Sample Code

private static void isMessageValid(String signing_cert_url,
            String signature, Map<String, String> message) {
        InputStream in = null;
        try {
            URL url = new URL(signing_cert_url);
            in = url.openStream();
            CertificateFactory cf = CertificateFactory.getInstance("X.509");
            X509Certificate cert = (X509Certificate) cf.generateCertificate(in);
            Signature sig = Signature.getInstance(cert.getSigAlgName());
            sig.initVerify(cert.getPublicKey());
            sig.update(buildSignMessage(message).getBytes("UTF-8"));
            byte[] sigByte = Base64.getDecoder().decode(signature);
            if (sig.verify(sigByte)) {
                System.out.println("Verify success");
            } else {
                System.out.println("Verify failed");
            }
        } catch (Exception e) {
            throw new SecurityException("Verify method failed.", e);
        } finally {
            if (in != null) {
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

private static String buildSignMessage(Map<String,String> msg) {
    String type = msg.get("type");
    String message = null;
    if ("Notification".equals(type)){
        message = buildNotificationMessage(msg);
    } else if ("SubscriptionConfirmation".equals(type) ||
    "UnsubscribeConfirmation".equals(type)){
        message = buildSubscriptionMessage(msg);
    }
    return message;
}

private static String buildSubscriptionMessage(Map<String, String> msg) {
    String stringMessage = "message\n";
    stringMessage += msg.get("message") + "\n";
    stringMessage += "message_id\n";
    stringMessage += msg.get("message_id") + "\n";
    stringMessage += "subscribe_url\n";
    stringMessage += msg.get("subscribe_url") + "\n";
    stringMessage += "timestamp\n";
    stringMessage += msg.get("timestamp") + "\n";
    stringMessage += "topic_urn\n";
    stringMessage += msg.get("topic_urn") + "\n";
    stringMessage += "type\n";
    stringMessage += msg.get("type") + "\n";
    return stringMessage;
}

private static String buildNotificationMessage(Map<String, String> msg)
    {
        String stringMessage = "message\n";
        stringMessage += msg.get("message").toString() + "\n";
        stringMessage += "message_id\n";
        stringMessage += msg.get("message_id").toString() + "\n";
        if (msg.get("subject") != null){
             stringMessage += "subject\n";
             stringMessage += msg.get("subject").toString() + "\n";
        }
        stringMessage += "timestamp\n";
        stringMessage += msg.get("timestamp").toString() + "\n";
        stringMessage += "topic_urn\n";
        stringMessage += msg.get("topic_urn").toString() + "\n";
        stringMessage += "type\n";
        stringMessage += msg.get("type").toString() + "\n";
        return stringMessage;
    }

package demo

import (
	"bytes"
	"crypto"
	"crypto/rsa"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"io/ioutil"
)

type Message struct {
	Signature        string  `json:"signature"`
	Subject          *string `json:"subject"`
	TopicUrn         string  `json:"topic_urn"`
	MessageId        string  `json:"message_id"`
	SignatureVersion string  `json:"signature_version"`
	Type             string  `json:"type"`
	Message          string  `json:"message"`
	SubscribeUrl     string  `json:"subscribe_url"`
	UnsubscribeUrl   string  `json:"unsubscribe_url"`
	SigningCertUrl   string  `json:"signing_cert_url"`
	Timestamp        string  `json:"timestamp"`
}

func VerifyMessage(pemFile string, message string) bool {
	msg := Message{}
	err := json.Unmarshal([]byte(message), &msg)
	if err != nil {
		fmt.Println("Convert json to struct failed")
		return false
	}
	pemContent, err := ioutil.ReadFile(pemFile)
	if err != nil {
		fmt.Println("Read pem file failed")
		return false
	}
	certDerblock, _ := pem.Decode(pemContent)
	if certDerblock == nil {
		fmt.Println("Decode pem file failed")
		return false
	}
	cert, err := x509.ParseCertificate(certDerblock.Bytes)
	if err != nil {
		fmt.Println("Parse cert failed")
		return false
	}

	msgString := buildMessage(&msg)
	msgHash := crypto.SHA256.New()
	msgHash.Write([]byte(msgString))
	msgHashSum := msgHash.Sum(nil)

	decodeSign, _ := base64.StdEncoding.DecodeString(msg.Signature)

	publicKey := cert.PublicKey.(*rsa.PublicKey)
	err = rsa.VerifyPKCS1v15(publicKey, crypto.SHA256, msgHashSum, decodeSign)
	if err != nil {
		fmt.Println("Verify failed")
		return false
	} else {
		fmt.Println("Verify success")
		return true
	}
}

func buildMessage(msg *Message) string {
	if msg.Type == "Notification" {
		return buildNotificationMessage(msg)
	} else if msg.Type == "SubscriptionConfirmation" || msg.Type == "UnsubscribeConfirmation" {
		return buildSubscriptionMessage(msg)
	}
	return ""
}

func buildNotificationMessage(msg *Message) string {
	buf := bytes.Buffer{}
	buf.WriteString("message\n" + msg.Message + "\n")
	buf.WriteString("message_id\n" + msg.MessageId + "\n")
       //The Subject field does not exist in msg, and this issue needs to be addressed.
	if msg.Subject != nil {
		buf.WriteString("subject\n" + *msg.Subject + "\n")
	}
	buf.WriteString("timestamp\n" + msg.Timestamp + "\n")
	buf.WriteString("topic_urn\n" + msg.TopicUrn + "\n")
	buf.WriteString("type\n" + msg.Type + "\n")
	return buf.String()
}

func buildSubscriptionMessage(msg *Message) string {
	buf := bytes.Buffer{}
	buf.WriteString("message\n" + msg.Message + "\n")
	buf.WriteString("message_id\n" + msg.MessageId + "\n")
	buf.WriteString("subscribe_url\n" + msg.SubscribeUrl + "\n")
	buf.WriteString("timestamp\n" + msg.Timestamp + "\n")
	buf.WriteString("topic_urn\n" + msg.TopicUrn + "\n")
	buf.WriteString("type\n" + msg.Type + "\n")
	return buf.String()
}