Encrypt a message or a file with AES-256-GCM using a passphrase only you know, then hand over something that is meaningless without it. Password managers charge a subscription to hold notes like these, "secure send" sites charge per transfer and quietly keep a copy, and file-lockers cap the free tier. Here the key is derived on your device from your passphrase, the ciphertext is written straight to your downloads, and the only person who can ever open it is someone holding that passphrase.
Your passphrase is turned into a key with PBKDF2-HMAC-SHA256 over 310,000 iterations and a fresh 16-byte random salt, then the data is encrypted with AES-256-GCM using a fresh 12-byte random IV. GCM is authenticated encryption: if the passphrase is wrong, or a single byte of the file has been altered, decryption fails outright rather than handing you garbage. That failure is the loud "wrong passphrase" error you get here.
The file format is small and deliberate: a magic marker, a version byte, the iteration count, the salt, the IV, then the ciphertext. The filename and MIME type are inside the ciphertext, so a locked file does not leak what it is called. Nothing about the passphrase is stored anywhere — not in the file, not in this page, not in your browser.
Three limits worth knowing. A passphrase is only as strong as it is unusual: three random words beats eight characters of cleverness, and the generator here builds the former. There is no recovery path — lose the passphrase and the data is gone, which is the whole point. And this protects the file's contents, not the fact that you encrypted something: if that matters to you, that is a different problem.
Everything runs on your device with the browser's own Web Crypto implementation. The vault needs a secure (https) page to expose those APIs; if you are reading this over plain http, the tool will say so instead of quietly pretending.