The Verge: Google Photos and the unguessable URL

So why is that public URL more secure than it looks? The short answer is that the URL is working as a password. Photos URLs are typically around 40 characters long, so if you wanted to scan all the possible combinations, you’d have to work through 10^70 different combinations to get the right one, a problem on an astronomical scale.

Or as we like to call it in the biz, “security by obscurity”.