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## PowerPoint Slideshow about 'Naming Cached PMKs' - eugene

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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

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

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

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

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

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

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

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

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

Advantages of Naming Cached PMKs for Fast Handoff

- Can be used with PSKs too!

Dan Harkins, Trapeze Networks

Discussion

Dan Harkins, Trapeze Networks

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