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authorRasmus Dahlberg <rasmus.dahlberg@kau.se>2021-10-02 00:04:16 +0200
committerRasmus Dahlberg <rasmus.dahlberg@kau.se>2021-10-02 00:54:13 +0200
commit16041237842bc729782acb828abd795b6f5935d4 (patch)
tree3852a25b11b53fd23a411b13a82d3c8b81492ce1 /doc
parentc466d2360c5ab4e042f6c778468b9073017f4bd6 (diff)
updated threat model
- Minor rephrasing and white-space changes to make raw text nicer. - Avoid using sigsum as "signed checksum" in text. Not helpful. - Removed paragraph about risk-averse attacker. It is not needed to make our points right now. In a future revision, we should re-add this and explain why it is interesting. It would also be a good idea to then cite the two papers that z4lem mentioned a while back, see archive. - Clarified that we need a threshold of witnesses that follow the cosigning protocol for security. It is a start on addressing rohonk's comment about which parties may (not) follow protocol and why. - Emphasized that sigsum logging is only more course-grained than CT if the data is actually lost. Hence, more course-grained _in isolation_. - Added links to slow-down and split-view attacks.
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@@ -147,41 +147,41 @@ to extend and/or modify. The last part contains documentation TODOs.
We consider a powerful attacker that gained control of a claimant's signing and
release infrastructure. This covers a weaker form of attacker that is able to
sign data and distribute it to a subset of isolated users. For example, this is
-essentially what the FBI requested from Apple in the San Bernardino case [\[FBI-Apple\]](https://www.eff.org/cases/apple-challenges-fbi-all-writs-act-order).
+essentially what the FBI requested from Apple in the San Bernardino case
+ [\[FBI-Apple\]](https://www.eff.org/cases/apple-challenges-fbi-all-writs-act-order).
The fact that signing keys and related infrastructure components get
-compromised should not be controversial these days [\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/).
+compromised should not be controversial these days
+ [\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/).
The attacker can also gain control of the transparent log's signing key and
infrastructure. This covers a weaker form of attacker that is able to sign log
data and distribute it to a subset of isolated users. For example, this could
have been the case when a remote code execution was found for a Certificate
-Transparency Log [\[DigiCert\]](https://groups.google.com/a/chromium.org/g/ct-policy/c/aKNbZuJzwfM).
-
-Any attacker that is able to position itself to control these components will
-likely be _risk-averse_. This is at minimum due to two factors. First,
-detection would result in a significant loss of capability that is by no means
-trivial to come by. Second, detection means that some part of the attacker's
-malicious behavior will be disclosed publicly.
-
-Following from our introductory goal we want to facilitate _disocvery_ of sigsum
-statements. Such discovery makes it possible to detect attacks on a claimant's
-signing and release infrastructure. For example, a claimant can detect an
-unwanted sigsum by inspecting the log. It could be the result of a compromised
+Transparency Log
+ [\[DigiCert\]](https://groups.google.com/a/chromium.org/g/ct-policy/c/aKNbZuJzwfM).
+
+Following from our introductory goal we want to facilitate _discovery_ of signed
+checksums. This makes it possible to detect attacks on a claimant's signing and
+release infrastructure. For example, a claimant can detect an unwanted checksum
+signature by inspecting the log. It could be the result of a compromised
signing key. The opposite direction is also possible. Anyone may detect that a
repository is not serving data and/or proofs of public logging.
+For security we need a collision resistant hash function and an unforgeable
+signature scheme. We also assume that at most a threshold of independent
+parties stop following protocol to protect against a malicious log that attempts
+[split-view](https://datatracker.ietf.org/doc/html/draft-ietf-trans-gossip-05)
+or
+[slow-down](https://git.sigsum.org/sigsum/tree/archive/2021-08-24-checkpoint-timestamp)
+attacks. This gives us a _trust-but-verify_ relationship towards the log.
+
It is a non-goal to disclose the data that a cryptographic checksum represents
-_in the log_. It is also a non-goal to allow richer metadata that is
-use-case specific. The type of detection that a sigsum log supports is
-therefore more _coarse-grained_ when compared to Certificate Transparency. A
+_in the log_. It is also a non-goal to allow richer metadata that is use-case
+specific. The type of detection that a sigsum log supports _in isolation_ is
+therefore more coarse-grained when compared to Certificate Transparency. A
significant benefit is that the resulting design becomes simpler, general, and
less costly to bootstrap into a reliable log ecosystem.
-For security we need a collision resistant hash function and an unforgeable
-signature scheme. We also assume that at most a threshold of seemingly
-independent parties are adversarial to protect against split-views
-[\[Gossip\]]().
-
## 3 - Design
We consider a _claimant_ that claims to distribute the _right_ opaque data with
cryptographic hash X. A claimant may add additional falsifiable claims.