The server holds only sealed messages
Every message is encrypted on your machine with the channel's keys before it leaves. Sigillum's server stores and passes along ciphertext, never the words.
You decide who is real
Each device shows a short code. Read it aloud to the person you're talking to; when the codes match, the device is marked verified. If a key changes, Sigillum says so in words beside the name.
Invite only
Nobody signs up uninvited. An admin hands out single-use invites, and people are added to channels by username. Someone added later sees nothing from before they joined.
Where it runs
Sigillum runs on Linux desktops today, for teams already invited. Windows and a browser version come next.
Want in? Ask the admin of your team for an invite.