The FireWall KNock OPerator implements an authorization scheme called
 Single Packet Authorization (SPA), based on Netfilter and libpcap.
 .
 Its main application is to protect services such as OpenSSH with
 an additional layer of security in order to make the exploitation of
 vulnerabilities (both 0-day and unpatched code) much more difficult.
 .
 This is the client program responsible for accepting password input
 from the user, constructing SPA packets that conform to the fwknop
 packet format, and encrypting packet data.