Currently, messages that I sent have a light green background and when they are received they show a green check mark. This makes me think “all is fine”. However, I did not know that my first message was not encrypted (and subsequent ones were not either). My chat partner uses Thunderbird without autocrypt and thus, he could import my key but I could not import his key. (There is no way that I see to do that manually.) Therefore, my messages to him keep being unencrypted. But they look green, and I thought they were encrypted.
I think it should be clearer when messages are unencrypted.
It is not enough to add a lock only when messages are encrypted.
I can imagine two ways of doing this:
show a crossed lock and mark my message in light blue instead of light green.
have a crossed key icon next to the name of my chat participant saying that I did not receive their key yet.
It might scare people to see a crossed lock. One could allow clicking on that lock and receiving the information that no secret key with the chat participant has been exchanged yet. And then maybe more information on why this is so.
Why I think this is important. I just told a journalist friend about delta chat because I think it’s a great idea (no central server, no additional account setup). But, now I imagine this journalist using the app believing that messages are encrypted while they actually aren’t. I understand your goal is to make things as simple as possible but here I do see a bit of a UX issue because the interface does not convey what’s really happening (I’m sending unencrypted messages).