Naming Cached PMKs

1. Naming Cached PMKs Dan Harkins Trapeze Networks Dan Harkins, Trapeze Networks

2. Current PMK Caching • Supplicant sets a “cached PMK” bit in the RSN Capabilities bitfield in the RSN IE in the associate request. • Authenticator sends associate response and • Begins 4-way handshake if it has a cached PMK for the supplicant • Begins 802.1X authentication if it does not have a cached PMK for the supplicant Dan Harkins, Trapeze Networks

3. Current PMK Caching • Relies on no ambiguity on which PMK to use in the 4-way handshake • Problematic to use for fast handoff • Proactive (push) techniques can provide an AP with a PMK for the supplicant prior to the associate request being received • Reactive (pull) techniques can allow the AP to retrieve a PMK for the supplicant • This can introduce ambiguity! Dan Harkins, Trapeze Networks

4. Name Cached PMKs! • pmkname = HMAC-SHA1-128(PMK, “Key Identifier” | AP-mac | STA-mac) • If supplicant sets “cached PMK” bit in associate request, a list of pmknames, and the number of pmknames, is appended to the request. • If authenticator has one of the named PMKs in the list it appends the pmkname to the first message of the 4-way handshake. Dan Harkins, Trapeze Networks

5. Name Cached PMKs! I have cached PMKs: fjkdkleifjcjd8w2 984oeruwonwru dbnier7owfurn7w 8qo8awq8t348h4 dbnier7owfurn7w Semantics: use PMK named by “dbnier7owfurn7w” in the 4-way handshake Dan Harkins, Trapeze Networks

6. Advantages of Naming Cached PMKs for fast handoff • No new key hierarchies • No new service primitives • No new PRFs • No new key exchanges • No new management frames • Minimal, simple, change to existing mechanisms– add a list, append a blob Dan Harkins, Trapeze Networks

7. Advantages of Naming Cached PMKs for Fast Handoff • Can work with any scheme for distributing PMKs • IAPP • Neighbor graphs • It doesn’t matter how the PMK got there, just that it got there. • Protocol does not assume existence of PMKs. Either side can delete a PMK from its cache for any reason and at any time. Dan Harkins, Trapeze Networks

8. A B A E C E C B D D Advantages of Naming Cached PMKs for Fast Handoff • STA authenticates to A, hibernates and wakes up at D where it authenticates again. PMKs were delivered by AS to B and E for first authentication and different PMKs were delivered to B and E for the second. The STA will assert both when it moves to B. Dan Harkins, Trapeze Networks

9. A B A E C E C B D D Advantages of Naming Cached PMKs for Fast Handoff • B will select one and initiate the 4-way handshake. If the STA moves to C it will again assert two named PMKs. Depending on the neighbor graph C may have one– in which case C will chose it– or none– in which case C will begin 802.1X authentication of the STA. Dan Harkins, Trapeze Networks

10. Advantages of Naming Cached PMKs for Fast Handoff • Can work with any scheme for deriving AP-specific PMKs. • Is independent of whatever key hierarchy may be defined. • It doesn’t matter how the key was derived as long as the STA and AS are using the same technique. AP is out-of-the-loop and therefore the protocol does not care. Dan Harkins, Trapeze Networks

11. Advantages of Naming Cached PMKs for Fast Handoff • Can be used with PSKs too! Dan Harkins, Trapeze Networks

12. Discussion Dan Harkins, Trapeze Networks

13. Motion! • Insert changes described in 03/484-r1 to draft. Dan Harkins, Trapeze Networks