DiffieHellman.java

public static void main(String[] args) throws Exception {
    Security.addProvider(new BouncyCastleProvider());
    KeyPairGenerator keygen = KeyPairGenerator.getInstance("ECDH", "BC");
    keygen.initialize(ECNamedCurveTable.getParameterSpec("P-256"), new SecureRandom());

    KeyPair keypair1 = keygen.generateKeyPair();
    KeyPair keypair2 = keygen.generateKeyPair();

    String secret1 = computeSecret((ECPrivateKey) keypair1.getPrivate(), (ECPublicKey) keypair2.getPublic());
    String secret2 = computeSecret((ECPrivateKey) keypair2.getPrivate(), (ECPublicKey) keypair1.getPublic());

    System.out.println(secret1);
    System.out.println(secret2);
}



public static String computeSecret(ECPrivateKey privateKey, ECPublicKey publicKey) throws Exception {
    KeyAgreement ka = KeyAgreement.getInstance("ECDH", "BC");
    ka.init(privateKey);
    ka.doPhase(publicKey, true); // true = lastPhase
    return Base64.encodeBase64String(ka.generateSecret());
}