path: root/archive/2021-06-21-witness-timestamp-verification
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authorRasmus Dahlberg <rasmus.dahlberg@kau.se>2021-06-22 18:48:42 +0200
committerRasmus Dahlberg <rasmus.dahlberg@kau.se>2021-06-22 18:48:42 +0200
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tree2ccda0bc6b4b516eac16c4be207c36750e2079d5 /archive/2021-06-21-witness-timestamp-verification
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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.