[caf]

Even better:

    1. During the HTTPS part of the communication, the server generates a single
       random key and sends it to the client.
    2. The client stores this string in local storage.
    3. For every request, the client generates a HMAC over the request parameters
       (including a monotonic sequence number) using the key.

Both of these schemes are still susceptible to a MITM, who can just insert a bit of javascript in any page received over HTTP, that reveals the temporary secret in local storage to anyone listening.

[paulbaumgart]

If you were to generate the HMAC over the entire request, would that help with ensuring authenticity?

[electrum]

It wouldn't be vulnerable to JavaScript if it was built into the browser -- like HttpOnly cookies.