Fix ActivityPub with Nextcloud

src/Module/Inbox.php

@@ -9,6 +9,7 @@ use Friendica\Protocol\ActivityPub;
 use Friendica\Core\System;
 use Friendica\Database\DBA;
 use Friendica\Util\HTTPSignature;
+use Friendica\Core\Logger;
 
 /**
  * ActivityPub Inbox

src/Util/HTTPSignature.php

@@ -204,6 +204,8 @@ class HTTPSignature
 
 		if (preg_match('/algorithm="(.*?)"/ism', $header, $matches)) {
 			$ret['algorithm'] = $matches[1];
+		} else {
+			$ret['algorithm'] = 'rsa-sha256';
 		}
 
 		if (preg_match('/headers="(.*?)"/ism', $header, $matches)) {