Added method validatePostback to validate postback checksum

lib/index.js

@@ -486,6 +486,37 @@ var KiteConnect = function(api_key, options) {
 		);
 	};
 
+	/**
+	 * Validate postback data checksum
+	 * @method validatePostback
+	 * @memberOf KiteConnect
+	 * @instance
+	 * @param {object} postback_data Postback data received. Must be an json object with required keys order_id, checksum and order_timestamp
+	 * @param {string} api_secret Api secret of the app
+	 * @returns {bool} Return true if checksum matches else false
+	 * @throws Throws an error if the @postback_data or @api_secret is invalid
+	 */
+	self.validatePostback = function(postback_data, api_secret) {
+		if (!postback_data || !postback_data.checksum || !postback_data.order_id ||
+			!postback_data.order_timestamp || !api_secret) {
+			throw new Error("Invalid postback data or api_secret");
+		}
+
+		var inputString = postback_data.order_id + postback_data.order_timestamp + api_secret;
+		var checksum;
+		try {
+			checksum = crypto.createHash("sha256").update(inputString).digest("hex");
+		} catch (e) {
+			throw(e)
+		}
+
+		if (postback_data.checksum === checksum) {
+			return true;
+		} else {
+			return false;
+		}
+	}
+
 	function parseHistorical(jsonData) {
 		var results = [];
 		for(var i=0; i<jsonData.data.candles.length - 1; i++) {