27 Jan 2022
Bulletproof TLS Newsletter is a free periodic newsletter bringing you commentary and news surrounding SSL/TLS and Internet PKI, designed to keep you informed about the latest developments in this space. Received monthly by more than 50,000 subscribers. Maintained by Hanno Böck.
The development of the WebTransport browser API has led to discussions about the nuances of the use of TLS without Web PKI certificates.
The WebTransport API has been proposed to give browsers a low latency mechanism to exchange messages with a server. One possible use case is gaming.
The specification has a feature that allows for identifying a server not via a certificate signed by a certificate authority from the Web PKI but by identifying the certificate with a hash. Because the hash is provided by code from a web page that itself needs to use HTTPS, the hash is indirectly authenticated. Chrome is expected to implement this feature soon, under the name serverCertificateHashes.
There were some lengthy discussions about the security properties of this mechanism in a GitHub issue posting tied to the WebTransport specification. Notably, a mechanism using certificates not issued via the Web PKI does not benefit from Certificate Transparency, and there is no revocation mechanism. Also, certificate authorities are usually expected not to issue certificates affected by known security vulnerabilities if they can identify them. However, it should be noted that with WebRTC, an existing specification already uses self-signed certificates that are not issued by the Web PKI.
One way the specification tries to mitigate this is by restricting the lifetime of the certificates to two weeks.
In a related blog post, Chrome developer Emily M. Stark discusses various ways of using non–Web PKI certificates in browsers, as well as the WebRTC and WebTransport certificate mechanisms.
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