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According to api.md witnesses must only cosign a tree head if it is not backdated or future-dated more than 12 hours.
* https://github.com/system-transparency/stfe/blob/c8400f98809fbc245f040598e8471fd833e5c1a5/doc/api.md#merkle-tree-head
Discussion raised by ln5
* Why +- 12 hours, and in particular why so much forward drift
* Initial thoughts were:
* Symmetry, easy to implement
* The point is not that it should be a precicely verified timestamp. Time moves forward roughly for liveliness.
* If we set this interval too short, say, 5m, then we would make it difficult to use a larger cosigning interval like an hour. Witnesses would have to do all of their signing the first 5 minutes. So with 12 hours we basically tried to set it "sufficiently large".
* Another related "threat" is that witnesses would stop signing because of a clock that is a little off. So by having a very flexible interval -> "reduced threat"
* Highlights of discussion
* Allowing large clock drift might not be helpful. It might not even be helpful to specify it is a strict implementation criteria. E.g., there is probably a distinction between "warning: time to look at your clock" and "error: we cannot sign this because timestamp is unfresh or someone's clock is wrong".
* Comments on clock drift is probably better as a recommendation, best practise, implementation hint, or similar in an Appendix or witness README.
* How unfresh timestamps should a witness sign?
* Right now we tried the "sufficiently large" approach
* Maybe it should be a configuration parameter, declared as log metadata.
* Catch: are we really envisioning an ecosystem where someone is going to say "we really want to run the log with a large cosigned tree head freauency".
* Probably not. Point is we want independent logs that behave the same.
* From UX perspective: cosigned tree head frequency F should be "short"
* After a leaf is merged in the tree, it takes [F,2F] time units before an inclusion proof can lead up to a cosigned tree head.
* From other perspective: cosigned tree head frequency should not be too short
* Witnesses need to poke the log more often to discover all tree heads
* More work and overhead, both for the log and witnesses
* Reliability of cosigning will be poor if witnesses miss signed tree heads
* Conclusion (ln5, rgdd)
* Set a fixed cosigned tree head frequency in api.md that all logs should use.
* 5 minutes to start with
* We can reconsider if it should be lower. It should probably not be higher.
* We can reconsider if it should be a configuration parameter. But a different design is needed for "much lower latency", like Syte et al. approach. The reason why we are not doing that is because of added complexities and costs.
* Don't consider clock drift considerations as an integral part of api.md. It is something that we should talk about in "implementation hints" or similar.
* A witness must not cosign a signed tree head that is older than the log's cosigned tree head frequency. As stated above, it is always 5 minutes.
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