The default WordPress “shortlink” URLs look like this: <code>http://www.example.com/blog/?p=123</code>, where “<code>123</code>” is actually a 7+ digit decimal number. This plugin shrinks your shortlinks by encoding that number into only 4 characters, and using the abbreviated URL prefix of your choice.
On my site, the default shortlinks are 35 bytes long, even though I have a very short domain name. This plugin shrinks them to 21 total bytes, which is comparable to most public URL-shortener services, and better than many.
E.g., this: <code>http://www.jwz.org/blog/?p=13240780</code> 
becomes: <code>http://jwz.org/b/ygnM</code>
This doesn’t affect your (long) permalinks: those can still be in whatever format you like.

Base64 Shortlinks

This plugin makes your shortlinks shorter!

Shivanand Sharma

Ruchika Karda

WP Shortlinker will help you achieve actual permanent links when inserting links in WordPress editor by changing the internal links to use shortlinks instead of permalinks.
Once you have installed and activated the plugin, you can go ahead and try inserting a link into WordPress editor from the kitchen sink. The plugin will automatically change it to the shortlink.

WP Shortlinker

WP Shortlinker changes the internal links to use shortlinks instead of permalinks inside the WordPress editor.

Base64 Shortlinks vs WP Shortlinker