Why you can trust this page
Your file and your passphrase never leave your machine. The security policy at the top of this file makes your browser refuse every kind of network connection — enforcement, not a promise. “Encrypt online” sites that upload your file first are asking you to trust a stranger with the very thing you are trying to protect.
The cryptography is the standard one, done by your browser. AES‑256‑GCM for the sealing, PBKDF2 with 600,000 rounds of SHA‑256 to turn your passphrase into a key — both performed by the browser’s built-in, audited WebCrypto engine. This page contains no home-made cipher, only the plumbing, and you can read all of it.
Tampering is detected, not just discouraged. GCM is authenticated encryption: change so much as one byte of the locked file and decryption refuses outright rather than quietly producing garbage.
The file format is documented inside this file, byte by byte, so anyone can decrypt a .sealed file independently with standard tools — you are never locked into this page. Keep a copy of it with your archives; it will work offline forever.
You are reading the whole program. This one file is the entire tool and its own source — open it in a text editor, or attach it to an AI and ask what it does. What you check is what runs, because there is only the one file.