How to connect kepware-ua-server in Eclipse Milo

Question

I have a client.der generated by Kepware opc ua configuration.How to use it convert to KeyPair?

    File clientFile = new File("C:\\Users\\Rex\\Desktop\\client.der");
    FileInputStream privatekeyfile = new FileInputStream(clientFile);
    Security.addProvider(new BouncyCastleProvider());
    PEMParser pemParser = new PEMParser(new InputStreamReader(privatekeyfile));
    Object object = pemParser.readObject();
    PEMDecryptorProvider pemDecryptorProvider = new JcePEMDecryptorProviderBuilder().build(certificate.getPublicKey().toString().toCharArray());
    JcaPEMKeyConverter converter = new JcaPEMKeyConverter().setProvider("BC");
    keyPair = converter.getKeyPair(((PEMEncryptedKeyPair) object).decryptKeyPair(pemDecryptorProvider));

Object object = pemParser.readObject(); the object always is null.