Excerpts from Alfredo Palhares's message of 2015-04-01 15:38:52 +0100:
> Hello fellow sup users,
> 
> How do I configure sup to sign all emails by default?
> 
> Regards,
> 
> -- 
> Alfredo Palhares
> GPG/PGP Key Fingerprint
> 68FC B06A 6C22 8B9B F110
> 38D6 E8F7 4D1F 0763 CAAD
> 

Hey Alfredo,

There is no setting for this, however there is a crypto-mode hook that
is run when you start editing message. In order to answer your question
I just wrote this:

$ cat /path/to/hooks/crypto-mode.rb                                                                                  
crypto_selector.set_to(:sign) if crypto_selector.can_set_to?(:sign)

This automatically selects the :sign if it is possible.