Cwa-documentation: Clarification regarding Event Registration and durability of QR-Codes

[MaZderMind]

Where to find the issue

The Document event_registration.md states at time of reading under Profiling of Venues, that

The proposed solution publishes warnings on CDN. A warning consists of the hashed ID of a venue and a time interval.

furthermore it states under QR Code Structure, that

The ID of a venue is derived as the SHA-256 hash of the concatenated byte representation of the string CWA-GUID and the byte representation of the Protocol Buffer message QRCodePayload. The cryptographicSeed adds sufficient entropy so that any modifications to the QR code result in a unique ID.

It also states under Profiling of Venues, that

To mitigate the risk, CWA encourages owners to regularly generate new QR codes for their venues. The more frequent QR codes are updated, the more difficult it is to keep a central database with venue data up-to-date.

Describe the issue

Considering the Sentences above, Venues might consider to rotate their QR-Codes very frequently, for example our Sinage System will display a different QR-Code every couple minutes, because an updated Code is generated via https://github.com/MaZderMind/cwa-qr and it will contain a different cryptographicSeed. This is in line with the encouragement quoted above.

However, the quoted Sentences also suggest that this will result in a different Venue-ID for people checking in with different QR-Codes, which would, judging from the quotes cited above, result in different Check-Ins using QR-Codes with different cryptographicSeeds to not be counted as the same Event.

Suggested change

If my assumption above would be correct, I would suggest to clarify the sentence under Profiling of Venues regarding how often a QR-Code should be changed at maximum and that this action will result of a different Event ID.

If my assumption is incorrect, I would guess that the Sentence regarding the contents of the Event-ID or another of the quoted Sentences needs clarification.

[MaZderMind]

Poking @thomasaugsten because he already hinted in this direction in https://github.com/corona-warn-app/cwa-wishlist/issues/399#issuecomment-824610365

[thomasaugsten]

The QR-Code should change more than once per day. Because every new QR-Code is a new Location/Event and user cannot warn each other.

[thomasaugsten]

I specified this in the documentation. The important fact is a new QR-Code should only regenerated when no visitor is in the location or event.

[MaZderMind]

Thank you very much, this indeed clarifies the Situation.