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-rw-r--r--doc/design.md18
1 files changed, 13 insertions, 5 deletions
diff --git a/doc/design.md b/doc/design.md
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@@ -284,11 +284,15 @@ its inclusion proof.
3. The provided tree head is from a known log with enough valid cosignatures.
Notice that there are no new outbound network connections for a verifier.
-Therefore, a proof of public logging is only as convincing as the tree head that
-an inclusion proof leads up to. Sigsum logs have trustworthy tree heads thanks to
-using a variant of witness cosigning. A verifier cannot be
-tricked into accepting data whose checksum have not been publicly logged
-unless the attacker controls more than a threshold of witnesses.
+Therefore, a verifier will not be affected by future log downtime since the
+signer already collected relevant proofs of public logging. Log downtime may be
+caused by temporary operational issues or simply because a shard is done.
+
+The lack of external communication means that a proof of public logging cannot
+be more convincing than the tree head an inclusion proof leads up to. Sigsum
+logs have trustworthy tree heads thanks to using a variant of witness cosigning.
+A verifier cannot be tricked into accepting data whose checksum have not been
+publicly logged unless the attacker controls more than a threshold of witnesses.
In a less ideal world sigsum logging can facilitate detection of attacks if a
verifier _fails open_ by enforcing the second and third criteria partially. For
@@ -353,6 +357,10 @@ set it as large as possible. If a verified timestamp is needed to reason about
the time of logging, you may use a cosigned tree head instead
[\[TS\]](https://git.sigsum.org/sigsum/commit/?id=fef460586e847e378a197381ef1ae3a64e6ea38b).
+A log operator that shuts down a completed shard will not effect verifiers. In
+other words, a signer can continue to distribute proofs that were once
+collected. This is important because a checksum does not necessarily expire.
+
#### 4.3 - What is the point of having a domain hint?
Domain hints help log operators combat spam. By verifying that every signer
controls a domain name that is aware of their public key, rate limits can be