Many new additions and clarifications based on review comments

docs/claimantmodel/Book.md

@@ -9,6 +9,10 @@ This longer form text on the Claimant Model serves as a more gentle on-ramp to u
 
 This text is broken into sections, but is intended to be read from start to finish.
 
+TODO: perhaps introduce the idea of logs implementing discoverability here and link to the later sections for those that want to skip to this topic. I'd expect that a significant audience for this guide will want to understand logs and before committing to reading so much will want some assurance that this is going to the correct destination.
+
+TODO: introduce [actors and roles](#actors-and-roles) around here; a basic understanding of the distinction is important.
+
 ## Motivation
 
 Prior to the Claimant Model being developed, new transparency projects were designed by copying and modifying
@@ -18,7 +22,7 @@ While this allows those familiar with CT to reach consensus on a rough design qu
 1. Those that don't already understand CT now have to read [RFC 6962](https://www.rfc-editor.org/rfc/rfc6962) to understand
    the domain that the other designers are drawing from
 1. There are still terms in CT that people disagree on the precise definition of (e.g. Auditor and Monitor)
-1. CT uses mechanisms such as SCTs that transparency systems should aim to avoid unless strictly necessary
+1. CT uses mechanisms such as [SCTs](#scts) that transparency systems should aim to avoid unless strictly necessary
 1. Any differences in the roles, interactions, actors, or motivations between CT and the new system being designed can
    have knock-on effects that can undermine the security properties achieved
 
@@ -31,6 +35,9 @@ information (`Claim`) provided to them by a trusted third-party (`Claimant`). Th
 Believer can not verify that the Claim is true, but believes it because they trust the Claimant.
 
 The situation above appears in countless settings in our everyday human lives.
+For example if you open a tab at a bar then any drinks are served as a trusted action, relying on you completing your implicit claim that you will pay for the tab before you leave.
+Even if you leave a payment card as a requirement for opening the tab, unless some amount is pre-authorized ahead of time, verification by the bank that the card is good for the value of the tab is deferred until after the actions have been performed.
+
 On a technical level, this also happens frequently, often invisibly to the user.
 For example, when a Chrome browser (`Believer`) receives a TLS certificate (`Claim`) for a domain, it only performs
 the connection (trusted action) if the certificate is issued by a Certificate Authority (`Claimant`) that chains to its
@@ -40,7 +47,12 @@ As another example, a software package manager (`Believer`) will only install so
 
 Above we state that it must be possible to verify that the Claim is true.
 This means that all Claims must be _falsifiable_, i.e. they are able to be proven false.
-We'll discuss _who_ can falsify Claims later.
+
+In both of the examples above, the Believer can verify the cryptographic primitives to operate as the verifier _providing that the Claimant is acting securely and honestly_.
+Explicitly, assuming that the CA is the only party with access to their signing key, and that they only issues certificates to legitimate domain owners, and the CA signing cert chains to the trusted root set, then verification can be performed by Chrome.
+Similarly, assuming that the package maintainer never loses control of their key material and that they only ever sign good packages, then the package manager can verify the signature.
+
+These assumptions can be weakened, but this will require another party to perform additional [verification](#verification).
 
 ### Exceptions
 
@@ -59,6 +71,8 @@ return on investment, or more simply, just confuse a situation that can be easil
 Any ecosystem where the Claimant Model is [applicable](#applicability) has potential for exploit.
 Specifically, a false Claim made under the identity of the Claimant would be believed by a Believer, and they would
 perform some trusted action they would not have made given a true Claim.
+This potential for exploit exists because there is an asymmetry between who believes a claim and who can verify it.
+The Claimant Model shines a light on this asymmetry and once a system has been described it should be clear how this tension will be resolved.
 
 ### Motivations for Betraying Trust
 
@@ -88,12 +102,10 @@ This is the motivation for [transparency logs](https://transparency.dev).
 Given enough time, potential motivation, and/or bad luck, any Claimant could issue a false Claim.
 The question is how to make a system with enough security given this fact.
 
-Earlier we established that Claims must be falsifiable, but that in general the Believer will not be able
-to perform this verification.
+Earlier we established that Claims must be falsifiable, but that in general the Believer will not be able to perform this verification.
 Thus we introduce the role of the `Verifier`, which will be able to verify all Claims in a system.
 
-TODO: link to sidebar discussing roles vs actors: the Verifier role does not imply there is a single actor that must
-be able to verify all the claims.
+TODO: expand on verification. In particular, link this to discoverablity. This is where logs should first be mentioned.
 
 ## More Precision
 
@@ -116,6 +128,9 @@ A Statement must be issued by the Claimant and it must be possible to construct
 In practical scalable systems, Statements are determined to have been issued by a Claimant by the presence of
 a digital signature over the Statement.
 
