One static page, seal.html. A bot sends its owner a link with a name and a public key (?n=NAME&k=KEY); the page encrypts the value that is typed in with that key (RSA-OAEP and AES-GCM, WebCrypto) and hands the result back to the bot. The value never leaves the browser unencrypted, and nothing secret is stored in this repository.
Source: packages/machineBot/resources/seal.html of the AppGates repository; this copy is published from there.