public static X509Certificate GetCertificate_v3(KeyPair keygen, Date startDate, Date expiryDate,
        String serial,  String Certification_Aut_Id) throws InvalidKeyException, SecurityException, SignatureException{

    X509V3CertificateGenerator v3CertGen =  new X509V3CertificateGenerator();
    v3CertGen.setSerialNumber(BigInteger.valueOf(System.currentTimeMillis()));
    v3CertGen.setIssuerDN(new X509Principal("CN=" + Certification_Aut_Id + ", O=o, L=L, ST=il, C= c"));
    v3CertGen.setNotBefore(startDate);
    v3CertGen.setNotAfter(expiryDate);
    v3CertGen.setSubjectDN(new X509Principal("CN=" + Certification_Aut_Id + ", O=o, L=L, ST=il, C= c"));
    v3CertGen.setPublicKey(keygen.getPublic());
    v3CertGen.setSignatureAlgorithm("SHA256withECDSA");
    X509Certificate cert = v3CertGen.generateX509Certificate(keygen.getPrivate());

    return cert;

}

public static void storeKeypair(String KSpwd, String PKpwd, String KSname, X509Certificate certificate,
        KeyPair keygen, String alias, String temp_local) throws KeyStoreException, NoSuchAlgorithmException, CertificateException, IOException{

    //Before a keystore can be accessed, it must be loaded.
        KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
        // get user password and file input stream
        char[] KSpassword = KSpwd.toCharArray();
        FileInputStream fis = new java.io.FileInputStream(KSname);
        ks.load(fis, KSpassword);
        fis.close();

        //writing the X509Certificate in a .cer file
        FileOutputStream fos1 = new FileOutputStream(temp_local + alias + ".cer");
        fos1.write( certificate.getEncoded() );
        fos1.flush();
        fos1.close();

    // Load the certificate chain (in X.509 DER encoding).
        FileInputStream certificateStream = new FileInputStream(temp_local + alias + ".cer");
        CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
        Certificate[] chain = {};
        chain = certificateFactory.generateCertificates(certificateStream).toArray(chain);

    // save my private key & certificate chain
        char[] PKpassword = PKpwd.toCharArray();
        ks.setEntry(alias, new KeyStore.PrivateKeyEntry(keygen.getPrivate(), chain),
                    new KeyStore.PasswordProtection(PKpassword)
                );

    //Store the KeyStore
     // Write out the keystore
        FileOutputStream fos = new FileOutputStream(KSname);
        ks.store(fos, KSpassword);
        fos.close();
}