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Merge pull request #571 from corona-warn-app/event-registration

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Event Registration: Create QR code locally without server and update QR code data structure
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Maximilian Lenkeit 2021-04-03 10:14:27 +02:00 committed by GitHub
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event_registration.md
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@ -2,9 +2,9 @@
Presence Tracing - in CWA also referred to as _Event Registration_ - aims at notifying people of a potential SARS-CoV-2 exposure if they have been to the same venue at a similar time as a positively tested individual. It addresses the potential of airborne transmission in spaces with poor ventilation despite maintaining physical distance. As such, it complements BLE-based proximity tracing with the Exposure Notification Framework.
CWA proposes a fully-automated decentral solution for Presence Tracing which works independent of local health authorities and host of the event. It integrates into the existing verification processes of CWA to issue warnings. The solution prioritizes the speed of issuing warnings over their accuracy. A higher degree of accuracy would require manual assessment by local health authorities and the respective resources to do so and is currently not on scope.
CWA proposes a fully-automated decentral solution for Presence Tracing which works independent of local health authorities and the collaboration of the host of a venue. It integrates into the existing verification processes of CWA to issue warnings. The solution prioritizes the speed of issuing warnings over their accuracy. A higher degree of accuracy would require manual assessment by local health authorities and the respective resources to do so and is currently not in scope. The solution also protects the privacy of both venues and users, as details about a venue such as description or address are not shared with the CWA infrastructure.
In summary, the proposed solution allows a _host_ to create a venue through CWA. All necessary signed data about the venue is encoded in a QR code which can be presented on a mobile device or printed out, for example to be posted at the entrance of the venue. An _attendee_ can check in to the venue by scanning the QR code. Check-ins are stored locally on the mobile device and deleted automatically after two weeks.
In summary, the proposed solution allows a _host_ to create a venue through CWA. All necessary data about the venue is encoded in a QR code which can be presented on a mobile device or printed out, for example to be posted at the entrance of the venue. An _attendee_ can check in to the venue by scanning the QR code. Check-ins are stored locally on the mobile device and deleted automatically after two weeks.
When an attendee tests positive for SARS-CoV-2, they can upload their check-ins along with their Diagnosis Keys to the CWA Server. The CWA Server publishes the relevant check-ins on CDN as _warnings_. Clients regularly download these warnings and match them against the local check-ins on the mobile device. If there is a match and the time an attendee spent at a venue overlaps with a warning for a sufficiently long time, the attendee receives a warning in CWA similar to how warnings are issued for BLE-based exposures.
@ -16,11 +16,11 @@ Several security and privacy threats have been identified for the proposed solut
### Profiling of Venues
The proposed solution publishes warnings on CDN. A warning consists of the GUID of a venue and a time interval. An adversary can collect these warnings and aggregate them to compile a list of venues with the most warnings (colloquially referred to as _most infectious venues_) or a list of venues with their most recent warning.
The proposed solution publishes warnings on CDN. A warning consists of the hashed ID of a venue and a time interval. An adversary can collect these warnings and aggregate them to compile a list of venues with the most warnings (colloquially referred to as _most infectious venues_) or a list of venues with their most recent warning.
This information is easy to collect, as warnings are publicly accessible and do not even required to make modifications to the CWA client.
This information is easy to collect, as warnings are publicly accessible and do not even require to make modifications to the CWA client.
The value of this information increases significantly once an adversary can link the GUID of a venue with the data included in the QR code such as the name or the address of the venue, or with metadata from other services, such as coordinates of the venue.
The value of this information increases significantly once an adversary can link the ID of a venue with the data included in the QR code such as the name or the address of the venue, or with metadata from other services, such as coordinates of the venue.
An adversary can collect this information for a single venue by scanning the QR code and extracting and storing the data outside of CWA. Collecting this information at scale requires coordinated effort by many individuals.
@ -32,13 +32,13 @@ However, we acknowledge that the proposed solution does not prevent this attack
### Profiling of Users
The proposed solution publishes warnings on CDN in packages on an hourly basis. A package includes multiple warnings. A warning consists of the GUID of a venue and a time interval. All the warnings that were created from the check-ins of a single user are included in one package. A package can include warnings of multiple users.
