fix signing tests on php 8

Author: smoench

src/Box.php

@@ -41,6 +41,7 @@
 use function KevinGH\Box\FileSystem\remove;
 use KevinGH\Box\PhpScoper\NullScoper;
 use KevinGH\Box\PhpScoper\WhitelistManipulator;
+use function openssl_error_string;
 use function openssl_pkey_export;
 use function openssl_pkey_get_details;
 use function openssl_pkey_get_private;
@@ -362,7 +363,7 @@ public function sign(string $key, ?string $password): void
 
         $resource = openssl_pkey_get_private($key, (string) $password);
 
-        Assertion::isResource($resource, 'Could not retrieve the private key, check that the password is correct.');
+        Assertion::notSame(false, $resource, sprintf('Could not retrieve the private key, check that the password is correct. "%s"', openssl_error_string() ?: ''));
 
         openssl_pkey_export($resource, $private);