May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 1

doc.: IEEE 802.11-04/0476r2

Submission

Pre-Keying

Jesse Walker and Emily Qi

Intel Corporation

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 2

doc.: IEEE 802.11-04/0476r2

Submission

Agenda• Problem Statement

• Design Goals

• Pre-Keying

• Usage

• Open Issues

• Q&A

• Straw Poll

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 3

doc.: IEEE 802.11-04/0476r2

Submission

Problem Statement• 802.11r seeks to optimize STA transition time from

one AP to another– VoIP requires << 50 msec 802.11 transition times,

including 802.11 security setup– 802.11i perceived as too expensive by many market

segments• 802.11k messages require protection prior to STA

transitioning from one AP to another– 802.11k measurement frames may be used before

association– 802.11i keys not available until after association

• Protection for Reassociation frames is desirable, too

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 4

doc.: IEEE 802.11-04/0476r2

Submission

Design Goals• Make 802.11i keys available before association

– “Make before break” architecture

• Reuse 802.11i framework to make keys available– Do not redesign 802.11i infrastructure

– Minimize amount of new invention

• Use an already proven, well-reviewed scheme

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 5

doc.: IEEE 802.11-04/0476r2

Submission

Pre-keying Overview• Reuse the 802.11i Pre-authentication mechanism for keying

– 802.11i 4-Way Handshake messages are encoded in 802.1X frames– Pre-authentication mechanism can be used to forward 802.1X frames

between a STA and a new AP via an AP already associated with the STA• Introduce two new 802.11i messages:

– Pre-Keying Request, sent from STA to targeted AP to request pre-keying• Identifies STA MAC Address, PMKID of PMK to use

– Pre-Keying Reject, send from targeted AP to STA if request cannot be honored

– AP may respond to Pre-Keying Request by initiating a 4-Way Handshake over the pre-authentication channel

• Introduce PTK caching– 4-Way Handshake via the Pre-authentication channel populates the PTKSA

cache– Inactive PTKSAs are timed out

• Move security policy agreement from Association to 4-Way Handshake– Add PTKSA cache timeout value to RSN IE sent AP STA

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 6

doc.: IEEE 802.11-04/0476r2

Submission

Ingredients: Pre-Authentication Channel

STA AP 1

AP 2

802.lX over 802.11

802.lX over DS

•All frames use 802.11 Pre-authentication Ethertype (0F-AC) instead of 802.1X Ethertype (88-8E)

•All frames are 802.1X frames

•STA AP 2 Frames have Src Addr = STA’s MAC address, Dest Addr = AP 2’s BSSID

•AP 2 STA have Src Addr = AP 2’s BSSID, Dest Addr = STA’s MAC address

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 7

doc.: IEEE 802.11-04/0476r2

Submission

Ingredients: PMK Caching

STA AP

STA PMK Cache

AP’s BSSID, PMKID, PMK

AP2’s BSSID, PMKID, PMK

STA’s MAC Addr, PMKID, PMK

AP PMK Cache

STA2’s MAC Addr, PMKID, PMK

If a STA and AP share a cached PMK, they needn’t reauthenticate

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 8

doc.: IEEE 802.11-04/0476r2

Submission

Ingredients: 4-Way Handshake

EAPOL-Key(ANonce)

Pick Random ANonce

EAPOL-Key(Unicast, SNonce, MIC, STA RSN IE)

EAPOL-Key(ANonce, MIC, AP RSN IE, GTK)

Pick Random SNonce, Derive PTK = EAPOL-PRF(PMK, ANonce | SNonce | AP MAC Addr | STA MAC Addr)

Derive PTK

EAPOL-Key(MIC)

Install TK, GTK Install TK, GTK

APSTA

PMKPMK

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 9

doc.: IEEE 802.11-04/0476r2

Submission

Some Observations• 802.11i 4-Way Handshake messages are encoded

as 802.1X messages– So could be forwarded over pre-authentication channel

by simply changing the Ethertype– 802.11i does not define how to send 4-Way Handshake

messages over the Pre-authentication– But it does not preclude this, either

• 802.11i implicitly ties PTK to association– But not explicitly: still works for IBSS case

• 802.11i 4-Way Handshake is self-protecting– Security unaffected by the message path

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 10

doc.: IEEE 802.11-04/0476r2

Submission

What’s Missing from 802.11i?• Minor change to Key Management state machines required to support pre-

keying– 4-Way Handshake, Group Key Handshake messages may be encapsulated using the

Pre-authentication Ethertype– Change state machines to track whether keying messages exchanged over normal or

over pre-authentication channel• STA needs a Request message to kick-start AP

– Must identify the STA and the PMK used• STA needs feedback if AP does not have the required PMK

– This can’t be secured so is only a hint• PTK rules need slight tinkering to permit pre-keying without association

– APs should not cache PTKs forever– PTKs can’t be used across associations

• RSN IE changes– STA needs feedback Re: PTK timeout– STA and AP have to negotiate security policy in 4-Way Handshake instead of

Reassociate– Need to advertise support for pre-keying

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 11

doc.: IEEE 802.11-04/0476r2

Submission

Usage• On first contact, STA uses existing 802.11i

– Discovery Open System Authentication Association 802.1X authentication 4-Way Handshake Data exchange

• After 4-Way Handshake completes STA may use pre-keying if desired to optimize AP-to-AP transition– Discovery Pre-key Reassociate Data exchange

• If desired, STA may use pre-keyed TK to protect other management messages prior to association– 802.11k Protected Action Frames

• If keys are in place prior to AP-to-AP transition, then they can be used to protect Reassociation– Protection of Disassociation, Deauthentication becomes meaningful

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 12

doc.: IEEE 802.11-04/0476r2

Submission

Open Issues• Identify modifications needed to 802.1X state machines to support pre-

keying?• Prevent same PTK being used across two associations

– PTK reuse across association breaks replay protection mechanism

• What if STA transitions to the new AP before pre-keying completes?• What if STA transitions to a different AP before pre-keying completes?• How to handle GTK updates?

– The AP can send GTK updates over the pre-authentication channel if the STA is not associated

– But what to do is STA moves? Security associations are stateful

• What to with pre-key request from an “already associated” STA?• Other information that can be transferred over the pre-authentication

channel?

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 13

doc.: IEEE 802.11-04/0476r2

Submission

Q&A• Where do the cached PMKs come from?

– Out of Scope. These can be provisioned by, e.g., pre-authentication, some IETF/IRTF “standard” back-end protocol, e.g. proactive keying, or by a proprietary key provisioning scheme, e.g., Cisco’s

• What about subnet boundary crossing?– Out of Scope. Since it is based on the pre-authentication channel, it is a

LAN-only solution.

• Why not use some other channel?– We know of no other candidates.

• Why reuse the 4-Way Handshake?– We don’t want to invent a new protocol. Getting a key establishment

scheme right is hard.

May 2004

Jesse Walker and Emily Qi, Intel CorporationSlide 14

doc.: IEEE 802.11-04/0476r2

Submission

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