The four-way handshake in 802.11i.
<a href="//commons.wikimedia.org/wiki/User:Mikm" title="User:Mikm">mikm</a>
2007-01-04
Own work based on: <a href="//commons.wikimedia.org/wiki/File:4-way-handshake_WPA2.png" title="File:4-way-handshake WPA2.png">4-way-handshake WPA2.png</a>