Hello,
For keeping safe and secure critical data online, we (a non profit organisation and a small group of managers) use the
encryptedpasswords plugin inside private namespaces.
It is possible using many times the same password in a page to keep an easy way to navigate the encrypted data with wiki syntax. You cannot protect medias with, only text.
It is not 100 % secure (nothing is 100 % secure) but as we use https and ModSecurity (Web Application Firewall) module and have a strong passwords politic for login, it seems enough for us to encrypt data in private namespaces to get the extra needed. Easy to set up and to use as a secure vault online for all the passwords needed (Bank and Insurance Accounts, tel numbers,...). As it is on a case-by-case basis, I am confident in this configuration, at this moment.
Among ways to harden your wiki (restrict acess by IP , two factor authentification, .... )
this page gives an overview about security in DokuWiki and
here a list of plugins tagged with security.
Hope this will help you to find a solution that fits to your security needs.