Rather than start geth with all the necessary flags and have blockchain syncing going on, you may wish to just run whisper functionality for private messaging in a lightweight way. You should always run your own node using current Whisper v6 or below, as the node administrator is able to sniff your keypairs as they are requested.
To do that, you can use Status's excellent docker image, which sadly has had its documentation taken down from here: https://status.im/docs/run_status_node.html
No worries, you can follow this gist instead.
- Create the
http-enabled.json
below (future improvement: add HTTPS support with certbot) - Run the docker invocation below
- Test it with a curl request
curl -X POST 'http://localhost:8545' -H 'Content-Type: application/json' --data '{"jsonrpc":"2.0","method":"shh_version","params":[],"id":1}'