How to DER encode an ECDH Public Key in BouncyCastle Java

Question

So I know how to encode/decode a public key in the BouncyCastle C# library into a byte array:

Encode:

PublicKey = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(p1.Public).GetDerEncoded();

Decode:

ECPublicKeyParameters pubKey = (ECPublicKeyParameters)PublicKeyFactory.CreateKey(OtherPublicKey);

I can't seem to figure out how to do this in the java version of the BouncyCastle Library since there doesn't seem to be a SubjectPublicKeyInfoFactory object in the Java version of the library. There does, however, seem to be a PublicKeyFactory class in Java so it looks like I can just use the same code but I don't know how to DER encode the public key in the java library. Can anyone help?? Thanks!

-----EDIT---------------------------------------------------------

Ok, so here is what I have so far in C#:

Create the ECDH instance:

public static ECDHBasicAgreement CreateECDHInstance(out byte[] PublicKey)
    {
        IAsymmetricCipherKeyPairGenerator g = GeneratorUtilities.GetKeyPairGenerator("ECDH");

        FpCurve curve = new FpCurve(
            new BigInteger("883423532389192164791648750360308885314476597252960362792450860609699839"), // q
            new BigInteger("7fffffffffffffffffffffff7fffffffffff8000000000007ffffffffffc", 16), // a
            new BigInteger("6b016c3bdcf18941d0d654921475ca71a9db2fb27d1d37796185c2942c0a", 16)); // b

        ECDomainParameters ecSpec = new ECDomainParameters(
            curve,
            curve.DecodePoint(Hex.Decode("020ffa963cdca8816ccc33b8642bedf905c3d358573d3f27fbbd3b3cb9aaaf")), // G
            new BigInteger("883423532389192164791648750360308884807550341691627752275345424702807307"), // n
            BigInteger.One); // h

        g.Init(new ECKeyGenerationParameters(ecSpec, new SecureRandom()));

        AsymmetricCipherKeyPair aKeyPair = g.GenerateKeyPair();
        ECDHBasicAgreement aKeyAgreeBasic = new ECDHBasicAgreement();
        aKeyAgreeBasic.Init(aKeyPair.Private);

        PublicKey = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(aKeyPair.Public).GetDerEncoded();

        return aKeyAgreeBasic;
    }

This creates and returns a ECDHBasicAgreement object perfectly and outputs a public key in der encoded byte array form. Here is what I have in java:

public void testECDH() throws Exception
{
    AsymmetricCipherKeyPairGenerator g = new ECKeyPairGenerator();

    Fp curve = new Fp(
        new BigInteger("883423532389192164791648750360308885314476597252960362792450860609699839"), // q
        new BigInteger("7fffffffffffffffffffffff7fffffffffff8000000000007ffffffffffc", 16), // a
        new BigInteger("6b016c3bdcf18941d0d654921475ca71a9db2fb27d1d37796185c2942c0a", 16)); // b

    ECDomainParameters ecSpec = new ECDomainParameters(
        curve,
        curve.decodePoint(Hex.decode("020ffa963cdca8816ccc33b8642bedf905c3d358573d3f27fbbd3b3cb9aaaf")), // G
        new BigInteger("883423532389192164791648750360308884807550341691627752275345424702807307"), // n
        BigInteger.ONE); // h

    g.init(new ECKeyGenerationParameters(ecSpec, new SecureRandom()));

    AsymmetricCipherKeyPair aKeyPair = g.generateKeyPair();
    ECDHBasicAgreement aKeyAgreeBasic = new ECDHBasicAgreement();
    aKeyAgreeBasic.init(aKeyPair.getPrivate());

    // The part that doesn't work
    //byte[] publickey = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(aKeyPair.getPublic()).GetDerEncoded();
}

In java, there doesn't seem to be a SubjectPublicKeyInfoFactory class or the equivalent that can take the aKeyPair.getPublic() and be able to generate a DER encoded byte array. Can anyone please help!??!!? I'm about at my wits end! Thanks!!!!

