Every passphrase and every message on this site is encrypted and decrypted entirely inside your own browser. Nothing you type is ever sent anywhere. You don't have to trust that claim — you can check it yourself, in under a minute, on the encrypt page or a real message link.
Open your browser's developer tools (usually the F12 key, or right-click anywhere on the page and choose "Inspect"), click the Network tab, then encrypt or decrypt something. Watch closely — no request goes out carrying your passphrase or your message. There's nothing to watch for, because it never leaves your device.
"View Page Source" (Ctrl/Cmd + U) shows you the real code
running in your browser, before you've typed anything into it — not a description of what it does,
the actual thing.