The proposed solution publishes warnings on CDN in packages on an hourly basis. A package includes multiple warnings. A warning consists of the hashed ID of a venue and a time interval. All warnings that were created from the check-ins of a single user are included in one package. A package can include warnings of multiple users.
An adversary can analyze the check-ins of a single package and try to build a profile of the users whose check-ins are included. This reveals limited information if the GUIDs of the venues cannot be linked to an actual venue (cf. [Profiling of Venues]), but can reveal significant information about the user the more GUIDs of venues can be identified.
An adversary can analyze the check-ins of a single package and try to build a profile of the users whose check-ins are included. This reveals limited information if the IDs of the venues cannot be linked to an actual venue (cf. [Profiling of Venues](#profiling-of-venues)), but can reveal significant information about the user the more venue IDs can be identified.
To mitigate the risk, CWA generates fake check-ins for each submission. These fake check-ins are generated upon submission of genuine check-ins so that even CWA cannot distinguish them.
However, we acknowledge that this does not prevent the attack if there is a central database of all venue GUIDs and venue metadata.
However, we acknowledge that this does not prevent the attack if there is a central database of all venue IDs and venue metadata.
### Targeting Specific Venues
@ -46,7 +46,7 @@ The proposed solution turns check-ins of the user into warnings and cannot verif
An adversary can target specific venues by obtaining the respective QR code and pretending a check-in. If the adversary also obtains the authorization to submit the check-ins to the CWA Server, false warnings would be issued for these venues.
The difficulty of this attack is dominated by the difficulty of obtaining authorization to submit check-ins. This is currently only possible with a confirmed positive test for SARS-CoV-2 or by obtaining a Tele TAN from the hotline. While a confirmed positive test is difficult obtain without putting oneself at risk, a valid Tele TAN can be obtained for example by Social Engineering.
The difficulty of this attack is dominated by the difficulty of obtaining authorization to submit check-ins. This is currently only possible with a confirmed positive test for SARS-CoV-2 or by obtaining a TeleTAN from the hotline. While a confirmed positive test is difficult obtain without putting oneself at risk, a valid TeleTAN can be obtained for example by Social Engineering.
To mitigate the risk, CWA only allows a certain number of check-ins per day. This prevents to scale such an attack by a single adversary across a multitude of venues.
@ -56,23 +56,35 @@ However, we acknowledge that this does not prevent to execute this attack for a
The QR code of a venue contains all required attributes for Presence Tracing, so that no server communication is necessary when an attendee checks in to a venue
The data structure is described by the following Protocol Buffer message `TraceLocation`:
The data structure is described by the Protocol Buffer message `QRCodePayload`:
```protobuf
message QRCodePayload {
uint32 version = 1;
TraceLocation locationData = 2;
CrowdNotifierData crowdNotifierData = 3;
// byte sequence of CWALocationData
bytes countryData = 4;
}
message TraceLocation {
// uuid
string guid = 1;
uint32 version = 2;
TraceLocationType type = 3;
// max. 150 characters
string description = 4;
// max. 150 characters
string address = 5;
uint32 version = 1;
// max. 100 characters
string description = 2;
// max. 100 characters
string address = 3;
// UNIX timestamp (in seconds)
uint64 startTimestamp = 6;
uint64 startTimestamp = 5;
// UNIX timestamp (in seconds)
uint64 endTimestamp = 7;
uint32 defaultCheckInLengthInMinutes = 8;
uint64 endTimestamp = 6;
}
message CrowdNotifierData {
uint32 version = 1;
bytes publicKey = 2;
bytes cryptographicSeed = 3;
uint32 type = 4; // exact semantic tbd
}
enum TraceLocationType {
@ -91,47 +103,46 @@ enum TraceLocationType {
LOCATION_TYPE_TEMPORARY_CLUB_ACTIVITY = 10;
LOCATION_TYPE_TEMPORARY_PRIVATE_EVENT = 11;
LOCATION_TYPE_TEMPORARY_WORSHIP_SERVICE = 12;
}
message CWALocationData {
uint32 version = 1;
TraceLocationType type = 2;
uint32 defaultCheckInLengthInMinutes = 3;
}
```
The `guid` attribute is generated by the CWA Server to ensure uniqueness across all CWA QR codes. The data structure is signed by the CWA Server with its private key to prevent tampering of the QR code or identity theft of the GUID of a venue.
