\section{Ethical Considerations}
\label{sec:section label}

We strictly adhere to ethical norms as well as the described ethical guidelines. We confirm our research considers potential negative impact and obeys legislation. Although we point out practical weaknesses in the design of deployed group communication protocols, namely Matrix, in Section \ref{sec:recs}, we emphasize these weaknesses concern only protocol design. Our research does not target a particular implementation nor product that embeds Matrix or other nested ratchet protocols. We plan to alert the stakeholders of our findings immediately after submission, and we do not plan to publicize our work onto a pre-print server until our findings are acknowledged. 

\section{Open Science}
\label{sec:section label}

We strictly adhere to and enthusiastically embrace the Open Science guidelines of USENIX Security 2026. All of our methodology, models, symbolic analysis in \textsc{ProVerif}, and nix flakes for environment reproducibility are thoroughly documented and openly shared in our companion artifact publicly available on Zenodo: \href{https://zenodo.org/records/16959099}{zenodo.org/records/16959099} 

Our companion artifact contains a nix flake, a nix lock file, a \texttt{README.md}, and a folder, \textbf{proverif}, that contains our \textsc{ProVerif} models of 3DH, X3DH, Signal, Olm, Sender Keys, and Megolm. Secrecy, authentication, post-compromise security, and forward secrecy properties are contained are grouped in the same model file, and offline deniability properties for both the initiator and responder are contained independently in a sub-folder \textbf{deniability}. Our README documents the many of the implementation-specific techniques and tricks that went into modeling Sender Keys, Megolm, and their sub-protocols.


