In 802.11i, a Pairwise Master Key (PMK) is the key that results from a successful authentication between a wireless station and an access point. The PMK is generally derived by the wireless station and the back-end EAP/AAA authentication server after a successful EAP authentication and sent to the wireless access point in a AAA message (In the context of EAP/AAA, the PMK is called Master Session Key 'MSK') secured using long-term security association between the authentications server and the access point.
The PMK is stored in the station and the access point with associated context information such as the access point's MAC addresses, the lifetime of the PMK and a unique identifier called PMKID. The collection of this information is called PMK Security Association (PMKSA). The PMKID is computed by applying a hash function (HMAC-SHA1-128) to the concatenation of the PMK, the label `PMK Name', the access point's MAC address (MAC_AP) and the station's MAC address (MAC_STA).
PMKID = HMAC-SHA1-128(PMK, "PMK Name" | MAC_AP | MAC_STA)
When associating with an access point, the station determines if it has a valid PMK with the target access point by checking if it has a PMKSA that matches the target access point's MAC address. If such PMK does not exist, the station and the access point perform authentication using EAP. If the station determines that it shares a PMK with the target AP, then the station proposes the use of the PMK by including the PMKID in the RSN Information Element of the (Re)Association Request message. Upon reciept of a (Re)Assiciation Request with a PMKID, the access point checks whether is has a valid PMKSA with the same PMKID. If so, it begins the four-way handshake exchange using the negotiated PMKSA.