Glossary · Encryption

MLS (RFC 9420)

MLS, or Messaging Layer Security, is the IETF standard published as RFC 9420 for end-to-end encrypted group messaging, designed so that adding or removing a member costs work proportional to the logarithm of the group size rather than to the group size itself.

Open betaThe platform is being built in the open, so parts of it are not there yet, behaviour changes between releases, and no availability figure is committed while it is in beta. What is still being built.

Also called

Messaging Layer Security

What MLS (RFC 9420) means

Before MLS, group encryption was usually built by encrypting each message separately for every recipient device — the approach the Signal protocol takes when extended to groups. That works, and it is what most encrypted messengers do, but the cost of every key change grows with the number of devices, which is why encrypted group chat has historically been small and slow. MLS replaces the pairwise fan-out with a binary tree of keys, so a change touches a path through the tree instead of every leaf.

The unit of membership in MLS is a device, not a person. Each device holds its own leaf, so a group of ten people with three devices each is a group of thirty members. Adding a device is a visible operation, and removing one changes the group key so that the removed device cannot decrypt anything sent afterwards. The state of a group at any moment is called an epoch, and every membership change advances it.

MLS became RFC 9420 in July 2023 after several years of work in the IETF working group of the same name, with contributions from the people behind Signal, Wire, Cisco, Google and Mozilla. Being a published standard rather than a proprietary design matters for a buyer: the protocol can be reviewed by anybody, has been analysed formally, and does not depend on any one vendor continuing to exist.

How Enclessa uses it

Enclessa uses MLS for direct messages and group direct messages. The implementation is OpenMLS, compiled once in Rust and shipped to every client, so the web client and the desktop client run identical cryptographic code rather than two implementations that agree until they do not. The Enclessa server implements only the MLS delivery service — it orders and forwards handshake messages and performs no cryptography of its own.

Where MLS (RFC 9420) is specified

Related terms

OpenMLSOpenMLS is an open-source implementation of the MLS protocol written in Rust, developed in the open by contributors from Phoenix R&D, Wire and Cryspen, and subjected to external security review.Read the entry End-to-end encryptionEnd-to-end encryption is a design in which a message is encrypted on the sending device and decrypted only on the receiving devices, so that the server carrying it holds no key that can open it.Read the entry Forward secrecyForward secrecy is the property that compromising a key today does not allow the messages sent before that compromise to be decrypted, because the keys that protected them have already been destroyed.Read the entry Post-compromise securityPost-compromise security is the property that a conversation heals after a device is compromised, so that once the compromised device is removed and the group re-keys, the attacker who held its keys can no longer read new messages.Read the entry Cross-signingCross-signing is a scheme in which each person holds one identity key that signs certificates for their own devices, so that verifying a person once is enough to trust every device they own rather than verifying each device separately.Read the entry Safety numbersA safety number is a short string derived from the public keys of both sides of an encrypted conversation, which the two people compare through a separate channel to confirm that no third party has interposed itself.Read the entry

Read further

The security page explains how Enclessa protects data and which certifications it does not hold. The trust centre covers the processing agreement, the subprocessors and the residency position. The FAQ answers the questions buyers ask most often.

Encrypted collaboration, hosted in Europe.

Create a workspace in a couple of minutes. It is yours at your-team.enclessa.app, hosted in the European Union, with encrypted direct messages from the first one you send.

Create your workspace See pricing

Open beta. Free plan, no payment card to start.