Messaging Layer Security
charter-ietf-mls-01

Several Internet applications have a need for group key establishment
and message protection protocols with the following properties:

o Message Confidentiality - Messages can only be read
by members of the group
o Message Integrity and Authentication - Each message
has been sent by an authenticated sender, and has
not been tampered with
o Membership Authentication - Each participant can verify
the set of members in the group
o Asynchronicity - Keys can be established without any
two participants being online at the same time
o Forward secrecy - Full compromise of a node at a point
in time does not reveal past messages sent within the group
o Post-compromise security - Full compromise of a node at a
point in time does not reveal future messages sent within the group
o Scalability - Resource requirements have good scaling in the
size of the group (preferably sub-linear)

Several widely-deployed applications have developed their own
protocols to meet these needs. While these protocols are similar,
no two are close enough to interoperate. As a result, each application
vendor has had to maintain their own protocol stack and independently
build trust in the quality of the protocol. The primary goal of this
working group is to develop a standard messaging security protocol for
human-to-human(s) communication with the above security and deployment
properties so that applications can share code, and so that there can be
shared validation of the protocol (as there has been with TLS 1.3).
Humans are assumed to have access to one or more general-purpose
computers.

It is not a goal of this group to enable interoperability/federation
between messaging applications beyond the key establishment,
authentication, and confidentiality services. Full interoperability
would require alignment at many different layers beyond security,
e.g., standard message transport and application semantics. The
focus of this work is to develop a messaging security layer that
different applications can adapt to their own needs.

While authentication is a key goal of this working group, it is not
the objective of this working group to develop new authentication
technologies. Rather, the security protocol developed by this
group will provide a way to leverage existing authentication
technologies to associate identities with keys used in the protocol,
just as TLS does with X.509.

In developing this protocol, we will draw on lessons learned from
several prior message-oriented security protocols, in addition to
the proprietary messaging security protocols deployed within
existing applications:

o S/MIME - https://tools.ietf.org/html/rfc5751
o OpenPGP - https://tools.ietf.org/html/rfc4880
o Off the Record - https://otr.cypherpunks.ca/Protocol-v3-4.1.1.html
o Double Ratchet - https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm

The intent of this working group is to follow the pattern of
TLS 1.3, with specification, implementation, and verification
proceeding in parallel. By the time we arrive at RFC, we
hope to have several interoperable implementations as well
as a thorough security analysis.

The specifications developed by this working group will be
based on pre-standardization implementation and deployment
experience, and generalizing the design described in:

o draft-omara-mls-architecture
o draft-barnes-mls-protocol

Note that consensus is required both for changes to the protocol mechanisms
from these documents and retention of the mechanisms from them. In particular,
because something is in the initial document set does not imply that there is
consensus around the feature or around how it is specified.