Answer (I think)

This is hand-wavey because how all this plugs together is hard to follow (for me) because documentation is missing, and the stack has a lot of layers :

1. ..... the write method for a message is in ssb-db/minimal.js (I've just heard this is true)
2. ssb-db/minimal.js#L179-L205 the append metod uses a queue
3. ssb-db/minimal.js#L117-L127 this is defined here, and takes an hmac_key
4. ssb-db/minimal.js#L74 which is var hmacKey = opts && opts.caps && opts.caps.sign

which in my case is null