Implement Proxy Attestation Service example

[ipetr0v]

We need to implement a new experimental example called Proxy Attestation Service.

Proxy Attestation Service works as a Certificate Authority for TEE applications. It attests applications and creates signed certificates for TEE applications, which they use to create TLS connections with clients.

The work cycle of the proxy is represented in 2 stages.
First stage corresponds to the backend server attestation:

• TEE application connects to the proxy
• Proxy attests the application using a corresponding remote attestation protocol
• Application sends the proxy a certificate signing request that contains TEE measurements
• Proxy sings it using its private key and sends the signature back to the application

Second stage corresponds to the client connection:

• Client remotely attests the proxy using a corresponding attestation protocol
• Proxy sends the client its root certificate
  • Client trusts that this is a correct certificate, since it was sent via a secure channel created during the attestation process
• Client connects to the application using TLS, and the application uses a TLS certificate previously signed by the proxy
• Client checks that the certificate was signed by the root certificate and establishes a secure connection