java sign public pgp key with bouncycastle

Question

I've got a doubt.. I have to sign a pgp public key using bouncycastle api supposedly. Now: to my understanding signing a key with another means ultimately adding to this public key a "certificate". Thus lacking any other way, I've gone blind searching in the library. my only find so far has been method generateCertification inside PGPSignatureGenerator. But this method generate a certification between a master PgpPublicKey and another PgpPublicKey.. And this strikes me as strange: I assumed that in order to trust another public key, that has to be signed with your own private pgp key just like in regular x.509 with CA certification in a manner.. This was assumption by some methods that I saw when trying to get some ideas from other library: didisoft for example has a similar method on a keystore where you have to provide the PgpPrivatekey keyuid...

Anyone has any hint or a piece of code to propose? Thanks in advance.

Answer 1

This can be used to check that one key gave default certification to another

  /**
 * Signs a public key
 *
 * @param publicKeyRing a public key ring containing the single public key to sign
 * @param id the id we are certifying against the public key
 * @param secretKey the signing key
 * @param secretKeyPassword the signing key password
 *
 * @return a public key ring with the signed public key
 */
public static PGPPublicKeyRing signPublicKey( PGPPublicKeyRing publicKeyRing, String id, PGPSecretKey secretKey,
                                              String secretKeyPassword ) throws PGPException
{
    try
    {
        PGPPublicKey oldKey = publicKeyRing.getPublicKey();

        PGPPrivateKey pgpPrivKey = secretKey.extractPrivateKey(
                new JcePBESecretKeyDecryptorBuilder().setProvider( provider )
                                                     .build( secretKeyPassword.toCharArray() ) );

        PGPSignatureGenerator signatureGenerator = new PGPSignatureGenerator(
                new JcaPGPContentSignerBuilder( secretKey.getPublicKey().getAlgorithm(), PGPUtil.SHA1 ) );

        signatureGenerator.init( PGPSignature.DEFAULT_CERTIFICATION, pgpPrivKey );

        PGPSignature signature = signatureGenerator.generateCertification( id, oldKey );

        PGPPublicKey newKey = PGPPublicKey.addCertification( oldKey, signature );

        PGPPublicKeyRing newPublicKeyRing = PGPPublicKeyRing.removePublicKey( publicKeyRing, oldKey );

        return PGPPublicKeyRing.insertPublicKey( newPublicKeyRing, newKey );
    }
    catch ( Exception e )
    {
        //throw custom  exception
        throw new PGPException( "Error signing public key", e );
    }
}


/**
 * Verifies that a public key is signed with another public key
 *
 * @param keyToVerify the public key to verify
 * @param id the id we are verifying against the public key
 * @param keyToVerifyWith the key to verify with
 *
 * @return true if verified, false otherwise
 */
public static boolean verifyPublicKey( PGPPublicKey keyToVerify, String id, PGPPublicKey keyToVerifyWith )
        throws PGPException
{
    try
    {
        Iterator<PGPSignature> signIterator = keyToVerify.getSignatures();
        while ( signIterator.hasNext() )
        {
            PGPSignature signature = signIterator.next();
            signature.init( new JcaPGPContentVerifierBuilderProvider().setProvider( provider ), keyToVerifyWith );
            if ( signature.verifyCertification( id.getBytes(), keyToVerify ) )
            {
                return true;
            }
        }
        return false;
    }
    catch ( Exception e )
    {
        //throw custom  exception
        throw new PGPException( "Error verifying public key", e );
    }
}

Answer 2

Here's a Codeexample to sign a public Key:

    PGPSecretKey mySecretKey;
    PGPPublicKey publicKeyToBeSigned; 
    PGPPrivateKey pgpPrivKey = mySecretKey
            .extractPrivateKey(new JcePBESecretKeyDecryptorBuilder()
                    .setProvider("BC").build("password for your private key"));
    PGPSignatureGenerator signatureGenerator = new PGPSignatureGenerator(
            new JcaPGPContentSignerBuilder(mySecretKey.getPublicKey()
                    .getAlgorithm(), PGPUtil.SHA512));
    signatureGenerator.init(PGPSignature.DIRECT_KEY, pgpPrivKey);

    PGPSignature signature = signatureGenerator.generateCertification(
            id, publicKeyToBeSigned);

This piece of code just creates the signature. You need to add it to your the public key then:

PGPPublicKey.addCertification(publicKeyToBeSigned, signature);

Hope that helps you :)