Getting GPG Decryption To Work In Java (Bouncy Castle)

Question

let me start by saying I'm extremely new to all of this. What I am trying to do is to use gpg from within Java in order to decrypt an encrypted file.

What I've done successfully:

• Had a colleague encrypt a file using my public key and his private key and successfully decrypted it.

• Went the other way

• Had another colleague try to decrypt a file that wasn't for him: fail (as expected)

My key was generated like this...

(gpg --version tells me I'm using 1.4.5 and I'm using Bouncy Castle 1.47)

gpg --gen-ley

Select option "DSA and Elgamal (default)"

Fill in the other fields and generate a key.

The file was encrypted using my public key and another's secret key. I want to decrypt it. I've written the following Java code to accomplish this. I'm using several deprecated methods, but I can't figure out how to properly implement the factory methods required to use the non-deprecated versions, so if anyone has an idea on implementations of those that I should be using that would be a nice bonus.

    Security.addProvider(new BouncyCastleProvider());

        PGPSecretKeyRingCollection secretKeyRing = new PGPSecretKeyRingCollection(new FileInputStream(new File("test-files/secring.gpg")));
        PGPSecretKeyRing pgpSecretKeyRing = (PGPSecretKeyRing) secretKeyRing.getKeyRings().next();
        PGPSecretKey secretKey = pgpSecretKeyRing.getSecretKey();
        PGPPrivateKey privateKey = secretKey.extractPrivateKey("mypassword".toCharArray(), "BC");

        System.out.println(privateKey.getKey().getAlgorithm());
        System.out.println(privateKey.getKey().getFormat());

        PGPObjectFactory pgpF = new PGPObjectFactory(
    new FileInputStream(new File("test-files/test-file.txt.gpg")));
        Object pgpObj = pgpF.nextObject();
        PGPEncryptedDataList encryptedDataList = (PGPEncryptedDataList) pgpObj;

        Iterator objectsIterator = encryptedDataList.getEncryptedDataObjects();

        PGPPublicKeyEncryptedData publicKeyEncryptedData = (PGPPublicKeyEncryptedData) objectsIterator.next();
        InputStream inputStream = publicKeyEncryptedData.getDataStream(privateKey, "BC");

So when I run this code I learn that my algorithm and format are as follows for my secret key:

Algorithm: DSA Format: PKCS#8

And then it breaks on the last line:

Exception in thread "main" org.bouncycastle.openpgp.PGPException: error setting asymmetric cipher
at org.bouncycastle.openpgp.operator.jcajce.JcePublicKeyDataDecryptorFactoryBuilder.decryptSessionData(Unknown Source)
at org.bouncycastle.openpgp.operator.jcajce.JcePublicKeyDataDecryptorFactoryBuilder.access$000(Unknown Source)
at org.bouncycastle.openpgp.operator.jcajce.JcePublicKeyDataDecryptorFactoryBuilder$2.recoverSessionData(Unknown Source)
at org.bouncycastle.openpgp.PGPPublicKeyEncryptedData.getDataStream(Unknown Source)
at org.bouncycastle.openpgp.PGPPublicKeyEncryptedData.getDataStream(Unknown Source)
at org.bouncycastle.openpgp.PGPPublicKeyEncryptedData.getDataStream(Unknown Source)
at org.bouncycastle.openpgp.PGPPublicKeyEncryptedData.getDataStream(Unknown Source)
at TestBouncyCastle.main(TestBouncyCastle.java:74)

Caused by: java.security.InvalidKeyException: unknown key type passed to ElGamal at org.bouncycastle.jcajce.provider.asymmetric.elgamal.CipherSpi.engineInit(Unknown Source) at org.bouncycastle.jcajce.provider.asymmetric.elgamal.CipherSpi.engineInit(Unknown Source) at javax.crypto.Cipher.init(DashoA13*..) at javax.crypto.Cipher.init(DashoA13*..) ... 8 more

I'm open to a lot of suggestions here, from "don't use gpg, use x instead" to "don't use bouncy castle, use x instead" to anything in between. Thanks!

Answer 1

If anyone is interested to know how to encrypt and decrypt gpg files using bouncy castle openPGP library, check the below java code:

The below are the 4 methods you going to need:

The below method will read and import your secret key from .asc file:

public static PGPSecretKey readSecretKeyFromCol(InputStream in, long keyId) throws IOException, PGPException {
    in = PGPUtil.getDecoderStream(in);
    PGPSecretKeyRingCollection pgpSec = new PGPSecretKeyRingCollection(in, new BcKeyFingerprintCalculator());

    PGPSecretKey key = pgpSec.getSecretKey(keyId);

    if (key == null) {
        throw new IllegalArgumentException("Can't find encryption key in key ring.");
    }
    return key;
}

The below method will read and import your public key from .asc file:

