replay attack on queries can leak confidential information

[brenzi]

currently, TrustedGetters have no nonce, therefore they can be replayed.

This is not critical in general as the signed payload is transmitted via TLS directly into the enclave and can't be intercepted by a MITM. But in presence of browser-required reverse proxy (because of declined self-signed certs or for load balancing) for relaying requests in web apps, this may allow the operator of the reverse proxy to leak confidential information.

options to look into:

• For convenience when polling balances, we may not want to use a per-request nonce, but rather a timestamp until which the query shall be valid. then the user can select more or less security with longer or shorter expiry
• return a session token for subsequent authentications? doesn't really add security