WebAuthn

WebAuthn (Web Authentication) is an effort by the World Wide Web Consortium (W3C), with input from the FIDO Alliance, to standardize an interface for public-key authentication of users to web-based applications and services. It implements an extension of the W3C's more general Credential Management API, which is an attempt to formalize the interaction between websites and in-browser password managers that store username/password combinations or other "federated" credentials such as authentication tokens generated by single sign-on services. However, WebAuthn is intended to work with multi-factor authentication, possibly in completely passwordless schemes that use gestures or biometric verification instead of requiring users to type in long, complex strings of characters.

WebAuthn is designed so that it can work with a range of public-key authenticator mechanisms, from pure software implementations to those using specialized hardware environments, such as a processor's trusted execution environment, a Trusted Platform Module, or an external hardware token accessed via USB, Bluetooth Low Energy, or near-field communications (NFC). This builds upon the FIDO Alliances's previous Universal Authentication Framework (UAF) and Universal 2nd Factor (U2F) standards, and is designed to work with the in-progress Client-to-Authenticator Protocol (CTAP) for communicating with hardware authentication devices. WebAuthn and CTAP are both outgrowths of the FIDO2 project.

Application and device support

The WebAuthn Level 1 standard was published as a Candidate Recommendation by the Web Authentication Working Group on March 20, 2018. The desktop version of Google Chrome has supported WebAuthn since version 67. Mozilla Firefox, which had not fully supported the previous FIDO U2F standard, included and enabled WebAuthn in Firefox version 60, released on May 9, 2018. The current Windows Insider release of Microsoft Edge implements a version of WebAuthn that works with both Windows Hello as well as external Security Keys. Support that matches the published standard is expected later in 2018.

Existing FIDO U2F Security Keys are largely compatible with the WebAuthn standard, though WebAuthn added the ability to reference a unique per-account "user handle" identifier, which older hardware tokens are unable to store. The first new token able to handle the new "FIDO2" style of key storage is the second-generation Security Key by Yubico, announced on April 10, 2018.

Dropbox announced support for WebAuthn logins (as a 2nd factor) on May 8, 2018.

API

The WebAuthn API extends the Credential Management navigator.credentials.create() and navigator.credentials.get() JavaScript methods so they accept a publicKey parameter. The create() method is used for registering public key authenticators as part of associating them with user accounts (possibly at initial account creation time, but more likely when adding a new security device to an existing account), and the get() method is used for authenticating (such as when logging in).

To check if a browser supports WebAuthn, scripts should check if the window.PublicKeyCredential interface is defined. In addition to PublicKeyCredential, the standard also defines the AuthenticatorResponse, AuthenticatorAttestationResponse, and AuthenticatorAssertionResponse interfaces in addition to a variety of dictionaries and other datatypes.

The API does not allow direct access to or manipulation of private keys, beyond requesting their initial creation.

Criticism

In August 2018, a team of researchers from Paragon Initiative Enterprises did a security audit of the upcoming WebAuthn standard. While they could not find any specific exploits, they revealed some serious weaknesses in the way the underlying cryptography is used and mandated by the standard.

The main points of criticism evolve around two potential issues that were problematic in other cryptographic systems in the past and therefore should be avoided in order to not fall victim to the same class of attacks:

• Support for RSA with PKCS1v1.5 padding is required by the WebAuthn standard. This particular scheme of padding is known to be vulnerable to specific attacks for at least twenty years and it has been successfully attacked in other protocols and implementations of the RSA cryptosystem in the past. It is difficult to exploit under the given conditions in the context of WebAuthn, but given that there are more secure cryptographic primitives and padding schemes, this is still a bad choice and is not considered to be best practice among cryptographers any more.

• The FIDO alliance standardized on an asymmetric cryptographic scheme called ECDAA. This is a version of direct anonymous attestation based on elliptic curves and in the case of WebAuthn is meant to be used to verify the integrity of authenticators, while also preserving the privacy of users, as it does not allow for global correlation of handles. However, ECDAA does not incorporate some of the lessons that were learned in the last decades of research in the area of elliptic curve cryptography, as the chosen curve has some security deficits that is inherent to this type of curves and reduces the security guarantees quite substantially. Furthermore the ECDAA standard involves random, non-deterministic, signatures, which already has been a problem in the past.

Despite these shortcomings the researches still encourage users to continue to use WebAuthn, but came up with some recommendations for potential implementors and developers of the standard and hope that this can be fixed in the upcoming standard, before it is finalized and therefore set in stone. Avoiding such mistakes as early as possible, will prevent the industry from any challenges that are introduced by broken standards and the need for backwards compatibility.

External links

• Web Authentication: An API for accessing Public Key Credentials Level 1
• Web Authentication Working Group
• Web Authentication API on MDN