Added protocol idea, wip.

protocol.md

+# The Authentication and Message Exchange Protocol
+
+[A]lice wants to communicate with [B]ob privately.
+
+## Protocol Steps
+
+1. Authenticate to the server and establish communication.
+   1. A tells the server to open a communication.
+      1. A calculates the challenge.
+      2. A sends the challenge to the server.
+   2. B tells the server to answer A's communication request.
+      1. B receives A's challenge.
+      2. B answers the solution to the server.
+      3. The server checks the solution and accepts the communication establishment.
+2. Generate and exchange an ephemeral key.
+3. Instantiate symmetric encryption using the ephemeral key.
+4. Authenticate each other directly.
+5. Check communication transcript (avoid malicious server).
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