    {
        InputStream keyIn = new BufferedInputStream(new FileInputStream(fileName));
        PGPSecretKey secKey = readSecretKey(keyIn);
        keyIn.close();
        return secKey;
    }

    /**
     * A simple routine that opens a key ring file and loads the first available key
     * suitable for signature generation.
     * 
     * @param input stream to read the secret key ring collection from.
     * @return a secret key.
     * @throws IOException on a problem with using the input stream.
     * @throws PGPException if there is an issue parsing the input stream.
     */
    static PGPSecretKey readSecretKey(InputStream input) throws IOException, PGPException
    {
        PGPSecretKeyRingCollection pgpSec = new PGPSecretKeyRingCollection(
            PGPUtil.getDecoderStream(input), new JcaKeyFingerprintCalculator());

        //
        // we just loop through the collection till we find a key suitable for encryption, in the real
        // world you would probably want to be a bit smarter about this.
        //

        Iterator keyRingIter = pgpSec.getKeyRings();
        while (keyRingIter.hasNext())
        {
            PGPSecretKeyRing keyRing = (PGPSecretKeyRing)keyRingIter.next();

            Iterator keyIter = keyRing.getSecretKeys();
            while (keyIter.hasNext())
            {
                PGPSecretKey key = (PGPSecretKey)keyIter.next();

                if (key.isSigningKey())
                {
                    return key;
                }
            }
        }

        throw new IllegalArgumentException("Can't find signing key in key ring.");
    }
}