Include `maxValue` in report authenticated data

[bmcase]

Right now the encrypted report only contains epsilon.

It should also include maxValue as we assume the noise is added by the Helpers knowing 2 * maxValue / epsilon

TODO: make sure this can also work with @martinthomson DAP extension for binding the additional DP data

[martinthomson]

maxValue is something that DAP needs to include in the VDAF configuration. It's fixed for the task, so there is no way for this to vary on a per-report basis.

I need to talk to David about updates to the L1 Bound Sum on the back of changes to Prio3 generally. Apparently we can get arbitrary bounds, not the $2^n-1$ that we currently have.

[csharrison]

maxValue is something that DAP needs to include in the VDAF configuration. It's fixed for the task, so there is no way for this to vary on a per-report basis.

Can we change that? Fixing maxValue likely means fixing to something large, which can reduce utility.

[martinthomson]

That might be a heavy lift for DAP. The choice of VDAF is something that is pretty heavily baked into the design.

You might imagine making this something that is done on a per-batch basis, but once you get down to aggregating a bucket of reports (which is the unit at which the aggregators do their work) I think you basically have to use the exact same VDAF configuration.

There are some batch modes where you might imagine asking the leader to only divert reports with the same VDAF configuration into the one bucket, but that isn't possible for some batch modes, so I wouldn't like to bet on being able to do that.

As you say, not ideal, but not something we're likely to be able to change.

(FWIW, the upshot of the current approach is likely that you end up with different tasks if you set different values for maxValue. That's not broken, but only if you consider having to deal with multiple tasks okay.)

[csharrison]

I'm not sure I follow precisely. Can you explain to me what I do if I am an advertiser that uses different values for maxValue? Do I instantiate different independent VDAFs that each separately fix a maxValue?

[martinthomson]

Presently, an advertiser that sets two values for maxValue will have the reports for each different value aggregated separately. With two lots of independent noise. Because each value of maxValue has to be a different VDAF, task, and aggregated histogram.

This is the present reality. DAP is not immutable. The good news is that if we change DAP, the browsers can tweak their implementation but the API doesn't need to change much more than maybe a new aggregation service type.

[csharrison]

No this is fine, I had misunderstood your previous comment. I am fine with this. @bmcase and I assumed that we would need to pass maxValue in the authenticated data for this to work. Do you agree?

[bmcase]

with the latest discussion on integrating with DAP we said this will go into the taskprov parameters which will suffice.

[martinthomson]

The current design includes the maximum value in the task configuration. This is bound to every report, through the AAD. In our design, it is also used to generate the task ID, so there is no way for reports with a different maximums to be mixed together when aggregating. Marking this as fixed by #384.