CloudKit Java Server to Server Client

package testclient.cloudkit;


import org.apache.http.HttpEntity;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.bouncycastle.crypto.digests.SHA256Digest;
import org.bouncycastle.openssl.PEMKeyPair;
import org.bouncycastle.openssl.PEMParser;
import org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter;

import java.io.IOException;
import java.io.StringReader;
import java.io.UnsupportedEncodingException;
import java.security.*;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Base64;
import java.util.Date;
import java.util.TimeZone;

public class CloudKitRequest {

    private String containerId;
    private String keyId;
    private String environment;
    private PrivateKey privateKey;
    private final String host = "api.apple-cloudkit.com";
    private String cloudKitResource;
    private String payload;
    private String operation;
    private String version = "1";


    public CloudKitRequest(String containerId, String keyId, String environment, String privateKeyText) {
        this.containerId = containerId;
        this.keyId = keyId;
        this.environment = environment;
        this.privateKey = loadPrivateKey(privateKeyText);
    }

    public CloudKitRequest setResource(String cloudKitResource) {
        this.cloudKitResource = cloudKitResource;
        return this;
    }

    public CloudKitRequest setEntity(String payload) {
        this.payload = payload;
        return this;
    }

    public CloudKitRequest setOperation(String operation) {
        this.operation = operation;
        return this;
    }

    public CloseableHttpResponse execute() {
        String dateString = getIsoDate();
        String requestUrl = "/database/" + version + "/" + containerId + "/" + environment + cloudKitResource;
        String concat = dateString+":"+hashRequestBody(payload)+":"+requestUrl;
        CloseableHttpClient httpclient = HttpClients.createDefault();
        final HttpPost httpPostRequest = new HttpPost("https://" + host + requestUrl);

        httpPostRequest.setHeader("X-Apple-CloudKit-Request-KeyID", keyId);
        httpPostRequest.setHeader("X-Apple-CloudKit-Request-ISO8601Date", dateString);
        httpPostRequest.setHeader("X-Apple-CloudKit-Request-SignatureV1", signRequest(concat));
        httpPostRequest.setHeader("Content-Type", "text/plain");

        try {
            httpPostRequest.setEntity( new StringEntity(payload));
            return httpclient.execute(httpPostRequest);
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e.getMessage());
        } catch (ClientProtocolException e) {
            throw new RuntimeException(e.getMessage());
        } catch (IOException e) {
            throw new RuntimeException(e.getMessage());
        }
    }

    private PrivateKey loadPrivateKey(String keyText) {

        PEMParser pemPrivateKeyReader = new PEMParser(new StringReader(keyText));
        Object pemObject = null;
        try {
            pemObject = pemPrivateKeyReader.readObject();
            if (pemObject instanceof PEMKeyPair) {
                KeyPair pair = new JcaPEMKeyConverter().setProvider("BC").getKeyPair((PEMKeyPair) pemObject);
                return pair.getPrivate();
            } else {
                throw new RuntimeException("Unexpected key spec");
            }
        } catch (IOException e) {
            throw new RuntimeException(e.getMessage());
        }
    }

    private String getIsoDate() {
        TimeZone tz = TimeZone.getTimeZone("UTC");
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
        df.setTimeZone(tz);
        return df.format(new Date())+"Z";
    }

    public static String hashRequestBody(String body) {
        SHA256Digest digest = new SHA256Digest();
        byte[] ret = new byte[digest.getDigestSize()];
        byte[] data = body.getBytes();
        digest.update(data, 0, data.length);
        digest.doFinal(ret, 0);
        return Base64.getEncoder().encodeToString(ret);
    }

    private String signRequest(String request) {
        byte[] requestBytes = new byte[0];
        try {
            requestBytes = request.getBytes("UTF-8");
            Signature ecdsaSignature = Signature.getInstance("SHA256withECDSA", "BC");
            ecdsaSignature.initSign(privateKey);
            ecdsaSignature.update(requestBytes);
            byte[] signature = ecdsaSignature.sign();
            return Base64.getEncoder().encodeToString(signature);
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e.getMessage());
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e.getMessage());
        } catch (SignatureException e) {
            throw new RuntimeException(e.getMessage());
        } catch (NoSuchProviderException e) {
            throw new RuntimeException(e.getMessage());
        } catch (InvalidKeyException e) {
            throw new RuntimeException(e.getMessage());
        }
    }
}

public class Main {

    static String privateKeyText =
            "-----BEGIN EC PRIVATE KEY-----\n" +
            "your prive key is here as from openssl" +
            "-----END EC PRIVATE KEY-----";

    public static void main(String[] args) throws UnsupportedEncodingException {

        if (Security.getProvider("BC") == null) {
            Security.addProvider(new BouncyCastleProvider());
        }

        CloseableHttpResponse response = new CloudKitRequest("iCloud.your-id-here",
                "your-key-id-here as created in the dashboard for serevr to server API",
                "development",
                privateKeyText)
        .setOperation("POST")
        .setEntity("{\"operations\":[{\"operationType\":\"create\",\"record\":{\"recordType\":\"Post\",\"fields\":{\"title\":{\"value\":\"A Post From The Server\"}}}}]}")
        .setResource("/public/records/modify")
        .execute();
    }
}