]> Unaffiliated

rohan.ietf@gmail.com

sec MLS ratchet_tree ratchet tree GroupInfo PartialGroupInfo The Messaging Layer Security (MLS) protocol needs to share its ratchet_tree object to welcome new clients into a group and in external joins. While, the protocol only defines a mechanism for sharing the entire tree, most implementations use various optimizations to avoid sending this structure repeatedly in large groups. This draft explores ways to convey these improvements in a standardized way and to convey parts of a GroupInfo object that are not visible to an intermediary server. About This Document Status information for this document may be found at . Discussion of this document takes place on the MLS Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction In the Messaging Layer Security (MLS) protocol , the members of a group are organized into a ratchet tree, the full representation of which is described in the ratchet_tree extension. The protocol specifies that the full ratchet_tree can be included in Welcome messages or shared externally, but describes no concrete way to convey externally. Likewise, when non-member clients want to join a group, they can do so using an external commit. They require the GroupInfo and the ratchet_tree. Many MLS implementations allow external commits to get the GroupInfo from a central server. In the MIMI architecture , this server is called the hub, and for brevity we will use that term generically to refer to any central server that provides either GroupInfo or ratchet_tree objects to new members (i.e. welcomed clients or external joining clients). When handshake messages (commits and proposals) are sent as PublicMessages, the hub can construct its own version of the ratchet_tree and most of the GroupInfo object as proposals and commits arrive.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document assumes familiarity with terms and structs from the MLS specification ().

Conveying the Ratchet Tree The ratchet tree can be conveyed inline in its entirety. Alternatively, this document describes how it can be referred to by reference, or "patched". ; case byReference: /* an HTTPS URL */ opaque ratchet_tree_url; opaque tree_signature; case delta: NodeChanges node_changes; }; } RatchetTreeOption; ]]>

• full indicates that the complete ratchet_tree extension is included in the RatchetTreeOption object.
• byReference indicates that the ratchet_tree is available from the ratchet_tree_url.
• delta indicates that the new ratchet_tree differs from the previous epoch. The changes are described in the node_changes.

; NodePlusIndex updated_nodes; Node new_nodes; } NodeChanges ]]> To describe changes between two ratchet trees in the NodeChanges "patch" format, first make a list of node indexes of nodes that have been completely removed, next make a list of nodes that were completely replaced and their node indexes, finally make a list of new nodes which were added after the right-most node in the tree.

Conveying the GroupInfo In some systems the GroupInfo is sent to a hub with a full ratchet_tree extension always included with every commit. This is used in systems where the hub may or may not track the membership of the group, but does not keep the entire ratchet_tree data structure. As group size increases, the size of the ratchet_tree extension in the GroupInfo scales roughly linearly. Even using basic credentials, this object gets large quickly. If x509 credentials are used, the size increases much more rapidly, and if a post-quantum ciphersuite (for example ) is used, the size will increase even more rapidly with each new member. In some systems that require unencrypted handshake messages, the hub tracks commits as they are sent and constructs changes to the ratchet_tree as each handshake is accepted. The hub could also recreate a GroupInfo from inspecting unencrypted handshake messages with the exception of the GroupInfo signature and the GroupInfo extensions. This document defines a PartialGroupInfo struct that contains these missing items. It can be included with a commit and any referenced proposals to reconstruct a GroupInfo and ratchet_tree from the GroupInfo and ratchet_tree included in the previous epoch. ; opaque Signature; } PartialGroupInfo; ]]> The value of the ratchet_tree_presence is defined as follows:

• no_ratchet_tree: the ratchet_tree extension appears in neither the current nor previous epochs.
• present: there is a ratchet_tree extension in both the current and previous epochs.
• removed: there was a ratchet_tree extension in the previous epoch but none in the current epoch.
• added: there is a ratchet_tree extension in the current epoch but there was none in the previous epoch.

The group_info_extensions object is the list of GroupInfo extensions, omitting any ratchet_tree extension (if present). The only other GroupInfo extension defined in the base protocol is external_pub, the public key of the external commiter. The group_info_extensions is often an empty list. The Signature in the PartialGroupInfo is the signature produced by the committer (represented by leaf index in the GroupInfo as the signer).

Security Considerations TODO Security

IANA Considerations This document has no IANA actions.

References Normative References Messaging applications are increasingly making use of end-to-end security mechanisms to ensure that messages are only accessible to the communicating endpoints, and not to any servers involved in delivering messages. Establishing keys to provide such protections is challenging for group chat settings, in which more than two clients need to agree on a key but may not be online at the same time. In this document, we specify a key establishment protocol that provides efficient asynchronous group key establishment with forward secrecy (FS) and post-compromise security (PCS) for groups in size ranging from two to thousands. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Cisco The More Instant Messaging Interoperability (MIMI) working group is defining a suite of protocols that allow messaging providers to interoperate with one another. This document lays out an overall architecture enumerating the MIMI protocols and how they work together to enable an overall messaging experience. Unaffiliated This document registers a new Messaging Layer Security (MLS) ciphersuite using the X-Wing hybrid post-quantum resistant / traditional (PQ/T) Key Exchange Mechanism (KEM). Phoenix R&D Phoenix R&D This document describes the MIMI Delivery Service.

Acknowledgments The PartialGroupInfo was first introduced in .