Fix random shared secret being of odd length

Author: Minishlink

Diff for: src/Encryption.php

@@ -65,7 +65,7 @@ public static function encrypt(string $payload, string $userPublicKey, string $u
 
         // get shared secret from user public key and local private key
         $sharedSecret = $curve->mul($userPublicKeyObject->getPoint(), $localPrivateKeyObject->getSecret())->getX();
-        $sharedSecret = hex2bin(gmp_strval($sharedSecret, 16));
+        $sharedSecret = hex2bin(str_pad(gmp_strval($sharedSecret, 16), 64, '0', STR_PAD_LEFT));
 
         // generate salt
         $salt = random_bytes(16);