{
  "transaction_id": "e9a0b0c8-6aad-4a63-8679-501bd2d28761",
  "object_id": "9de9b5e3-1720-11e8-8790-0242ac110002",
  "destination": "https://myawesomeapp.com/webhooks",
  "timestamp": 1517236809256,
  "event_timestamp": 1517236809256,
  "flow": "publish_advert",
  "event_type": "advert_posted_success",
  "error_message": "",
  "data": { ... }
}

package com.olx.example;

import com.olx.ecosystem.util.HmacSha1SignatureUtil;
import java.security.GeneralSecurityException;
import java.util.Objects;

public class Client {

  private static final String HMAC_SHA1_ALGORITHM = "HmacSHA1";
  
  public void handleNotification(final String signatureHeader,
    final WebhookNotification notification, final String secret)
    throws GeneralSecurityException {
    String toSign = String.format("%s,%s", 
      notification.getObjectId(), notification.getTransactionId());
    String signature = calculateRFC2104HMAC(toSign, secret);
    if (!Objects.equals(signature, signatureHeader)) {
      throw new SecurityException(
        "Signature in header (" + signatureHeader + ") does not match expected signature (" + signature + ")");
    }
    ... // do whatever you need to do with this notification
  }

  private String calculateRFC2104HMAC(final String data, final String key) throws GeneralSecurityException {
    if (data == null || StringUtils.isBlank(key)) {
      return null;
    }
    Key signingKey = new SecretKeySpec(key.getBytes(), HMAC_SHA1_ALGORITHM);
    Mac mac = Mac.getInstance(HMAC_SHA1_ALGORITHM);
    mac.init(signingKey);
    return Hex.encodeHexString(mac.doFinal(data.getBytes()));
  }
  
  ...
}

<?php 

  function handleNotification($signatureHeader, $notification, $secret) {
    $signature = hash_hmac("sha1", 
         "$notification->objectId,$notification->transactionId", $secret);
    if($signature != $signatureHeader) {
      throw new \Exception("Signature in header ($signatureHeader) does not match expected signature ($signature)");
    }

    ... // do whatever you need to do with this notification
  }
...
?>

package "example"

import (
	"crypto/sha1"
	"crypto/hmac"
	"encoding/hex"
	"fmt"
)

func HandleNotification(signatureHeader string, message Notification, secret string) error {
	h := hmac.New(sha1.New, []byte(secret))
  msg := fmt.Sprintf("%s,%s", message.ObjectId, message.TransactionId)
	h.Write([]byte(msg))
	signature := hex.EncodeToString(h.Sum(nil))

	if signature != signatureHeader {
    return fmt.Errorf("Signature in header (%s) does not match expected signature (%s)", signatureHeader, signature)
	}

  ... // do whatever you need to do with this notification
	return nil
}

using System;
using System.Security.Cryptography;

  public void handleNotification(string signatureHeader, Notification notification, string secret) {
    secret = secret ?? "";

    var encoding = new System.Text.ASCIIEncoding();
    byte[] keyByte = encoding.GetBytes(secret);

    string toSign = String.Format("{0},{1}", notification.ObjectId, notification.TransactionId);
    byte[] messageBytes = encoding.GetBytes(toSign);

    using (var hmacsha256 = new HMACSHA1(keyByte))
    {
      byte[] hashmessage = hmacsha256.ComputeHash(messageBytes);
      string signature = BitConverter.ToString(hashmessage).Replace("-", string.Empty).ToLower();
      
      if (!signature.Equals(signatureHeader)) {
        throw new Exception(
          "Signature in header (" + signatureHeader + ") does not match expected signature (" + signature + ")");
      }
    }
    ... // do whatever you need to do with this notification
  }
...

import hmac
import hashlib

...
def handle_notification(signature_header, notification, secret):
	msg = "{},{}".format(notification["object_id"], notification["transaction_id"]).encode("utf-8")
	signature = hmac.new(secret, msg=msg, digestmod=hashlib.sha1).hexdigest()
	if signature_header != signature:
		ex_msg = "Signature in header ({}) does not match expected signature ({})".format(signature_header, signature)
		raise Exception(ex_msg)
  
  ... // do whatever you need to do with this notification

...