Introduction
Modern Internet users are used to the convenience of signing in with major websites they often use. "Sign in with Google", "Sign in with Facebook", and "Sign in with Apple" are staples of many websites' login forms.
Meanwhile, here in FFXIV-land, over and over again, the developers of fan websites and apps face the same problem: they want to allow users to register characters on their accounts, or to sign up as specific characters, but there is no official way to verify that a character indeed belongs to the person at the keyboard.
For That Other MMO, this is a Solved Problem. Blizzard itself provides a programmatic API that gives third-party websites and apps an official, documented way to verify ownership of World of Warcraft characters and let users sign in with Blizzard accounts like they would with Google or Facebook.
Nothing comparable exists for FFXIV. Every fansite ends up reinventing the wheel, which usually involves asking users to paste a special code into their characters' Lodestone field. It would be nice if there was an established service that could take this burden upon itself, and give third-party apps a good, straightforward API with which to implement single sign-on.
Central Archives is one such service. While it began as a roleplay portal, now its scope has expanded, and it also acts as an identity provider for any FFXIV players, be they roleplayers or not.
The XIVAuth Project
The design of the Central Archives identity provider was heavily inspired by XIVAuth, a project by KazWolfe of the Dalamud team to develop an identity provider for FFXIV websites and Dalamud plugins. As of this writing, XIVAuth is still in development, so for now, Central Archives fills this niche. When XIVAuth is ready, it will be a more comprehensive and feature-rich solution, and the two identity providers will integrate with each other instead of competing.