diff --git a/src/VAPID.php b/src/VAPID.php
index 5a40cd9..9e9e424 100644
--- a/src/VAPID.php
+++ b/src/VAPID.php
@@ -54,7 +54,7 @@ public static function validate(array $vapid): array
                 throw new \ErrorException('Failed to convert VAPID public key from hexadecimal to binary');
             }
             $vapid['publicKey'] = base64_encode($binaryPublicKey);
-            $vapid['privateKey'] = base64_encode(str_pad(Base64Url::decode($jwk->get('d')), 2 * self::PRIVATE_KEY_LENGTH, '0', STR_PAD_LEFT));
+            $vapid['privateKey'] = base64_encode(str_pad(Base64Url::decode($jwk->get('d')), self::PRIVATE_KEY_LENGTH, '0', STR_PAD_LEFT));
         }
 
         if (!isset($vapid['publicKey'])) {