I'm flattered you assume I have this knowledge, but I don't. I would presume the key is derived from your sign-in information, but I honestly don't know.

What I do know is that you don't have access to messages that happened before you first signed in to the device you're using. My message history is complete on the phone I created my Wire account on, but I installed it on a laptop a few days later and as such is missing those first days. Messages get delivered to all your active devices without problem though.

The source code for all their different client varieties (iOS, Android, Web/Electron) and server software can be found here if you're interested [0].

[0]: https://github.com/wireapp