security in jxta

Hi all,
I was wondering how the security options work in jxta.

As far as I understand it, a secure unicast pipe is encrypted with tls, 
and the sender can reliably know whether the receiver got the message.

But I found no way to provide any certificates or something similar, and 
  since anyone can bind to a pipe endpoint (?) how does the sender even 
know that the correct receiver has got the message and not someone else, 
or the receiver that the message orginated from the real sender?

Likewise, the constructor for a messageElement accepts a signature as a 
paramter, but what format is that signature supposed to have, and how 
does it work?

I am sorry, these are probably a rather obvious question, but I am 
unable to find any information about that.

Is there any example code for a jxta communication using encrypted and 
signed messages?