mkm: add README with build instructions and protocol description

components/mission_key_management/README.md

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+# Mission Key Management Server
+
+This server processes key requests and distributes keys to other components.
+Any component can connect to the MKM, request a key, and attest to the code
+that it's running; the MKM will then send the key if allowed by the MKM's
+built-in policy.
+
+
+## Building
+
+To build the MKM server:
+
+```sh
+make
+```
+
+Or, to build an ARM binary for use inside the OpenSUT VMs:
+
+```sh
+make TARGET=aarch64
+```
+
+
+## Protocol
+
+The protocol that components use to communicate with the MKM works as follows:
+
+1. The client connects to the MKM over TCP.
+2. The client component sends a key ID (1 byte), indicating which key it is
+   requesting.
+3. The MKM sends a random challenge (32 bytes).
+4. The client obtains an attestation matching the challenge (by communicating
+   with its trusted boot daemon) and sends the attestation (32 bytes).
+5. If the attestation is valid and MKM policy allows the component to receive
+   the requested key, the MKM sends the key (32 bytes).
+
+If an error occurs, such as an invalid attestation or a policy violation, the
+MKM simply closes the connection without sending the key.
+
+Since all messages have a fixed size and occur in a fixed order, the protocol
+does not use any headers or delimiters for messages.
+
+The MKM server listens on localhost (127.0.0.1) port 6000 by default.  To
+change this, set the `MKM_BIND_ADDR` and/or `MKM_PORT` environment variables.
+For example, `MKM_BIND_ADDR=0.0.0.0 MKM_PORT=6001 ./mkm` will cause it to
+listen on port 6001 on all network interfaces.