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!

Robert Chapin

As of WordPress 3.0, most of this plugin’s original features have been incorporated into WordPress itself. WordPress adds shortlinks to most pages by default. This plugin simply adds a settings page where you can easily make some minor adjustments.
WordPress shortlinks are useful for micro-blogging, by using your own domain instead of a 3rd-party short URL service.
The URLs are automatically added to the HTTP and HTML headers of each post, page, attachment, and category.
A template tag enables you to display a human-readable link in addition to the automatically generated headers. Use of human-readable links is highly encouraged, because Google is known to favor them and rank them higher than some longer URLs. This has the effect of boosting the rank of URLs that were deemed “too long” by Google’s standards. Also, in many mobile web browsers, the easiest way for a visitor to find a short link is by seeing it somewhere on the page.
The template tag idea can be extended further with CSS code for print media, which could ensure that each article’s short link URL is printed along with the article. This will make it much easier for the user to return to the article after reading a hard copy.
<h3>Theme</h3>
Current documentation should be found at <a href="https://codex.wordpress.org/Function_Reference/the_shortlink" rel="nofollow ugc">Function Reference/the shortlink</a>
Here is a basic reference for the template functions provided by Simple Short Links. WordPress 3.0+ users, see also wp-includes/link-template.php for more details about the_shortlink().
<pre><code>/**
 * Template Tag for Displaying the Short Link
 *
 * Must be called from inside "The Loop"
 *
 * Call like <?php the_shortlink(__('Shortlinkage FTW')); ?>
 *
 * @since 1.1
 * @param string $text Optional The link text or HTML to be displayed. Defaults to 'This is the short link.'
 * @param string $title Optional The tooltip for the link. Must be sanitized. Defaults to the sanitized post title.
 */
function the_shortlink($text = '', $title = '');
</code></pre>
A second, similar tag is now available in case you need to display a self-referring short link on a category page.
<pre><code>/**
 * Template Tag for Displaying the Short Link for a Category
 *
 * Should be called from outside "The Loop"
 *
 * Call like <? the_single_shortlink(__('Shortlinkage FTW')); ?>
 *
 * @since 1.3
 * @param string $text Optional The link text or HTML to be displayed. Defaults to 'This is the short link.'
 * @param string $title Optional The tooltip for the link. Must be sanitized. Defaults to the sanitized category name.
 */
function the_single_shortlink($text = '', $title = '');
</code></pre>
<h4>The CSS Printing Trick</h4>
Try adding something like this to your theme’s CSS file to make the URL for the short link appear when printed.
<pre><code>@media print {
 .shortlink a:link:after {
 content: " " attr(href);
 }
}
</code></pre>
The CSS example assumes the_shortlink() is used inside of a DIV or P element with a class attribute called “shortlink”.

Simple Short Links

Adjust the WordPress shortlinks format with an extra settings page.

Base64 Shortlinks vs Simple Short Links