Paste a message, get a link back with a random 256-bit key baked in. Whoever has that link can read the message — no account, no login, no identity check on either side. That key is the entire access-control model, the same way a Dropbox share link or a session cookie already is: possession is authorization.
Your text is written to cinder under a randomly generated key. You choose how it's picked up: burn-after-read (the first person to open the link consumes it) or a short time-to-live. Either way, once it's gone, there's nothing left to recover — cinder keeps everything in memory only, never on disk.
Not an account system, not a chat history, not a place to keep anything. This is cindernote's original job and the whole family's starting point — cindertransfer, cinderchat, and cindertunnel all grew out of the same relay, the same key model, extended for different jobs.