Sunday, February 2, 2014

STA-AP: WPS STA PIN Procedure

WPS (Wi-Fi Protected Setup) is a WFA Certification program that is intended to allow the users to easily connect to a secure home or small office Wi-Fi network. It is also used for establishing the connection between two Wi-Fi Direct devices.

Connection establishment between a STA and AP using WPS can be done in of the the following ways:

  • Push Button Configuration (PBC): By pressing a physical/virtual button on the STA and AP
  • PIN
    • Enter STA PIN in the AP
    • Enter AP PIN in the STA
      • A good description of this method and its vulnerabilities can be found here
  • NFC (Near Field Communication)
    • This is "out of band" method (PBC and PIN are "in band"). 
    • An example of this is the "S-Beam"

This article is about what happens between an STA and AP in the "STA PIN in the AP" method.


Basic steps in WPS

Connection establishment to a WPA2 network using "In Band" WPS procedure consists of the following steps:

  • Authentication and Association (Round 1)
  • WPS EAP Procedure
    • EAP Start, Identity request/response
    • WPS registration procedure(M1 to M8)
    • EAP Done (Ends with an EAP Failure as this is not real Network authentication) 
  • Deauthentication
  • Authenticaton and Association (Round 2)
  • EAPOL 4-way handshake (WPA2 Keys)

WPS PIN procedure may fail in the following cases:
  • STA(Enrollee) initiates the procedure before AP (registrar) has the PIN
    • This is not a "total" failure. STA understands that it has to wait and retry.
  • User enters an incorrect PIN in the AP


WPS STA PIN : Sequence Diagram

The below sequence diagram is based on tests run between a mobile phone (STA) and an AP. The sniffer logs for the tests can be found here.


1 comment:

  1. Thoughtful explanation.I Strucked why AP sends deauth at first time when STA tries to associate.After see ur blog I got struck free.Thanks..

    ReplyDelete