Original text:
3.4.1. <del> Attestation is an important part of the issuance protocol.  In Privacy Pass, attestation is the process by which an Attester bears witness to, confirms, or authenticates a Client so as to verify a property about the Client that is required for Issuance.  Clients explicitly trust Attesters to perform attestation correctly and in a way that does not violate their privacy. </del>  RFC9334 describes an architecture for attestation procedures.  Using that architecture as a conceptual basis, Clients are RATS attesters

Reviewer comments:
In a successful issuance exchange, the attester provides the issuer with information about the client. The text in Section 3.2.1 points out that the way in which information about the client is combined could reduce anonymity set size in ways that affect privacy. Given that we have a chain of custody, I imagine that the privacy property here is about what the issuer learns about a client. More information means more information that might then propagate from the issue onward to the origin. This means that the attester needs to limit what it passes along to only that which is relevant. The client needs to trust the attester to do this. ... or does it? The text in Section 3.2.1 implies strongly that this anonymity set size is important, but the constraints on the client-issuer interaction, especially consistency rules, very much limits what the issuer can pass along to the origin. So maybe the system depends less on trusting the attester and more on the combination of the cryptographic properties of the token and the consistency system that mitigates against misuse. None of this appears in the privacy considerations. Should it?
On this point: "Clients do not explicitly trust Issuers." (Section 3.2.2) This would invalidate much of the discussion of anonymity sets in attestations. This is quite confusing. The client doesn't need to trust the issuer, because the issuer is unable to pass information on to others. However, the client cannot trust the issuer with information, so it has to trust the attester not to pass stuff along AND it has to use an anonymizing proxy to interact with the issuer so that nothing leaks. This is quite contradictory overall.
There are issuance protocols wherein the issuer learns information about the origin by design, like the . In this variant, if the attester were to pass information about the client on to the issuer, then the issuer would be able to reconstruct the (client, origin) pair, breaking the unlinkability goals. I agree that in the case where the issuer learns nothing about the origin in the protocol then reliance on the attester is lessened, but we need to make sure we accommodate future issuance protocols here. I'll think about ways in which we can try to make this more clear. This might go into the privacy considerations, as suggested.

Revised text:
3.4.1.  <ins> In Privacy Pass, attestation is the process by which an Attester bears witness to, confirms, or authenticates a Client so as to verify properties about the Client that are required for Issuance.  Issuers trust Attesters to perform attestation correctly. </ins> RFC9334 describes an architecture for attestation procedures.  Using that architecture as a conceptual basis, Clients are RATS attesters