Is there a way to guarantee that a static HTTP page is unchanged from when it was last reviewed
I'm making a simple web service that (skipping other details) allows a user to upload a message that can be retrieved by another user but can be decrypted only with a certain key. The message is encrypted and decrypted on the client side and can't be decrypted on the server unless the key is somehow known.
The code of the web page can be reviewed to check that there are no backdoors that would send the key to the server or anywhere else. However, that needs to be checked every time the page is served, because theoretically the backdoor can be added at any time, since what is happening on the server is not transparent to the client. Even if the user reviews the source code of the server side, there is no guarantee that the code has not been modified.
Is there a way to guarantee that the initial page the user accesses has not been changed?
As an example, maybe there is a service where one can upload an HTML page, and the service would be a guarantor that the page has never changed from that moment, then I could redirect my users to that page from that service.
I wish there were some user-friendly checksum kind of thing in browsers for these cases, but from what I understand it is not a thing.
