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Diffstat (limited to 'archive/2021-06-21-witness-timestamp-verification')
| -rw-r--r-- | archive/2021-06-21-witness-timestamp-verification | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/archive/2021-06-21-witness-timestamp-verification b/archive/2021-06-21-witness-timestamp-verification new file mode 100644 index 0000000..51a90bc --- /dev/null +++ b/archive/2021-06-21-witness-timestamp-verification @@ -0,0 +1,33 @@ +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. |