Client to Authenticator Protocol

Short description: Authentication protocol

"CTAP" redirects here. For the diagnostic radiology study, see Computed tomography of the abdomen and pelvis. For the peptide, see CTAP (peptide).

File:CTAP (Client to Authenticator Protocol).png

The Client to Authenticator Protocol (CTAP) or X.1278[1] enables a roaming, user-controlled cryptographic authenticator (such as a smartphone or a hardware security key) to interoperate with a client platform such as a laptop.

## Standard

CTAP is complementary to the Web Authentication (WebAuthn) standard published by the World Wide Web Consortium (W3C).[2] WebAuthn and CTAP are the primary outputs of the FIDO2 Project, a joint effort between the FIDO Alliance and the W3C.[3]

CTAP is based upon previous work done by the FIDO Alliance, in particular the Universal 2nd Factor (U2F) authentication standard. Specifically, the FIDO U2F 1.2 Proposed Standard (July 11, 2017) became the starting point for the CTAP Proposed Standard, the latest version 2.2 of which was published on February 28, 2025.[4]

The CTAP specification refers to two protocol versions, the CTAP1/U2F protocol and the CTAP2 protocol.[5] An authenticator that implements CTAP2 is called a FIDO2 authenticator (also called a WebAuthn authenticator). If that authenticator implements CTAP1/U2F as well, it is backward compatible with U2F.

The protocol uses the CBOR binary data serialization format.

The standard was adopted as ITU-T Recommendation X.1278.[6][1]

## References

1. ↑ 1.0 1.1 "X.1278: Client to authenticator protocol/Universal 2-factor framework". https://www.itu.int/rec/T-REC-X.1278.
2. ↑ Balfanz, Dirk; Czeskis, Alexei; Hodges, Jeff et al., eds (4 March 2019). "Web Authentication: An API for accessing Public Key Credentials Level 1". World Wide Web Consortium (W3C). https://www.w3.org/TR/2019/REC-webauthn-1-20190304/.
3. ↑ "FIDO2: Moving the World Beyond Passwords". FIDO Alliance. https://fidoalliance.org/fido2/.
4. ↑ "Client to Authenticator Protocol (CTAP)". https://fidoalliance.org/specs/fido-v2.2-ps-20250228/fido-client-to-authenticator-protocol-v2.2-ps-20250228.html.
5. ↑ Brand, Christiaan; Czeskis, Alexei; Ehrensvärd, Jakob et al., eds (January 30, 2019). "Client to Authenticator Protocol (CTAP)". FIDO Alliance. https://fidoalliance.org/specs/fido-v2.0-ps-20190130-pub/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html.
6. ↑ ITU (2018-12-18). "New ITU standards to overcome the security limitations of passwords" (in english). https://news.itu.int/new-itu-standards-to-overcome-the-security-limitations-of-passwords/.

## External links

* FIDO Specifications Overview
* FIDO Specifications

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