Journalists communicating with sources in censored regions
Whistleblowers sharing information securely
You and your peer agree on an encryption key (any string).
This is unacceptably unsecure for the usecases you mention. There is a reason why the most secure messaging apps don't use symetric encryption, don't use passphrases, and they also possess forward secrecy.
It's pointless to push this as a censhorship circumvention method when many other methods exist that already do so 10x better, in a secure way, over decentralized, hidden and unblockable infrastructure. (Tor's meek-azure bridges use microsoft's infrastructure, which nobody is able to block because everybody depends on it, even China).
I appreciate the project, and I am always happy to see people learning, progressing, and publishing their results, but you need to be honest about the weaknesses of your software compared to established solutions. It's not impossible for you to one day produce a secure messaging app, but today is not the day. Right now, using this is just a fast way to get killed.