diff --git a/src/Message/AbstractRequest.php b/src/Message/AbstractRequest.php
index 715e1bc..8d213b5 100644
--- a/src/Message/AbstractRequest.php
+++ b/src/Message/AbstractRequest.php
@@ -47,9 +47,9 @@ protected function guardParameters()
         );
     }
 
-    protected function createSignatureFromString($fullStringToHash)
+    protected function createSignatureFromString($fullStringToHash, $merchantKey)
     {
-        return hash('sha256', $fullStringToHash);
+        return hash_hmac('sha512', $fullStringToHash, $merchantKey);
     }
 
     // private function hex2bin($hexSource)
diff --git a/src/Message/CompletePurchaseRequest.php b/src/Message/CompletePurchaseRequest.php
index 5808109..30e12d6 100644
--- a/src/Message/CompletePurchaseRequest.php
+++ b/src/Message/CompletePurchaseRequest.php
@@ -45,6 +45,6 @@ protected function signature($merchantKey, $merchantCode, $paymentId, $refNo, $a
 
         $paramsInArray = [$merchantKey, $merchantCode, $paymentId, $refNo, $amount, $currency, $status];
 
-        return $this->createSignatureFromString(implode('', $paramsInArray));
+        return $this->createSignatureFromString(implode('', $paramsInArray), $merchantKey);
     }
 }
diff --git a/src/Message/PurchaseRequest.php b/src/Message/PurchaseRequest.php
index 5c9f869..d99fdfd 100644
--- a/src/Message/PurchaseRequest.php
+++ b/src/Message/PurchaseRequest.php
@@ -20,7 +20,7 @@ public function getData()
             'UserContact' => $this->getCard()->getNumber(),
             'Remark' => '',
             'Lang' => '',
-            'SignatureType' => 'SHA256',
+            'SignatureType' => 'HMAC-SHA512',
             'Signature' => $this->signature(
                 $this->getMerchantKey(),
                 $this->getMerchantCode(),
@@ -44,7 +44,6 @@ private function signature($merchantKey, $merchantCode, $refNo, $amount, $curren
 
         $paramsInArray = [$merchantKey, $merchantCode, $refNo, $amount, $currency];
 
-        return $this->createSignatureFromString(implode('', $paramsInArray));
+        return $this->createSignatureFromString(implode('', $paramsInArray), $merchantKey);
     }
-
 }