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 result in a unique ID.
The combination of signature and TraceLocation is represented in the following Protocol Buffer message `SignedTraceLocation`:
```protobuf
message SignedTraceLocation {
// byte representation of a TraceLocation
bytes location = 1;
// byte representation of the signature of the TraceLocation
bytes signature = 2;
}
```
A SignedTraceLocation is base32-encoded and included in a URL. The URL is the content of the QR code and structures as follows:
A `QRCodePayload` is base64-encoded and included in a URL. The URL is the content of the QR code and structures as follows:
```text
HTTPS://E.CORONAWARN.APP/C1/<SIGNED_TRACE_LOCATION_BASE32>
https://e.coronawarn.app?v=1#<base64_encoded>
# example:
HTTPS://E.CORONAWARN.APP/C1/BIPEY33...
CWA Germany:
https://e.coronawarn.app?v=1#Y3dh...
NotifyMe CH:
https://qr.notify-me.ch?v=2#bm90aWZ5bWU=
CLEA FR:
https://tac.gouv.fr?v=1#Y2xlYQ==
```
The base32 encoding allows to leverage the input mode _alphanumeric_ when generating the QR code and produces a QR code with a lower density compared to base64 encoding.
### QR Code Compatibility with Other Contract Tracing Apps in Germany DRAFT
### Interoperability with Other Contact Tracing Apps DRAFT
Other contact tracing apps that leverage QR code for Presence Tracing can integrate with CWA by creating QR codes according to the following pattern:
Other contact tracing apps in Germany that leverage QR code for Presence Tracing can integrate with CWA by creating QR codes according to the following pattern:
```text
<URL>/<VENDOR_DATA>#[VENDOR_ADDITIONAL_DATA]/CWA1/<ENCODED_SIGNED_TRACE_LOCATION>
<URL>/<VENDOR_DATA>#[VENDOR_ADDITIONAL_DATA]/CWA1/<ENCODED_PAYLOAD>
```
| Parameter | Description |
|---|---|
| `<URL>` | The URL associated with the respective contact tracing apps, with or without a partial path. |
| `<VENDOR_DATA>` | Any vendor-specific data such as venue ids. This data may be passed to the vendor-specific app upon interaction by the user if a deeper integration is required. |
| `<VENDOR_DATA>` | Any vendor-specific data such as the venue id in the vendor's system. This data may be passed to the vendor-specific app upon interaction by the user if a deeper integration is required. |
| `[VENDOR_ADDITIONAL_DATA]` | Additional vendor-specific data (optional). |
| `<ENCODED_SIGNED_TRACE_LOCATION>` | A representation of the Protocol Buffer message SignedTraceLocation encoded in base64. Note that the signature must have been created by the CWA Server. |
| `<ENCODED_PAYLOAD>` | A representation of the Protocol Buffer message `QRCodePayload` encoded in base64. Note that the signature must have been created by the CWA Server. |
**Note:** Any contact tracing apps that integrate with CWA must ensure that they do not process any information from the CWA part of the QR code.
@ -143,12 +154,12 @@ https://presence-tracing.app/386d0384-8aaa-41b6-93c2-d3399894d0ee#/CWA1/CiRmY2E.
URL: https://presence-tracing.app
VENDOR_DATA: 386d0384-8aaa-41b6-93c2-d3399894d0ee
VENDOR_ADDITIONAL_DATA: ∅
ENCODED_SIGNED_TRACE_LOCATION: CiRmY2E...
ENCODED_PAYLOAD: CiRmY2E...
# with optional data
https://check-in.pt.app/386d0384-8aaa-41b6-93c2-d3399894d0ee#42/CWA1/CiRmY2E...
URL: https://check-in.pt.app
VENDOR_DATA: 386d0384-8aaa-41b6-93c2-d3399894d0ee
VENDOR_ADDITIONAL_DATA: 42
ENCODED_SIGNED_TRACE_LOCATION: CiRmY2E...
ENCODED_PAYLOAD: CiRmY2E...
```

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