@freakazoid It looks like they have an explanation of their encryption here: https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/encryption_details.html
It appears that you are right that they are base64-encoding the blocks: https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/encryption_details.html#encrypt-the-blocks
Seems an odd choice, but apparently it's a standard option in PHP's openssl_encrypt.