3 research outputs found
Security Analysis and Improvements for the IETF MLS Standard for Group Messaging
Secure messaging (SM) protocols allow users to communicate securely
over untrusted infrastructure. In contrast to most other secure
communication protocols (such as TLS, SSH, or Wireguard), SM
sessions may be long-lived (e.g., years) and highly asynchronous.
In order to deal with likely state compromises of users during the
lifetime of a session, SM protocols do not only protect authenticity
and privacy, but they also guarantee forward secrecy (FS) and
post-compromise security (PCS). The former ensures that
messages sent and received before a state compromise remain secure,
while the latter ensures that users can recover from state
compromise as a consequence of normal protocol usage.
SM has received considerable attention in the two-party
case, where prior work has studied the well-known double-ratchet
paradigm in particular and SM as a cryptographic primitive in
general. Unfortunately, this paradigm does not scale well to the
problem of secure group messaging (SGM). In order to address
the lack of satisfactory SGM protocols, the IETF has launched the
message-layer security (MLS) working group, which aims to
standardize an eponymous SGM protocol.
In this work we analyze the TreeKEM protocol, which is at the
core of the SGM protocol proposed by the MLS working group.
On a positive note, we show that TreeKEM achieves PCS in isolation
(and slightly more). However, we observe that the current version
of TreeKEM does not provide an adequate form of FS. More precisely,
our work proceeds by formally capturing the exact security of
TreeKEM as a so-called continuous group key agreement (CGKA)
protocol, which we believe to be a primitive of independent
interest. To address the insecurity of TreeKEM, we propose a simple
modification to TreeKEM inspired by recent work of Jost et al.
(EUROCRYPT \u2719) and an idea due to Kohbrok (MLS Mailing List). We
then show that the modified version of TreeKEM comes with almost no
efficiency degradation but achieves optimal (according to MLS
specification) CGKA security, including FS and PCS. Our work also
lays out how a CGKA protocol can be used to design a full SGM
protocol.
Finally, we propose and motivate an extensive list of
potential future research directions for the area
Formal Verification Applications for the TreeKEM Continuous Group Key Agreement Protocol
The features of Secure Group Messaging, the security guarantees of Message Layer Security, and the TreeKEM protocol designed to satisfy these guarantees and features are explored. A motivation and methodology for verification via explicit model checking is presented. Subsequently, a translation of the TreeKEM protocol into a Promela reference model is described, examining the nuances explicit model checking brings. Finally the results of the formal verification methods are discussed
Formal Models and Verified Protocols for Group Messaging: Attacks and Proofs for IETF MLS
Group conversations are supported by most modern messaging applications, but the security guarantees they offer are significantly weaker than those for two-party protocols like Signal. The problem is that mechanisms that are efficient for two parties do not scale well to large dynamic groups where members may be regularly added and removed. Further, group messaging introduces subtle new security requirements that require new solutions. The IETF Messaging Layer Security (MLS) working group is standardizing a new asynchronous group messaging protocol that aims to achieve strong guarantees like forward secrecy and post-compromise security for large dynamic groups. In this paper, we define a formal framework for group messaging in the F language and use it to compare the security and performance of several candidate MLS protocols up to draft 7. We present a succinct, executable, formal specification and symbolic security proof for TreeKEMB, the group key establishment protocol in MLS draft 7. Our analysis finds new attacks and we propose verified fixes, which are now being incorporated into MLS. Ours is the first mechanically checked proof for MLS, and our analysis technique is of independent interest, since it accounts for groups of unbounded size, stateful recursive data structures, and fine-grained compromise