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Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
byte[] privateKeyBytes = .....;
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PrivateKey rsaPrivateKey = keyFactory.generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));
KeyStore store = KeyStore.getInstance("PKCS12", "BC");
store.load(null, null);
Certificate[] certificates = { certificate };
store.setKeyEntry("key", rsaPrivateKey, null, certificates);

ByteArrayOutputStream bOutput = new ByteArrayOutputStream();
store.store(bOutput, password);
return bOutput.toByteArray();