Re: [Privacy-pass] Issuer directory representation (#369)

[Colin Bendell <[e-mail]>]

On Thu, 15 Jun 2023 at 19:28, Christopher Wood <[e-mail]> wrote:

> To hopefully close the loop in this, I put together this PR that
> conceptually lifts the concept of key commitments from PSTs into the
> current Privacy Pass issuer directory:
>

I initiated this thread after contrasting the similar key-sets exposed
forPrivacyPass and Private-State-Tokens. As an outsider here, I'd like to
see PrivacyPass expand the set of tokens so that PST can remove the
definition from the W3C spec and depend exclusively on the PrivacyPass
definition. To this end, the issuer directory should also include:
* expiry
* batch-size
* key-id / version

Expiration is important for PST because it is used by the UA to select from
a set of possible keys. HTTP Cache-Control does not serve the same function
as a certificate expiration. The cache-control header reflects the ttl of
the directory, but the expiration for each public key. If we were to use
HTTP cache policies, all public keys would be treated as if they had the
same expiration.

Batch-Size is interesting for VOPRF workflows where the issuance request
includes multiple nonces. In this case the compute work for the issue
request needs to be known apriori. Can the issuer handle 1 or 10 or 100
token batches?

key-id or version is not needed for the protocol, but as a dev ergonomics
it is nice to have a common implementation. Is it called 'version' or 'id'
or 'key-id'?


> I think we can pursue alternative representations separately.
>

Sounds good.

/colin

>