As I wrote in the fedi thread, email addresses are not great to use but it will always be better than nothing. So at least, there could be that method. But Keyoxide will still need to fetch that information about your account. Let's see if they have an API, preferably public.

---edit

Awesome!

    I don't understand this story of credits that appear throughout the documentation. Are these API request credits? Likely, given one needs an API authentication secret.

    Oh never mind, look at this: https://gateway.threema.ch/de/products I don't speak german but it would appear this API is paid for.

    With enough funding, we could propose this but right now, that's not really viable I think ☹️

    ---edit

    I think sending messages costs credit, not the lookup requests.

      yarmo Ohhh, there's a much easier way: Keyoxide registers a free Gateway API account, and to verify your Threema ID you just send a certain message to Keyoxide via Threema itself! Receiving messages is free of charge.

      Problem: “The number of incoming messages must not exceed the number of outgoing messages by more than a factor of 10.”

      English: https://gateway.threema.ch/en/products

      Maybe Threema are willing to make an exception for Keyoxide as a non-profit organisation?

          I now have a Threema Gateway account and ID but no clue how to obtain an API authentication secret token...

          Pixelcode Maybe Threema are willing to make an exception for Keyoxide as a non-profit organisation?

          If we ever hit a limit, that is definitely something we can try and ask them!

              yarmo I think the limit would be 10 incoming messages a.k.a. 10 Keyoxide verifications, if you don't send any messages (which would cost money).

                Maybe I don't get an API authentication token because I need to pay the 800 credits setup fee? I really don't understand what button I need to press to get the token. Documentation just assumes you have one.

                  yarmo Yeah, I'm pretty sure that's it. Threema has provided me with a test ID, up to me to set up a real ID. A basic ID will suffice but I can't spare the setup fee right now. So we can try contacting them or wait a bit till I have more funds.

                      yarmo I'd recommend asking them straight away, since you need to expect that quite a few users will want to try that out. Costs can only grow. 🤷

                        Alright, I have asked the Threema team, now let's wait and hope they are willing to help us out. I'll let you know when I hear back.

                        Threema has given me the credits to set up a Gateway account \o/ thanks folks! I will look into what kind of infrastructure is needed for an E2E account but again, I do believe it's overkill for our purpose.

                        It might be good to have if we ever want to use Threema messages for more advanced purposes — for example, verify non-OpenPGP keys by sending the fingerprint via message. This has the giant drawback if making this information "centralized", only Keyoxide (the project's instance) could access it, not other instances. Unless it's built into a separate public service, but that's a different discussion.