Understand Passkeys — with a Cup of Coffee

A single-page interactive web app that explains how passkeys (WebAuthn) work and lets you try them out in your browser.

What is this?

An educational tool that covers:

• Theory — what passkeys are, how public-key cryptography replaces passwords, and why they resist phishing.
• Registration & authentication flows — step-by-step ASCII diagrams showing the browser/server/authenticator exchange.
• Live demo — register and log in with a real passkey right in the page. All data is stored in server memory and resets on restart.

Tech stack

• Backend: Go, using go-webauthn/webauthn
• Frontend: Single HTML file with vanilla JS, no framework
• Storage: In-memory (no database)

Getting started

Run locally

go run main.go

The server starts on http://localhost:8080 by default.

Environment variables

Variable	Default	Description
PORT	8080	HTTP listen port
RP_ID	localhost	WebAuthn Relying Party ID (domain)
RP_ORIGIN	http://localhost:<PORT>	Allowed origin for WebAuthn requests

Docker

docker build -t understand-passkey .
docker run -p 8080:8080 understand-passkey

API endpoints

Method	Path	Description
POST	/api/register/begin	Start passkey registration
POST	/api/register/finish	Complete registration
POST	/api/login/begin	Start passkey login
POST	/api/login/finish	Complete login
GET	/api/users	List registered users