Sorting FIDO/CTAP/WebAuthn terminologies

Ackermann Yuriy
Oct 28, 2020 · 3 min read

CTAP1 my U2F you FIDO2 CTAP2.

If you are confused about all various FIDO terms, you are not alone. Over the years FIDO has expanded from two, to a dozen working groups. Standards started overlapping, having backwards compatibility and everything went terminologically speaking, bonkers.

Image for post
Image for post
TL;DR image

So here is a breakdown:

  • FIDOFast IDentity Online, or FIDO Alliance. As I explained earlier it’s a consortium that develops secure, open, phishing proof, passwordless authentication standards. FIDO Protocol Family is a set of protocol that was developed by FIDO Alliance. UAF — Universal Authentication Framework. U2F — Universal Second Factor, and FIDO2. When I say use “FIDO” I generally mean “Use any of these three protocols”, as they are all conceptually the same protocol, with the difference being structural (UAF — TLV, U2F — RAW, FIDO2 — CBOR).
  • CTAP — Client to Authenticator Protocols — A set of low level definitions to the transport protocols(HID, NFC, BLE) that describe how CTAP enabled devices must communicate using them with the client. These characteristics are shared by U2F and FIDO2 authenticators, and respectively they are called CTAP1 and CTAP2.
  • U2FUniversal 2nd Factor — together with UAF, first phishing proof authentication protocol on the web. Originally presented in 2014 publication. It is web centric, so it supports USB, NFC and BLE transports. It contains specifications for low level transports, message format, JS API and other.
  • CTAP1 — A formal name of low lever part of the U2F protocol. U2F uses raw message format aka many glued together buffers.
  • U2F JS API — A legacy U2F only API, that was introduced in 2014 in Google Chrome. It is now outdated and companies should move away from it as it is only supports U2F and is only supported by few browsers, where WebAuthn is a modern standard that supports both U2F and FIDO2 standards.
  • FIDO2 — A new, modern, simple, secure, phishing proof, passwordless authentication protocol. Its core specifications are WebAuthn(the client API) and CTAP(the authenticator API).
  • CTAP2 — A formal name of low level part of the FIDO2 protocol. FIDO2 uses CBOR to encode response structures aka JSON but TLV-ish. CTAP2 specification is then defines how FIDO2 requests converted into CTAP1/U2F requests, and CTAP1/U2F responses into CBOR/FIDO2 responses. But U2F is NOT a part of FIDO2.
  • WebAuthn — A browser JS API that describes an interface for creating and managing public key credentials. It’s a new, W3C led specification that is build with modern web requirements. WebAuthn can talk to both CTAP2/FIDO2 and CTAP1/U2F authenticators. Based on criterial it will take encode requests for the desired standards, and decode/re-encode responses based on the protocol. So despite CTAP1/U2F conversion is defined in CTAP2 specs, it is implemented on WebAuthn side.
Image for post
Image for post


Is U2F a part of FIDO2?

— No, it is not. U2F is a separate standard for a phishing proof second factor authentication. FIDO2 is simply backwards compatible with existing U2F authenticators.

FIDO2 is WebAuthn?

— No, it is not. WebAuthn is just another specification in FIDO2 protocol, so correct way to say is “My server supports authentication with FIDO2”. It’s a common misconception due to WebAuthn is being the most public piece of FIDO2 project. As a compromise you could say “My server supports authentication with FIDO2/WebAuthn”

Is there more I can read about FIDO2?

— Shameless self advertising 1: — My introduction to WebAuthn API

— Shameless self advertising 2: — My introduction to verifying FIDO2 responses

— Shameless self advertising 3: — A collection of curated resources on FIDO2/WebAuthn


This article is licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). So you are free to read, share, etc. If you are interested in commercial use of this article, or wish to translate it to a different language, please contact me at ackermann(dot)yuriy(at)gmail(dot)com.

WebAuthn Works

WebAuthn Works Limited

Medium is an open platform where 170 million readers come to find insightful and dynamic thinking. Here, expert and undiscovered voices alike dive into the heart of any topic and bring new ideas to the surface. Learn more

Follow the writers, publications, and topics that matter to you, and you’ll see them on your homepage and in your inbox. Explore

If you have a story to tell, knowledge to share, or a perspective to offer — welcome home. It’s easy and free to post your thinking on any topic. Write on Medium

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store