@SuppressWarnings("rawtypes")
    public static PGPPublicKey readPublicKeyFromCol(InputStream in) throws IOException, PGPException {
        in = PGPUtil.getDecoderStream(in);
        PGPPublicKeyRingCollection pgpPub = new PGPPublicKeyRingCollection(in, new BcKeyFingerprintCalculator());
        PGPPublicKey key = null;
        Iterator rIt = pgpPub.getKeyRings();
        while (key == null && rIt.hasNext()) {
            PGPPublicKeyRing kRing = (PGPPublicKeyRing) rIt.next();
            Iterator kIt = kRing.getPublicKeys();
            while (key == null && kIt.hasNext()) {
                PGPPublicKey k = (PGPPublicKey) kIt.next();
                if (k.isEncryptionKey()) {
                    key = k;
                }
            }
        }
        if (key == null) {
            throw new IllegalArgumentException("Can't find encryption key in key ring.");
        }
        return key;
    }

The below 2 methods to decrypt and encrypt gpg files:

public void decryptFile(InputStream in, InputStream secKeyIn, InputStream pubKeyIn, char[] pass) throws IOException, PGPException, InvalidCipherTextException {
        Security.addProvider(new BouncyCastleProvider());

        PGPPublicKey pubKey = readPublicKeyFromCol(pubKeyIn);

        PGPSecretKey secKey = readSecretKeyFromCol(secKeyIn, pubKey.getKeyID());

        in = PGPUtil.getDecoderStream(in);

        JcaPGPObjectFactory pgpFact;


        PGPObjectFactory pgpF = new PGPObjectFactory(in, new BcKeyFingerprintCalculator());

        Object o = pgpF.nextObject();
        PGPEncryptedDataList encList;

        if (o instanceof PGPEncryptedDataList) {

            encList = (PGPEncryptedDataList) o;

        } else {

            encList = (PGPEncryptedDataList) pgpF.nextObject();

        }

        Iterator<PGPPublicKeyEncryptedData> itt = encList.getEncryptedDataObjects();
        PGPPrivateKey sKey = null;
        PGPPublicKeyEncryptedData encP = null;
        while (sKey == null && itt.hasNext()) {
            encP = itt.next();
            secKey = readSecretKeyFromCol(new FileInputStream("PrivateKey.asc"), encP.getKeyID());
            sKey = secKey.extractPrivateKey(new BcPBESecretKeyDecryptorBuilder(new BcPGPDigestCalculatorProvider()).build(pass));
        }
        if (sKey == null) {
            throw new IllegalArgumentException("Secret key for message not found.");
        }

        InputStream clear = encP.getDataStream(new BcPublicKeyDataDecryptorFactory(sKey));

        pgpFact = new JcaPGPObjectFactory(clear);

        PGPCompressedData c1 = (PGPCompressedData) pgpFact.nextObject();

        pgpFact = new JcaPGPObjectFactory(c1.getDataStream());

        PGPLiteralData ld = (PGPLiteralData) pgpFact.nextObject();
        ByteArrayOutputStream bOut = new ByteArrayOutputStream();

        InputStream inLd = ld.getDataStream();

        int ch;
        while ((ch = inLd.read()) >= 0) {
            bOut.write(ch);
        }

        //System.out.println(bOut.toString());

        bOut.writeTo(new FileOutputStream(ld.getFileName()));
        //return bOut;

    }

    public static void encryptFile(OutputStream out, String fileName, PGPPublicKey encKey) throws IOException, NoSuchProviderException, PGPException {
        Security.addProvider(new BouncyCastleProvider());

        ByteArrayOutputStream bOut = new ByteArrayOutputStream();

        PGPCompressedDataGenerator comData = new PGPCompressedDataGenerator(PGPCompressedData.ZIP);

        PGPUtil.writeFileToLiteralData(comData.open(bOut), PGPLiteralData.BINARY, new File(fileName));

        comData.close();

        PGPEncryptedDataGenerator cPk = new PGPEncryptedDataGenerator(new BcPGPDataEncryptorBuilder(SymmetricKeyAlgorithmTags.TRIPLE_DES).setSecureRandom(new SecureRandom()));

        cPk.addMethod(new BcPublicKeyKeyEncryptionMethodGenerator(encKey));

        byte[] bytes = bOut.toByteArray();

        OutputStream cOut = cPk.open(out, bytes.length);

        cOut.write(bytes);

        cOut.close();

        out.close();
    }

Now here is how to invoke/run the above:

try {
             decryptFile(new FileInputStream("encryptedFile.gpg"), new FileInputStream("PrivateKey.asc"), new FileInputStream("PublicKey.asc"), "yourKeyPassword".toCharArray());

            PGPPublicKey pubKey = readPublicKeyFromCol(new FileInputStream("PublicKey.asc"));

            encryptFile(new FileOutputStream("encryptedFileOutput.gpg"), "fileToEncrypt.txt", pubKey);




        } catch (PGPException e) {
            fail("exception: " + e.getMessage(), e.getUnderlyingException());
        }