----------EDIT 2 -------------------------------------------------------------------------

Ok, so here's where I'm at now:

public void test2() throws Exception
{
    ECKeyPairGenerator g = new ECKeyPairGenerator();

    Fp curve = new Fp(
            new BigInteger("883423532389192164791648750360308885314476597252960362792450860609699839"), // q
            new BigInteger("7fffffffffffffffffffffff7fffffffffff8000000000007ffffffffffc", 16), // a
            new BigInteger("6b016c3bdcf18941d0d654921475ca71a9db2fb27d1d37796185c2942c0a", 16)); // b

    ECDomainParameters ecP = new ECDomainParameters(
            curve,
            curve.decodePoint(Hex.decode("020ffa963cdca8816ccc33b8642bedf905c3d358573d3f27fbbd3b3cb9aaaf")), // G
            new BigInteger("883423532389192164791648750360308884807550341691627752275345424702807307"), // n
            BigInteger.ONE); // h

    g.init(new ECKeyGenerationParameters(ecP, new SecureRandom()));

    // Generate key pair
    AsymmetricCipherKeyPair aKeys = g.generateKeyPair();

    JCEECPublicKey jpub = new JCEECPublicKey("EC", (ECPublicKeyParameters)aKeys.getPublic());
    JCEECPrivateKey jpriv = new JCEECPrivateKey("EC", (ECPrivateKeyParameters)aKeys.getPrivate());

    KeyPair aKeyPair = new KeyPair(jpub, jpriv);

    ECDHBasicAgreement aKeyAgree = new ECDHBasicAgreement();

    aKeyAgree.init(aKeys.getPrivate());

    byte[] encoded = aKeyPair.getPublic().getEncoded();
    // The part that breaks now (Exception DERNull)
    ECPublicKeyParameters decoded = decodeECPublicKeyParameters(encoded);
}

public static ECPublicKeyParameters decodeECPublicKeyParameters(byte[] pkByte) throws IOException {
    return (ECPublicKeyParameters) PublicKeyFactory.createKey(pkByte);
    }

So I've been able to get the public/private keys into JCEEC Key objects and have been able to encode them. When I attempt to decode them I get a DERNull exception. I ran some other tests and generated keys using the regular native java KeyPairGenerator and was able to encode/decode the keys so I know that this method does work. I think there is just something missing when I convert the AsymmetricCipherKeys into the JCEEC Keys. I did notice there is another parameter in the JCEECPublicKey construct, the third parameter of ECKeySpec. Only trouble is, I'm not exactly sure how to get an ECKeySpec out of the code I have so far (or if thats even the problem to begin with). Any other suggestions? Thanks!!!

Answer 1

Have you tried using the Bouncycastle SubjectPublicKeyInfo class? Something like:

byte [] derEncoded;
//... 
SubjectPublicKeyInfo pkInfo = new SubjectPublicKeyInfo((ASN1Sequence)ASN1Object.fromByteArray(derEncoded))

EDIT:

There is an easy if somewhat unsatisfying way. You can use the JCEPublicKey class and it has a getEncoded() method that produces (I think) the correct answer.

EDIT 2:

I'm learning as I go :) It turns out you must identify the elliptic curve in the algorithm parameters, which makes sense. Here is a slight change that does that.

    g.init(new ECKeyGenerationParameters(ecP, new SecureRandom()));

    // Generate key pair
    AsymmetricCipherKeyPair aKeys = g.generateKeyPair();

    ECParameterSpec ecSpec = new ECParameterSpec(ecP.getCurve(), ecP.getG(), ecP.getN());
    JCEECPublicKey jpub = new JCEECPublicKey("EC",
            (ECPublicKeyParameters) aKeys.getPublic(), ecSpec);
    JCEECPrivateKey jpriv = new JCEECPrivateKey("EC",
            (ECPrivateKeyParameters) aKeys.getPrivate(), jpub,  ecSpec);