+For example, in CT the Claim is the logical statement "I am authorized to certify $pubKey for $domain" made by the CA, and the Statement is an X.509 certificate containing $pubKey and $domain, and signed by the CA.
+Given an X.509 certificate it would be possible to construct the Claim from it by extracting the relevant fields.
+
 ### Composite Claims
 
 Thus far we've referred to the Believer taking action based on a single Claim.
@@ -124,7 +139,61 @@ In particular, this is useful when a claim has multiple sub-claims that can be v
 
 TODO: example
 
+### Actors and Roles
+
+TODO: write up about actors and roles. I got the idea initially from https://en.wikipedia.org/wiki/Paxos_(computer_science)#Typical_deployment but looking at this again it doesn't talk about actors, and there isn't much easily appearing in searches about the distinction between these concepts.
+
+Roles are an abstract description of a participant in a system. The Actor is the person that will perform this Role. It _may_ be the case that all actors play precisely one role, and that all roles are played by precisely one actor. However, in general this Role:Actor mapping should be considered many-to-many. This can mean:
+
+* One actor plays multiple roles: this is the case for [signature transparency](https://www.sigstore.dev/) use-cases; the owner of the signing material is both the claimant (signing stuff) and the verifier (checking that all usages of the signing material were known to the rightful owner)
+* One role is fulfilled by multiple actors: this commonly occurs for the Verifier role where there are [composite claims](#composite-claims); multiple different actors each verify a sub-claim ([example](https://github.com/google/trillian/blob/master/docs/claimantmodel/experimental/cmd/render/internal/models/armorydrive/full.md)).
+
 ## Other Reading
 
 - [Transparency: An Implementer's Story](https://transparency.dev/articles/transparency-an-implementers-story/) provides a short,
   narrative-driven journey that would be useful to those understanding how to upgrade their existing security posture to include logs.
+
+## Appendices
+
+While writing this documentation the need for other supporting documentation is frequently requested in the comments.
+These topics are important but are outside the scope of what this documentation is aiming to provide.
+As a pragmatic compromise, such topics will be _very briefly_ summarized here along with a feature request bug tracker to write up the topic in more detail.
+Once such documentation is complete, this appendix stub should be removed and all hyperlinks updated.
+
+### SCTs
+
+Signed Certificate Timestamps (SCTs) are a design choice from Certificate Transparency that is *not recommended* as a default choice in new transparency applications.
+This is not to say that this pattern should never be used, but alternatives should be strongly considered first.
+
+When an item is added to a log, it is usually an asynchronous operation, i.e. there is a delay between the item being sent to the log and the item being provably committed to by the Merkle structure of the log.
+It is standard practice that logs publish a Maximum Merge Delay (MMD) for what this delay can be.
+In the case of CT this is 24h.
+
+A long merge delay makes it easier for log operators to operate the logs within [SLA](https://en.wikipedia.org/wiki/Service-level_agreement), but this comes at the expense of Believers not being able to rely on proving inclusion of any claim that is fresher the MMD in the worst case.
+
+SCTs are a promise that is issued when a claim is sent to the log that claims that the log operator promises to include the claim by a given integration time.
+This promise is signed by the log and returned to the user that logged the claim.
+If you're getting that feeling that this itself now feels like a Claim, then bingo:
+
+<dl>
+<dt>Claim<sup>PROMISE</sup></dt>
+<dd><i>${claim} will be committed to by the Merkle structure of this log by $integrationTime</i></dd>
+<dt>Claimant<sup>PROMISE</sup></dt>
+<dd>Log Operator</dd>
+<dt>Believer<sup>PROMISE</sup></dt>
+<dd>Believer<sup>LOG</sup></dd>
+<dt>Verifier<sup>PROMISE</sup></dt>
+<dd>Anyone with the promise at $integrationTime</dd>
+<dt>Arbiter<sup>PROMISE</sup></dt>
+<dd>Arbiter<sup>LOG</sup></dd>
+</dl>
+
+Using the Claimant Model to describe promises succinctly shows the inherent problem with them; promises need to be verified.
+Anyone _can_ verify an inclusion promise, but making it so that everyone will verify any promise they have seen is generally not feasible.
+For example, in CT this would require that every browser that has been shown a certificate containing one or more SCTs would verify the inclusion in all logs that had issued SCTs after the MMD had passed.
+Even if this was feasible to implement and users were happy to sacrifice having a lightweight browser for something that performed this additional work (using more disk space, network, battery etc in the meantime), there would then need to be a mechanism for these browsers to be able to report SCTs that did not verify.
+
+The other option is that SCTs are made discoverable to a party of dedicated verifiers who can report non-fulfilled promises to the arbiter.
+This disoverability problem would require a log for SCTs, which then appears to be precisely the same problem as logging certificates in the first place!
+
+TODO(#3014): complete this stub and document solutions for working around the need for SCTs.