Guidelines for URL Storage and Comparison

This document contains some thoughts about the storage and comparison of URLs, common operations crucial to the correct function of security software like Microsoft SmartScreen. Importantly, URLs are also displayed on security surfaces to enable the user to make a decision based on their contents. Eight years ago, I wrote the Guidelines for URL Display. Background The Web allows linking and retrieval of various resources via an address known as a URL. A URL (Uniform Resource Locator) is an identifier used to locate a resource on the Internet. (Note: “URI” stands for Uniform Resource Identifier, and in the real world, the two terms are used interchangeably). It’s tempting to think of URLs as plain strings because almost all clients (like web browsers) and servers accept URL input as strings. However, despite the existence of various standards for the representation of URLs, there is considerable variation in the handling of URLs that creates inconsistency and variability in the…