27 July 2023
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. Written by Ivan Ristić.
Messaging Layer Security (MLS), a new standard that supports end-to-end encryption in messaging applications, has been released as RFC 9420. The name is an obvious riff on Transport Layer Security (TLS), but that’s not where the similarities end. MLS had been in development for about five years, which is similar to the time it took to produce TLS 1.3. Both standards were built on the protocols that came before them. Both standards engaged academia and a wider expert base to analyze the designs before they were considered complete. Is this what the new model for development of cryptographic protocols looks like?
To understand why we need MLS, we need to go back as far as 1991 and start with Pretty Good Privacy (PGP). Developed by Phil Zimmerman, PGP was the first attempt to provide end-to-end encryption for messaging—more specifically, for email. Although PGP provided some basic security, its usability was poor and cryptography weak by today’s standards. Crucially, its security relied on a master key. If that key became compromised, all communication, past and future, would be compromised as well.
Other standards followed. Off-the-Record (OTR) protocol focused on security of real-time (chat) messages, introducing forward secrecy and deniability. The Signal protocol further improved things by adding support for offline communication as well as post-compromise security. The Signal protocol in particular went on to achieve great success, being implemented not only by Signal, but also by WhatsApp, Facebook Messenger, Skype, and others.
Although the Signal protocol is a de facto standard at the moment, MLS aims to take things further with improved group communication, proper standardization, and a reference implementation in Rust under a permissive open-source license. If multiple vendors all adopt this protocol, that would open doors to some possible federation and interoperability across messaging applications. A number of prominent organizations participated in the development effort. Google already stated that they will implement MLS in Google Messages. And Matrix is experimenting with it already.
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