Encrypt a memo with a passphrase. Post the ciphertext on chain. Only those who hold the phrase can read what it said. The chain remembers everything; only some of it is in plain text.
Strength comes from length and unpredictability. Don't reuse a phrase.
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Cipher: AES-256-GCM with a key derived via PBKDF2-SHA256 (200k iterations, salt embedded). The on-chain memo stores UM1:salt:iv:ct (base64) — anyone can see it; only those with the passphrase can decrypt. No metadata about sender or recipient is stored beyond the optional recipient field — the wallet that signed the tx is the de-facto sender.
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If you paste a transaction signature, we'll pull the memo from chain and try to decrypt the UM1 prefix. If it isn't a Cipher Post message, you'll get the raw bytes back so you can see what it actually was.
A WORD ON SAFETY. A passphrase is only as strong as it is unguessable. Share it through a channel that isn't this chain. The Undercity remembers what you wrote even after you've forgotten — if you lose the phrase, the bytes are still there, but you can't read them. Neither can we.