To chat privately using Open Whisper Systems Signal app:
Send a Signal message to +61431926131, containing just the word "Hello". This serves to initialise the Signal conversation's safety numbers, without revealing much to a MITM.
Sign a copy of the conversation's safety numbers using a key associated with your Keybase account. Optionally, if you don't want to reveal your Keybase identity to a MITM, you may encrypt the signed message.
To do this, you may use the following Keybase command line, pasting the safety numbers on standard input:
keybase sign -b | keybase encrypt --no-self --anonymous mbrcknl
Send the result to me using Signal, and wait for me to do the same in return.
Decrypt my return message, verify its signature, and check that the result matches the safety numbers for the Signal conversation.
To do this, you may use the following Keybase command line, pasting the contents of my message on standard input:
keybase decrypt | keybase verify
You should see both:
If all goes well, carry on chatting. Otherwise, delete the Signal conversation and start again.