Numeric Association
In the numeric association model, devices associate without using a wired cable, but require the user to confirm the association by verifying a code number manually. After locating a nearby wireless USB host, the device may show a short code number on its display, which the user will then verify with the host.
The specification also has provisions for devices that do not have input capabilities. In this case, the host simply displays the number and the user clicks "OK," if it matches. Alternately, hosts can require the user to enter a predefined code number supplied with the device, a method familiar to users of many existing wireless technologies today. Any of these methods establish a connection context similar to that obtained in the cable association model.
Security and encryption
Once association is complete using either of these two methods, the devices are now authorized to communicate using the connection context that is established during the association process. During each session, the devices will initially derive a "session" key (also called a "pair-wise temporal key" or PTK) which is based on the pre-established connection context.
Wireless traffic between devices is encrypted using the session key, and therefore any receiver, including any analysis tools, must have prior knowledge of the connection context in order to determine the session key and decrypt the wireless traffic. However if the connection key is known, the analysis tool can determine the session key by listening in to the wireless traffic as the devices begin to communicate.
Analyzing secure traffic
Protocol layer testing for wireless technologies generally involves a radio-based "sniffer" to eavesdrop on the exchange between devices. Analysis tools must perform this task with minimal impact on the real-world operating environment of the devices. The WiMedia specifications do not require encryption of the PHY and MAC header information. This allows devices (and analysis tools) to identify framing and routing information even if the packets are encrypted. But all certified wireless USB logical data transfers will be encrypted.
LeCroy's protocol analysis platforms are designed to address this security problem. For devices that use the cable association model, an integrated plug-in for capturing the cable association sequence is available.
It automatically records and passes the connection context to the radio-based analysis engine contained within the analyzer. The analyzer radio channel can then automatically follow the 4-way handshake and use the connection context to derive the "session" key (or PTK). With this unique session key in the hardware-recording engine, the analyzer can automatically decrypt the scrambled wireless traffic.
Support for the Numeric Association model is provided through a software interface that allows the user to manually enter the connection context for paired devices.
In both cable and numeric association, the pairing information is preserved in non-volatile memory. This allows the test tool to automatically recognize when these devices establish future connections and automatically derive a new PTK whenever a new 4-way handshake exchange takes place between the devices.
Important points:
- The session keys are regenerated every time the two devices power up. The analyzer must be powered on and record the 4 way handshake sequence at this stage to automatically derive the PTK and decrypt the scrambled traffic.
- In some cases, developers can use a diagnostic key (CK= 0) for debug environments. But most validation labs will require testing with commercial products using real CK values.
- In the Numeric model, the user must enter the Connection Host ID and the Connection Key in the analyzer software for a given device. The CK is generally a known value that is pre-programmed in firmware (but is never transmitted over the air).
- If a host-device pair have already established a connection context using a cable association model prior to the introduction of the analyzer, it will be necessary to repeat the cable association while monitoring the USB traffic between the host and the device in order for the analyzer to capture the connection context.
Testing of both the association process and the security process will be required to gain certification of wireless USB devices. This will be enforced through the USB-IF certification program, which will allow companies to test device and host compliance to all Certified Wireless USB specifications.
About the author
Mike Micheletti is the Wireless USB Product Manager at LeCroy's Protocol Solutions Group.
