[ryao]

Their documentation suggests that is only needed by contacts only mode and they wrote some code to get the needed certificates from macOS:

https://github.com/seemoo-lab/airdrop-keychain-extractor

[lxgr]

That still requires you to have (access to) a Mac and an iCloud account.

It might be possible to reimplement the required Apple API, but as demonstrated by the iMessage/Beeper saga, they usually shut such things down pretty quickly.