diff --git a/src/Utility.php b/src/Utility.php
index 9f3a527..820852f 100644
--- a/src/Utility.php
+++ b/src/Utility.php
@@ -78,28 +78,26 @@ public function generateOnboardingSignature($data, $secret){
     
     private function encrypt($dataToEncrypt, $secret) {
         try {
-            // Use the first 16 bytes of the secret as the key
             $key = substr($secret, 0, 16);
-    
-            // Use the first 12 bytes of the key as IV
-            $iv = substr($key, 0, 12);
-    
-            // Encrypt the data using AES-128-GCM
+
+            // Generate a fresh random 12-byte nonce per call (fixes AES-GCM nonce reuse).
+            // A static IV derived from the key allows keystream recovery and tag forgery
+            // (NIST SP 800-38D §8.3 Forbidden Attack) using only two captured ciphertexts.
+            $iv = random_bytes(12);
+
             $cipher = 'aes-128-gcm';
-            $tag = ''; // Authentication tag will be filled after encryption
+            $tag = '';
             $encryptedData = openssl_encrypt($dataToEncrypt, $cipher, $key, OPENSSL_RAW_DATA, $iv, $tag, '', 16);
-    
+
             if ($encryptedData === false) {
-                throw new Exception('Encryption failed');
+                throw new \Exception('Encryption failed');
             }
-    
-            // Concatenate encrypted data with the authentication tag
-            $finalData = $encryptedData . $tag;
-    
-            // Convert to hex string
-            return bin2hex($finalData);
-        } catch (Exception $e) {
-            throw new Exception('Encryption failed: ' . $e->getMessage());
+
+            // Output format: iv (12 bytes) || ciphertext || tag (16 bytes), hex-encoded.
+            // Receiver must read the first 24 hex chars as the IV before decrypting.
+            return bin2hex($iv . $encryptedData . $tag);
+        } catch (\Exception $e) {
+            throw new \Exception('Encryption failed: ' . $e->getMessage());
         }
     }
 
diff --git a/tests/OAuthTokenClientTest.php b/tests/OAuthTokenClientTest.php
index aeeeeda..cceb3b3 100644
--- a/tests/OAuthTokenClientTest.php
+++ b/tests/OAuthTokenClientTest.php
@@ -118,15 +118,21 @@ public function testRevokeTokenExecutesValidationFailure(){
     }
 
     public function testGenerateOnboardingSignature(){
-        // encrypted from Java sdk;
-        $expectedSignature = "37ad80c568a44f6999aa8f80bb5080dbc50eed353d325cb94d624bf82a9a36d12e4fd00490bc06271e06628c889c6b1c2a48e2f355f8598210d1b1c8c1c42dfcd02502f1515294028fd4";
         $api = new Api("key", "secret");
         $secret = "mzhK9zRdA2QoLxhlSR6Pg721";
         $attributes = [
             "submerchant_id" => "avaBWdazt7LoYu",
-            "timestamp" => 1741098479 
+            "timestamp" => 1741098479
         ];
-        $actualSignature = $api->utility->generateOnboardingSignature($attributes, $secret);
-        $this->assertEquals($expectedSignature, $actualSignature);    
+
+        $sig1 = $api->utility->generateOnboardingSignature($attributes, $secret);
+        $sig2 = $api->utility->generateOnboardingSignature($attributes, $secret);
+
+        // Output is hex: 12-byte IV + ciphertext + 16-byte tag — minimum 28 bytes = 56 hex chars.
+        $this->assertMatchesRegularExpression('/^[0-9a-f]+$/', $sig1);
+        $this->assertGreaterThanOrEqual(56, strlen($sig1));
+
+        // Each call must produce a unique ciphertext (random nonce per call).
+        $this->assertNotEquals($sig1, $sig2);
     }
 }