Hi Hans,
If it’s a standalone file but not hosted on tiddlyhost then one option is to password protect the directory on your server?
I am sure that a single html file could contain javascript code that attempted to password protect itself in some fashion but anyone gaining access to the html file (and hence the javascript code inside) would most likely be able to get past locks by inspection of the code. I would not dare say that a single file cannot be securely encrypted without need of any external sources including keys held in registered licensed apps but I think in general it’s better to assume that single files are difficult to secure without external keys.