ECDH + Bouncy Castle

public class SessionController {
    private static final String BOB = "bob";
    private static final String ALICE = "alice";
    private static final String KEY_PUBLIC = "KEY_PUBLIC";
    private static final String KEY_PRIVATE = "KEY_PRIVATE";
    private static final String PROVIDER = "SC";
    private static final String CURVE = "secp256K1";
    private static final String ALGORITHM = "ECDH";
    private final ECParameterSpec ecNamedCurveParameterSpec = ECNamedCurveTable.getParameterSpec(CURVE);

    static {
        Security.addProvider(new BouncyCastleProvider());
    }

    @AfterInject
    void afterInject() {
        clearKey();
        generateKey();
        readKey();
    }

    void generateKey() {
        try {
            KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(ALGORITHM, PROVIDER);
            keyPairGenerator.initialize(new ECGenParameterSpec(CURVE), new SecureRandom());

            Log.d(SessionController.class.getSimpleName(), "<<<<<<<<<<GENERATE KEYS>>>>>>>>>>");
            //Alice
            Log.d(SessionController.class.getSimpleName(), "<<<<<<<<<<Alice>>>>>>>>>>");

            KeyPair pairA = keyPairGenerator.generateKeyPair();
            String publicKeyAlice = bytesToHex((((ECPublicKey) pairA.getPublic()).getQ().getEncoded(true)));
            String privateKeyAlice = bytesToHex((((ECPrivateKey) pairA.getPrivate()).getD().toByteArray()));

            Hawk.put(ALICE + KEY_PUBLIC, publicKeyAlice);
            Hawk.put(ALICE + KEY_PRIVATE, privateKeyAlice);

            Log.d(SessionController.class.getSimpleName(), "Alice Pub: " + publicKeyAlice);
            Log.d(SessionController.class.getSimpleName(), "Alice Prv: " + privateKeyAlice);

            //Bob
            Log.d(SessionController.class.getSimpleName(), "<<<<<<<<<<Bob>>>>>>>>>>");

            KeyPair pairB = keyPairGenerator.generateKeyPair();
            String publicKeyBob = bytesToHex((((ECPublicKey) pairB.getPublic()).getQ().getEncoded(true)));
            String privateKeyBob = bytesToHex((((ECPrivateKey) pairB.getPrivate()).getD().toByteArray()));

            Hawk.put(BOB + KEY_PUBLIC, publicKeyBob);
            Hawk.put(BOB + KEY_PRIVATE, privateKeyBob);

            Log.d(SessionController.class.getSimpleName(), "Bob Pub:   " + publicKeyBob);
            Log.d(SessionController.class.getSimpleName(), "Bob Prv:   " + privateKeyBob);

        } catch (Exception exception) {
            Log.e(SessionController.class.getSimpleName(), exception.getMessage());
            Crashlytics.logException(exception);
        }
    }

    void readKey() {
        try {
            Log.d(SessionController.class.getSimpleName(), "<<<<<<<<<<READ KEYS>>>>>>>>>>");

            //Alice
            Log.d(SessionController.class.getSimpleName(), "<<<<<<<<<<Alice>>>>>>>>>>");
            byte[] publicKeyAlice = hexToBytes(Hawk.get(ALICE + KEY_PUBLIC));
            byte[] privateKeyAlice = hexToBytes(Hawk.get(ALICE + KEY_PRIVATE));

            Log.d(SessionController.class.getSimpleName(), "Alice Pub: " + Arrays.toString(publicKeyAlice));
            Log.d(SessionController.class.getSimpleName(), "Alice Prv: " + Arrays.toString(privateKeyAlice));


            //Bob
            Log.d(SessionController.class.getSimpleName(), "<<<<<<<<<<Bob>>>>>>>>>>");
            byte[] publicKeyBob = hexToBytes(Hawk.get(BOB + KEY_PUBLIC));
            byte[] privateKeyBob = hexToBytes(Hawk.get(BOB + KEY_PRIVATE));

            Log.d(SessionController.class.getSimpleName(), "Bob Pub: " + Arrays.toString(publicKeyBob));
            Log.d(SessionController.class.getSimpleName(), "Bob Prv: " + Arrays.toString(privateKeyBob));


            doECDH("Alice secret: ", convertByteToPublicKey(publicKeyBob), convertByteToPrivateKey(privateKeyAlice));
            doECDH("Bob secret: ", convertByteToPublicKey(publicKeyAlice), convertByteToPrivateKey(privateKeyBob));
        } catch (Exception exception) {
            Log.e(SessionController.class.getSimpleName(), exception.getMessage());
            Crashlytics.logException(exception);
        }
    }

    public void clearKey(){
        Hawk.clear();
    }

    private String bytesToHex(byte[] bytes) {
        try {
            return new String(Hex.encode(bytes), "UTF-8");
        } catch (UnsupportedEncodingException unsupportedEncodingException) {
            Log.e(SessionController.class.getSimpleName(), unsupportedEncodingException.getMessage());
            Crashlytics.logException(unsupportedEncodingException);
        }

        return "";
    }

    private byte[] hexToBytes(String hex) {
        int length = hex.length();
        byte[] data = new byte[length / 2];

        for (int i = 0; i < length; i += 2) {
            data[i / 2] = (byte) ((Character.digit(hex.charAt(i), 16) << 4)
                    + Character.digit(hex.charAt(i + 1), 16));
        }

        return data;
    }

    private PublicKey convertByteToPublicKey(byte[] data) throws Exception {
        return KeyFactory.getInstance(ALGORITHM, PROVIDER).generatePublic(new ECPublicKeySpec(ecNamedCurveParameterSpec.getCurve().decodePoint(data), ecNamedCurveParameterSpec));
    }

    private PrivateKey convertByteToPrivateKey(byte[] data) throws Exception {
        return KeyFactory.getInstance(ALGORITHM, PROVIDER).generatePrivate(new ECPrivateKeySpec(new BigInteger(data), ecNamedCurveParameterSpec));
    }

    private void doECDH(String name, PublicKey publicKey, PrivateKey privateKey) throws Exception {
        KeyAgreement keyAgreement = KeyAgreement.getInstance(ALGORITHM, PROVIDER);
        keyAgreement.init(privateKey);
        keyAgreement.doPhase(publicKey, true);

        byte[] secret = keyAgreement.generateSecret();
        Log.d(SessionController.class.getSimpleName(), name + bytesToHex(secret));
    }
}