diff --git a/Controllers/FHIRcastClientController.cs b/Controllers/FHIRcastClientController.cs
index a63aca5..86e9796 100644
--- a/Controllers/FHIRcastClientController.cs
+++ b/Controllers/FHIRcastClientController.cs
@@ -87,8 +87,23 @@ public IActionResult Post([FromForm] ClientModel model)
         /// <param name="verification">Hub's verification response to our subscription attempt</param>
         /// <returns></returns>
         [HttpGet("{subscriptionId}")]
-        public IActionResult Get(string subscriptionId, [FromQuery] SubscriptionVerification verification)
+        public IActionResult Get(string subscriptionId)
         {
+            if (!Request.Query.ContainsKey("hub.challenge"))
+            {
+                this.logger.LogDebug($"Missing hub.challenge");
+                return NotFound();
+            }
+
+            if (!Request.Query.ContainsKey("hub.topic"))
+            {
+                this.logger.LogDebug($"Missing hub.topic");
+                return NotFound();
+            }
+
+            string challenge = Request.Query["hub.challenge"];
+            string topic = Request.Query["hub.topic"];
+
             //Received a verification request for non-pending subscription, return a NotFound response
             if (!pendingSubs.ContainsKey(subscriptionId)) { return NotFound(); }
 
@@ -96,12 +111,12 @@ public IActionResult Get(string subscriptionId, [FromQuery] SubscriptionVerifica
 
             //Validate verification subcription with our subscription. If a match return challenge
             //otherwise return NotFound response.
-            if (SubsEqual(sub, verification))
+            if (topic == sub.Topic)
             {
                 //Move subscription to active sub collection and remove from pending subs
                 activeSubs.Add(subscriptionId, sub);
                 pendingSubs.Remove(subscriptionId);
-                return this.Content(verification.Challenge);
+                return this.Content(challenge);
             }
             else
             {
@@ -148,7 +163,7 @@ public async Task<IActionResult> Subscribe(string subscriptionUrl, string topic,
                 UID = subUID,
                 Callback = new Uri(this.Request.Scheme + "://" + this.Request.Host + "/client/" + subUID),
                 Events = events.Split(";", StringSplitOptions.RemoveEmptyEntries),
-                Mode = SubscriptionMode.Subscribe,
+                Mode = SubscriptionMode.subscribe,
                 Secret = secret,
                 LeaseSeconds = 3600,
                 Topic = topic
@@ -179,14 +194,14 @@ public async Task<IActionResult> Subscribe(string subscriptionUrl, string topic,
             return View("FHIRcastClient", internalModel);
         }
 
-        [Route("unsubscribe/{subscriptionId}")]
+        [Route("Unsubscribe/{subscriptionId}")]
         [HttpPost]
         public async Task<IActionResult> Unsubscribe(string subscriptionId)
         {
             this.logger.LogDebug($"Unsubscribing subscription {subscriptionId}");
             if (!activeSubs.ContainsKey(subscriptionId)) { return View("FHIRcastClient", internalModel); }
             Subscription sub = activeSubs[subscriptionId];
-            sub.Mode = SubscriptionMode.Unsubscribe;
+            sub.Mode = SubscriptionMode.unsubscribe;
 
             var httpClient = new HttpClient();
             var result = await httpClient.PostAsync(sub.HubURL,
diff --git a/Model/Subscriptions.cs b/Model/Subscriptions.cs
index 9d9bb0d..40f284c 100644
--- a/Model/Subscriptions.cs
+++ b/Model/Subscriptions.cs
@@ -62,8 +62,8 @@ public class SubscriptionVerification : SubscriptionWithLease {
     }
 
     public enum SubscriptionMode {
-        Subscribe,
-        Unsubscribe,
+        subscribe,
+        unsubscribe,
     }
 
     public class SubscriptionComparer : IEqualityComparer<Subscription> {
diff --git a/Rules/SubscriptionValidator.cs b/Rules/SubscriptionValidator.cs
index b352215..e707889 100644
--- a/Rules/SubscriptionValidator.cs
+++ b/Rules/SubscriptionValidator.cs
@@ -18,40 +18,49 @@ public async Task<ClientValidationOutcome> ValidateSubscription(Subscription sub
                 throw new ArgumentNullException(nameof(subscription));
             }
 
