Markdown Version | Session Recording | Session Materials

mls

Summary

The MLS Working Group meeting focused on addressing outstanding issues in the MLS architecture draft and discussing various extensions, including those related to instant messaging (Mimi) and post-quantum cryptography. A significant portion of the meeting was dedicated to a detailed examination of an attack involving invalid commits and external rejoins, and potential mitigations. The group also discussed several extensions, including AppSync, semi-private messages, and different credential types.

Key Discussion Points

• Invalid Commits and External Rejoins:

  • Brendan presented a PR addressing vulnerabilities related to invalid commits and the external rejoin mechanism.
  • The discussion revolved around a "forced rejoin attack" where a malicious delivery service (DS) can manipulate group state by choosing invalid commits.
  • Concerns were raised about post-compromised security (PCS) being compromised by the ability to revert to compromised epochs.
  • Various mitigations were proposed, including epoch checking, transcript verification, and public key bookkeeping.
  • The debate centered on whether the PR should only flag the risks of automated rejoin behaviors, or also include potential solutions.
  • Richard suggested focusing the architectural discussion on the risks of automated re-syncs.

• Extensions Framework:

  • Rowan raised an issue regarding safe extensions and AAD and a PR was submitted.

• AppSync Extension:

  • Rowan presented the AppSync extension, aimed at ensuring agreement on application state within the group.
  • There was discussion around the level of MLS's involvement in defining common representations of state.
  • Raphael noted potential overlap with other extensions related to group context and extension state updates.

• Semi-Private Message Extension:

  • Rowan introduced the semi-private message extension, allowing selective sharing of private message content with external receivers known to the group.
  • Raphael raised concerns about the lack of protection for the group ID.
  • Conrad and Britta supported the inclusion of this feature to the toolbox.

• Other Extensions:

  • Richard presented the Different Credentials draft.
  • Richard presented the Replaced Proposal draft, allowing re-origination of updates. Concerns about potential rollbacks were raised.
  • Richard presented Tree Free draft, which allows client to operate with hard login guarantees instead of using the Ratchet Tree

• Post-Quantum MLS:

  • Britta provided an overview of a proposed approach for post-quantum MLS, involving two sessions (post-quantum and standard) and using exporter keys to inject post-quantum guarantees into the standard ratchet.

Decisions and Action Items

• PR 261 (Invalid Commits/External Rejoins):

  • It was decided to adopt a PR to address the issues of invalid commits and external rejoins.
  • A vote was held on including approaches within the PR, 4 for yes and 8 for no, and 5 for no opinions
  • Richard, Britta, Brendan, and Rowan agreed to further discuss the PR to determine appropriate text.
  • The goal is for the document to be completed during the current week.

• Different Credentials Extension:

  • It was decided that Richard should proceed with putting this directly into the current extension draft.

Next Steps

• Richard, Britta, Brendan, and Rowan to collaborate and edit the PR.
• Richard to create a PR to include the different credential drafts into the extension draft.
• The WG to review edited PR and hopefully accept it.