How to protect my plugin in svn?

Hi, I have a plugin called KnxDT WordPress Bookmarks and I have a few doubts about the svn system to upload the plugin content:

– Ok, I upload my plugin using the svn but in no one moment it request me a password or something. Can anyone install svn on his computer and update my plugin files? How can I set the permissions using my wordpress plugin password or something?

EDIT: When I commit my local plugin folder I set my user and password but when I start I get this message:

Error: Commit failed (details follow):
Error: Server sent unexpected return value (403 Forbidden) in response to CHECKOUT”
Error: request for ‘/!svn/ver/40406/knxdt-bookmarks-wordpress-plugin/tags’
Finished!

– The other question is: How the WordPress could ‘know’ if a new version is available to THAT plugin? I know that the TAG directory could do that BUT How do you relation THAT plugin with THAT svn url?

I don’t know if you unsertood my question. I hope that, excuse my newbie questions and bad english. Thank so much WordPress comunity.