...which mechanism is this....
I have to leave it to Anders to answer that.
I don't think there is a name for the mechanism we use. It's a password form and the credentials are stored in the session.
...is it as secure as .htacces ?
It should be just as secure as the sign in mechanism on the jalbum.net site. You can use https for the album and the password form if you like.
I sometimes wish I knew a way of circumvent the password form, but I don't... And I wrote the code for our password protection...