Add suggestion on how malicious issuers can be detected.

index.html

@@ -1167,15 +1167,30 @@ <h3>Malicious Issuers and Verifiers</h3>
 tracking or sharing the presentation of particular credentials.
       </p>
       <p>
+A malicious [=verifier=] might intentionally attack group privacy by sharing
+information from presented credentials with a malicious [=issuer=]. This
+sort of collusion is difficult to detect as it is typically performed via a
+secure communication channel between the [=issuer=] and the [=verifier=].
+      </p>
+      <p>
 A malicious [=issuer=] might intentionally attack group privacy by creating a
-unique status list per credential issued in order to establish a one-to-one mapping to track
-when a [=verifier=] processes a specific credential. Similarly, they could establish
-another a one-to-one mapping by using a different cryptographic key for every credential
-issued that is tracked in a status list.
+unique status list for each issued credential, in order to establish a one-to-one
+mapping to track when a [=verifier=] processes each mapped credential. Similarly,
+they could establish a one-to-one mapping by using a different
+cryptographic key for each issued credential that is tracked by a given status list.
       </p>
       <p>
-A malicious [=verifier=] might intentionally attack group privacy by sharing
-information from presented credentials with a malicious [=issuer=].
+This sort of collusion can be detected by [=holder=] software that serves
+multiple [=holders=] (e.g., a [=holder=] app that runs on a server) if it
+has, for example, an opt-in process that finds that some global identifier(s) 
+used within a [=verifiable credential=] are not adequately shared by other
+credentials.
+[=Holders=] could then be warned when presenting a [=verifiable credential=]
+that contains some global identifier(s) that are unique to that credential.
+Such an opt-in service could represent some additional privacy concerns;
+whether this potential exposure via the [=holder=] software is justified by
+the awareness of possible global identifier correlation can only be evaluated
+by the users of such a system.
       </p>
     </section>