Manual:$wgSessionInsecureSecrets/de-formal

Details
MediaWiki will not work out of the box if the PHP openssl extension is not available on the server. If for some reason you cannot install one of these extensions, you can set  to "true" to make MediaWiki work again. This comes at the cost of storing sensitive session data insecurely.

If one of the two extensions is installed,  is not set, but none of the cipher algorithms, which MediaWiki supports, is available in that extension, you will get this error message:

Warning: openssl_encrypt: Unknown cipher algorithm in /wiki/includes/session/Session.php on line 424

This can be fixed like so: In order for OpenSSL to be able to work correctly, you will need one of these cipher algorithm: aes-256-ctr or aes-256-cbc. If you are using mcrypt, it needs to support the rijndael-128 algorithm and either ctr or cbc mode.