What is in the SafeWA QR Codes?

Earlier this month, the Western Australian government introduced the SafeWA contact tracing app, which relies on users scanning a QR code at a venue or event in order to be added to the online register. The app doesn't request location permission, so it is solely linking your SafeWA user account with the information in the QR code.

The QR codes were quite large, so I was kind of curious what data was held inside them. So I tried scanning one with a different barcode scanning app, which showed a standard URL-style QR code. Here is what was in a code displayed outside the Coles supermarket in Claremont:

https://safewa.health.wa.gov.au/qr-code/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ2ZW51ZUlkIjoiNWZkYzVkYjViMjdjNzY5MDJhN2NiMzU4Iiwic2NhbkxvY2F0aW9uSWQiOiI1ZmRjNWRiNWIyN2M3NjEyYzU3Y2IzNWEiLCJpYXQiOjE2MDgyNzc0MjksImV4cCI6MjIzODk5NzQyOX0.ruc9OkZ0KgjF8z00BBhMzUIh-kJb1DhxhW9nNqvVi5w?scanLocation=5fdc5db5b27c7612c57cb35a&venue=5fdc5db5b27c76902a7cb358

There's two query parameters in the URL holding what looks like hexadecimal encoded data -- scanLocation and venue. The location identifier shares the prefix 5fdc5db5b27c76 with the venue ID: I'm not sure if that means the identifiers are encoding some common data, or whether they were generated using a UUID-style algorithm that generates IDs with a common authority prefix.

Before the query parameters, we have a large chunk of encoded data. Interestingly, it consists of three dot separated chunks with the first two starting with "ey". That's a strong indication that we're looking at a JSON Web Token. Here, the header is:

{"alg":"HS256","typ":"JWT"}

And the payload is:

{"venueId":"5fdc5db5b27c76902a7cb358","scanLocationId":"5fdc5db5b27c7612c57cb35a","iat":1608277429,"exp":2238997429}

The last blob is an HMAC-SHA256 signature of the first two parts. So we've essentially got a signed duplicate of the query string parameters together with what looks like issue and expiry time stamps. Assuming these are standard UNIX time stamps, the token was issued at 3:43pm on 18th December. The expiry date has been set 7300 days after the issue date, which would be 20 years if every year was 365 days long.

As they're using an HMAC signature, presumably the SafeWA app has no way to verify the token: if it did, then anyone with a copy of the app could extract the key and generate their own signed JWT blobs. If the JWT is sent back to the server verbatim, I wonder how much trouble it would be to just check that the (venue, scanLocation) pair is valid?

So there are a few ways they could simplify the QR codes:

  1. If the JWT signatures are actually necessary, dropping the query parameters would remove 20% of the data in the code.
  2. If the signature is unnecessary, dropping the JWT would remove 68% of the data in the code.
  3. Having the QR code take the form of a URL would be useful if the app was set up to claim that URL prefix, since it would allow you to start the check-in process through any barcode scanning app. They haven't done that though, so the URL prefix could be removed for a shorter plain text QR code.

Lastly, visiting the URL from the QR code directly in the a web browser currently just redirects you to the SafeWA home page and tells you to install the app. It seems like a missed opportunity not to let people sign their attendance at that URL directly, in case they don't have the app installed or if it is malfunctioning. It could open the door for people spoofing the Health Dept website, but it's not clear that's worse than the the status quo where some venues still seem to be running their own contact registers.

Update: I played around with generating QR codes generated from modified versions of the URL to see what the app would accept. The app would accept a QR code with the query parameters stripped out, and fails if the JWT is stripped from the URL. So it is definitely using the JWT token to determine the parameters. It also seems to accept tokens with the signature stripped off, so it seems possible that it doesn't actually care about validity.