I would like for User-agents to index my relative pages only without accessing any directory on my server.

As initial thought, i had this version in mind:

User-agent: *
Disallow: */*

Sitemap: http://www.mydomain.com/sitemap.xml

My Questions:

1. Is it correct to block all directories like that - Disallow: */*?
2. Would still search engines be able to see and index my sitemap if i disallowed all directories?
3. What are the best practices for securing the robots.txt file?

For Reference:

Here is a good tutorial for robots.txt

#Add this if you want to stop Alexa from indexing your site.
User-agent: ia_archiver
Disallow: /

#Add this to stop duggmirror 
User-agent: duggmirror
Disallow: /

#Add this to allow specific agents 
User-agent: Googlebot
Disallow: 

#Add this to allow all agents while blocking specific directories
User-agent:  *
Disallow: /cgi-bin/
Disallow: /*?*