**Session Date/Time:** 17 Mar 2025 10:00

# keytrans

## Summary

This keytrans session covered recent protocol document changes, focusing on time authentication and its impact on security.  A proposal for self-balancing prefix trees to improve efficiency in non-privacy-sensitive applications was discussed.  Finally, the session included a presentation on the formal verification efforts of the keytrans protocol.

## Key Discussion Points

*   **Time Authentication:**
    *   Discussed issues with the original "clock label" approach to time authentication due to potential forking and misbehavior concealment.
    *   Introduced the solution of making timestamps a first-class feature of the protocol, associating them directly with log entries.
    *   Explained the concept of "maximum lifetime" for log entries, enabling provable and secure pruning of the tree.
    *   Presented the idea of "reasonable monitoring window" and "distinguished log entries" to coordinate label monitoring and reduce state retention requirements.
*   **Self-Balancing Prefix Trees:**
    *   Motivation: Improving efficiency for non-privacy-sensitive applications by avoiding unnecessary VRF usage.
    *   Problem: Typical balanced search trees store data in intermediate nodes, doubling the size of cryptographic search proofs.
    *   Solution: Allow multiple starting points (cursors) within a search range, with the transparency log indicating the most efficient cursor to use.
    *   Data Density: Efficiency depends on data density, not just total range of values
    *   Debate ensued whether hashing was a better strategy for dealing with tree balance
*   **Formal Verification:**
    *   Goals: Verify the client side of a Go reference implementation, proving that all clients agree on their view of the log.
    *   Property: For all identical queries, the results of `verifyLatest` should be the same, independent of the server response.
    *   Challenges: The specification, while well-written, relies on non-obvious insights into algorithms and graph theory.  Efforts are underway to make it less ambiguous.
    *   Signatures in tree head should include a domain separator
    *   Extension field should be added for client configuration information

## Decisions and Action Items

*   **Action Item:** Felix to open issues and pull requests regarding specification ambiguities to facilitate concrete discussions.
*   **Action Item:** Collect a list of use cases that would benefit from the self-balancing prefix trees.
*   **Action Item:** Engage in a follow-up discussion on the formal analysis effort, particularly focusing on the opened issues and pull requests.

## Next Steps

*   Continue refining the specification based on feedback from implementers and formal verification efforts.
*   Further explore the use cases and requirements for self-balancing prefix trees.
*   Advance the formal verification of the key transparency protocol.