You can use *Pidgin* to connect to [[!wikipedia Internet_Relay_Chat desc="IRC"]] or [[!wikipedia XMPP]] (also known as Jabber) servers. You can have several accounts connected at the same time.
The messages you send do not have digital signatures that are checkable by a third party. Anyone can forge messages after a conversation to make them look like they came from you. However, during a conversation, your correspondent is assured the messages are authentic and unmodified.