From fef460586e847e378a197381ef1ae3a64e6ea38b Mon Sep 17 00:00:00 2001
From: Rasmus Dahlberg <rasmus.dahlberg@kau.se>
Date: Tue, 28 Sep 2021 22:15:31 +0200
Subject: noted that verified timestamps have more use-cases
A believer can be convinced that a sigsum was logged after time T. This
is because witnesses do Verifier(append-only) and Verifier(Freshness).
Outline: a claimant is about to log a sigsum.
1. Fetch the most recent cosigned tree head.
- Timestamp is T
- Tree size is N
2. Submit sigsum for logging.
3. Wait for inclusion at index N+k, k=>0.
4. Wait for next cosigned tree head.
- Timestamp is T', where T' > T
- Tree size is N', where N' > N+k
5. Download inclusion proof for tree size N'.
Now you can convince a believer that a sigsum is publicly logged. Just
reveal inclusion proof which leads up to the second cosigned tree head.
Next, you can reveal the first cosigned tree head that _have not merged
that entry yet_. This follows from the first cosigned tree head size,
and makes it obvious that the entry must have been merge after time T.
---
doc/api.md | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
(limited to 'doc')
diff --git a/doc/api.md b/doc/api.md
index ea5a2a0..3f3db55 100644
--- a/doc/api.md
+++ b/doc/api.md
@@ -78,7 +78,9 @@ struct tree_head {
```
`timestamp` is the time since the UNIX epoch (January 1, 1970 00:00 UTC) in
seconds. It is included so that monitors can be convinced of _freshness_ if
-enough witnesses added their cosignatures, see below.
+enough witnesses added their cosignatures. A claimant may also use timestamps
+to prove to a believer that some logged data is current. See timestamp
+verification in Section 2.3.2.
`tree_size` is the number of leaves in a log.
@@ -207,7 +209,7 @@ Output on success:
### 3.3 - get-tree-head-cosigned
Returns the latest cosigned tree head. Used together with `get-inclusion-proof`
and `get-consistency-proof`. Ensures that verifiers see the same statements as
-believers.
+believers. May also be used to convince a believer about when logging happened.
```
GET <base url>/sigsum/v0/get-tree-head-cosigned
--
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