Add post-handshake proto messages to Oak Session

Post-handshake is needed to bind the Noise Session to the Attestation
Evidence. And it can also optionally be used to bind Endorsements and
expected peer Reference Values (as suggested in go/oak-noise-crypto-attestation-binding).

Bug: 343170960
Change-Id: I1123a0b3014832b9c67caeb3948d600947b73db5

proto/session/session.proto

@@ -33,24 +33,53 @@ message AttestResponse {
   repeated oak.session.v1.EndorsedEvidence endorsed_evidence = 1;
 }
 
-// Request message for the crypto handshake request needed to establish a set of
-// session keys.
-message HandshakeRequest {
+// Noise handshake message containing fields for all handshake patterns.
+// <http://www.noiseprotocol.org/noise.html#handshake-patterns>
+message NoiseHandshakeMessage {
   // Noise Protocol ephemeral public key 'e'.
   // <http://www.noiseprotocol.org/noise.html#overview-of-handshake-state-machine>
   bytes ephemeral_public_key = 1;
+  // Noise Protocol static public key 's'.
+  // <http://www.noiseprotocol.org/noise.html#overview-of-handshake-state-machine>
+  //
+  // Note: For some Noise patterns (such as XX and IX) static public key may be
+  // encrypted with the chaining key to hide peer's identity.
+  // <http://www.noiseprotocol.org/noise.html#handshake-patterns>
+  bytes static_public_key = 2;
   // Payload encrypted with the current chaining key.
-  bytes ciphertext = 2;
+  bytes ciphertext = 3;
+}
+
+// Message to be signed as part of the attestation binding.
+message AttestationBindingMessage {
+  bytes handshake_hash = 1;
+  bytes endorsements_hash = 2;
+  bytes peer_reference_values_hash = 3;
+}
+
+// Message that binds the Noise session (and optionally Attestation Endorsement
+// and peer Reference Values) to the Attestation Evidence.
+message AttestationBinding {
+  // Signature of the serialized `AttestationBindingMessage` Protobuf message.
+  bytes signature = 1;
+}
+
+// Request message for the crypto handshake request needed to establish a set of
+// session keys.
+message HandshakeRequest {
+  oneof handshake_type {
+    NoiseHandshakeMessage noise_handshake_message = 1;
+  }
+  AttestationBinding attestation_binding = 2;
 }
 
 // Response message for the crypto handshake request needed to establish a set
 // of session keys.
 message HandshakeResponse {
-  // Noise Protocol ephemeral public key 'e'.
-  // <http://www.noiseprotocol.org/noise.html#overview-of-handshake-state-machine>
-  bytes ephemeral_public_key = 1;
-  // Payload encrypted with the current chaining key.
-  bytes ciphertext = 2;
+  oneof handshake_type {
+    NoiseHandshakeMessage noise_handshake_message = 1;
+  }
+  AttestationBinding attestation_binding = 2;
 }
 
 // Request message for the Oak protocol attested secure session.