-            SubscriptionBase callbackParameters = null;
+
+            HttpResponseMessage response = null;
+            string challenge = Guid.NewGuid().ToString("n");
+
             if (outcome == HubValidationOutcome.Canceled) {
                 logger.LogDebug("Simulating canceled subscription.");
 
-                callbackParameters = new SubscriptionCancelled
+                SubscriptionBase callbackParameters = new SubscriptionCancelled
                 {
                     Reason = $"The subscription {subscription} was canceled for testing purposes.",
                 };
             } else {
                 logger.LogDebug("Verifying subscription.");
 
-                callbackParameters = new SubscriptionVerification
+                SubscriptionVerification callbackParameters = new SubscriptionVerification
                 {
                     // Note that this is not necessarily cryptographically random/secure.
-                    Challenge = Guid.NewGuid().ToString("n"),
-                };
-            }
+                    Challenge = challenge,
+                    LeaseSeconds = subscription.LeaseSeconds,
+                    Callback = subscription.Callback,
+                    Events = subscription.Events,
+                    Mode = subscription.Mode,
+                    Topic = subscription.Topic,
+                 };
 
-            // Default parametres for both cancel/verify.
-            callbackParameters.Callback = subscription.Callback;
-            callbackParameters.Events = subscription.Events;
-            callbackParameters.Mode = subscription.Mode;
-            callbackParameters.Topic = subscription.Topic;
+                logger.LogDebug($"Calling callback url: {subscription.Callback}");
+                string verifyUrl = subscription.Callback + "?" +                   
+                     $"&hub.mode={callbackParameters.Mode.ToString().ToLower()}" +
+                     $"&hub.topic={callbackParameters.Topic}" +
+                     $"&hub.challenge={callbackParameters.Challenge}" +
+                     $"&hub.events={string.Join(",", callbackParameters.Events)}" +
+                     $"&hub.lease_seconds={callbackParameters.LeaseSeconds}";
+                response = await new HttpClient().GetAsync(verifyUrl);
+            }
 
-            logger.LogDebug($"Calling callback url: {subscription.Callback}");
-            var callbackUri = new SubscriptionCallback().GetCallbackUri(subscription, callbackParameters);
-            var response = await new HttpClient().GetAsync(callbackUri);
+       
 
             if (!response.IsSuccessStatusCode) {
                 logger.LogInformation($"Status code was not success but instead {response.StatusCode}");
                 return ClientValidationOutcome.NotVerified;
             }
             if (outcome == HubValidationOutcome.Valid) {
-                var challenge = ((SubscriptionVerification)callbackParameters).Challenge;
+
                 var responseBody = (await response.Content.ReadAsStringAsync());
                 if (responseBody != challenge) {
                     logger.LogInformation($"Callback result for verification request was not equal to challenge. Response body: '{responseBody}', Challenge: '{challenge}'.");
diff --git a/Rules/ValidateSubscriptionJob.cs b/Rules/ValidateSubscriptionJob.cs
index 816461d..c21465b 100644
--- a/Rules/ValidateSubscriptionJob.cs
+++ b/Rules/ValidateSubscriptionJob.cs
@@ -16,7 +16,7 @@ public ValidateSubscriptionJob(ISubscriptionValidator validator, ISubscriptions
         }
 
         public async Task Run(Subscription subscription, bool simulateCancellation) {
-            if (subscription.Mode == SubscriptionMode.Subscribe) {
+            if (subscription.Mode == SubscriptionMode.subscribe) {
                 HubValidationOutcome validationOutcome = simulateCancellation ? HubValidationOutcome.Canceled : HubValidationOutcome.Valid;
                 var validationResult = await this.validator.ValidateSubscription(subscription, validationOutcome);
                 if (validationResult == ClientValidationOutcome.Verified) {
@@ -26,7 +26,7 @@ public async Task Run(Subscription subscription, bool simulateCancellation) {
                 } else {
                     this.logger.LogInformation($"Not adding unverified subscription: {subscription}.");
                 }
-            } else if (subscription.Mode == SubscriptionMode.Unsubscribe) {
+            } else if (subscription.Mode == SubscriptionMode.unsubscribe) {
                 this.subscriptions.RemoveSubscription(subscription);
             }
         }