Put files in, choose a passphrase, and CAPSULE writes a single HTML page that decrypts itself. The person you send it to opens it in any browser, offline if they like, types the passphrase and saves the originals. No software on their end, no account, no server holding a copy. AES-256-GCM with a PBKDF2 key, all with the browser's own cryptography. Included in the Sovereign Pass.
No files yet.
The passphrase is the whole strength. There is no recovery. Send it by a different route from the capsule (say it on a call, or use a different app), and never in the same email.
A capsule opens on its own in any browser. If you would rather open one on this page, choose the file and enter the passphrase.
It is real encryption in a convenient shape. AES-256-GCM, which also detects any change to the sealed data, with a key derived from the passphrase by PBKDF2-SHA256 over 600,000 rounds and a random salt. The page carries the salt and the sealed bytes; the passphrase is never in the file.
It does not hide that it is encrypted. The page says plainly what it is. Anyone who sees the file knows it is a sealed container; they just cannot open it without the passphrase.
It is only as strong as the passphrase. The rounds make guessing slow, not impossible. A short or common passphrase will fall; four or five unrelated words will hold.
Size is limited by memory. The whole capsule is built and opened in the browser's memory, so the total is capped at 40 MB. For larger transfers use VAULT, which moves a file directly between two browsers.
The recipient's browser matters. Any current desktop or mobile browser can open a capsule from a local file. Some email previewers and locked-down viewers strip scripts; the recipient should save the file and open it directly.
Nothing leaves this tab. Files, passphrase and the finished capsule stay in browser memory until you download the capsule.