Proactive Key Caching (Opportunistic Key Caching); Certificate-Based Authentication - ASCOM VoWiFi System System Description

System Description
Ascom VoWiFi System
handover). The amount of time needed for the key exchange is implementation dependant
and differs between AP manufacturers.
Like WPA, WPA2 has a Pre-Shared Key mode (PSK), WPA2 Personal, designed for SOHO
(small office/home office) wireless networks that cannot afford the cost and complexity of
an 802.1X authentication server.
Key Caching is another feature that was added to WPA2. This allows a mobile device to
"cache" the master keys it gains through a successful authentication with an AP, and re-use
it in a future association with the same AP. This fast-roaming technique is called pre-
authentication and it offers a way for the mobile device to pre-authenticate with an AP
before it needs to handover, establishing a Pairwise Master Key (PMK) that can be cached
for a period of time, usually some hours. Now, when the mobile device wishes to handover, it
can bypass the whole 802.1X re-authentication sequence by presenting its cached PMK. The
authenticator will recognize the PMK and move straight to the next step, deriving session
keys.
2.3.6

Proactive Key Caching (Opportunistic Key Caching)

Even with key caching (described in section
authenticate with each AP it wishes to get service from. Even though this event only needs
to occur once for each AP (for as long as the keys are considered valid and are cached on
both the mobile device and the AP), it introduces significant latency and overheads. To
resolve this issue, the so called Proactive Key Caching (PKC) (also called Opportunistic Key
Caching) was designed. PKC allows a station to re-use a PMK it had previously gained
through a successful authentication process with the first AP, eliminating the need for the
station to authenticate against new APs when roaming.
When a mobile device moves from one AP to another AP, the client re-computes a PMKID
using the previously used PMK and presents it during the association process. The Controller
will search its PMK cache to determine if it has such an entry. If it does, it will by-pass the
802.1X authentication process and immediately initiate the WPA2 key exchange. If it does
not, it will go through the standard 802.1X authentication process.
Proactive Key Caching is vendor-specific, but work is underway to develop a standard.
2.3.7

Certificate-Based Authentication

The authentication security can be improved using certificates. A certificate is issued by a
certification authority to validate the identity of the owner of the certificate. Any party
wishing to validate the certificate uses a root certificate that identifies the certification
authority.
The following authentication protocols that are compliant with 802.1X and Extensible
Authentication Protocol (EAP) use certificate-based authentication:
• Protected EAP-Microsoft Challenge Handshake Authentication Protocol (PEAP-
MSCHAPv2): The network has a certificate, and the mobile device has a root certificate
that it uses to check that the network's certificate is valid, thereby ensuring that he
network is trusted.
• EAP-Transport Layer Security (EAP-TLS): The network and the mobile device has a
certificate each, and they both have the common root certificate. The mobile device uses
the root certificate to check that the network's certificate is valid, thereby ensuring that
the network is trusted. If so, the network uses its root certificate to check that the mobile
device's certificate is valid, thereby ensuring that it is authorized to access the network.
EAP-TLS is more secure than PEAP-MSCHAPv2, but requires that all mobile devices have
unique certificates.
7 September 2011 / Ver. G
2.3.5 WPA2™), a mobile device must
TD 92313EN
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