1. Broadcast Management Frame Protection
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Broadcast Management Frame Protection
Date: March 06, 2006
Authors:
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S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

2. Main Idea Authentication of Broadcast Management Frames (BMF)
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Main Idea
Authentication of Broadcast Management Frames (BMF)
MIC for Broadcast Management Frames are sent in Maintenance Beacon
Key for MIC is disclosed in Broadcast Management Frames
Keys form hash chain and are assigned to the Beacon Intervals
Key should be sent in different interval than MIC (after)
Two intervals should be defined
Maintenance Beacon waiting period
Broadcast Frames waiting period
If MIC or Key is received in the wrong interval, then they should be ignored
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

3. Waiting Periods Maintenance Beacon is sent immediately after Beacon
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Waiting Periods
Maintenance Beacon is sent immediately after Beacon
Beacon could be delayed
Delay is caused by unfinished transmission started in previous Beacon Interval (medium is busy)
Maintenance Beacon waiting period
Start: nominal beacon transmission time
End: max delay + PIFS + max Beacon + SIFS + Maintenance Beacon
Max delay = RTS + CTS + MPDU of maximum length + ACK + 3*SIFS
Broadcast Management Frames waiting period
Start: end of Maintenance Beacon waiting period
End: end of Beacon interval
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

4. Authentication Details
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Authentication Details
BMi – beacon sent in i-th beacon interval
BMFij – j-th broadcast management frame sent in i-th beacon interval
Check that
Maintenance Beacon was received during appropriate Maintenance Beacon waiting period
Broadcast Management Frames were received during appropriate Broadcast Management Frames waiting period
Disclosed key belongs to the hash chain
Disclosed key is assigned to the current Beacon Interval
MIC is valid and was calculated using disclosed key
Otherwise all received BMFs are dropped
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

5. Month Year
doc.: IEEE yy/xxxxr0
January 2006
Hash Chain - I
Keys t
Kj-1 Kj K0
Kj-1=H(Kj)
K0 – public key
j j 1 Kn
IGTK is used as a seed to calculate a “seed” chain SKn such that
SKn = H(IGTK, n)
The SKs are used as pseudo random seeds to precompute the hash chain such that
Ki=H(Ki+1)
Kn = seed from the “seed” chain
AP discloses one key during each beacon interval
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

6. Hash Chain - II Trivial approaches for calculating current key
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Hash Chain - II
Trivial approaches for calculating current key
Store only Kn and calculate Ki=Hi(Kn) (high complexity)
Store the whole chain (high memory requirements)
Alternative Approach
Markus Jakobsson, Fractal Hash Sequence Representation and Traversal
Memory requirements: O(logn)
Complexity: O(logn) hash function evaluation per key
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

7. Month Year
doc.: IEEE yy/xxxxr0
January 2006
Time Synchronization
Synchronization is performed using timestamp received in Beacon
But Beacon is not protected
Attacker could
sent bogus Beacon, which desynchronizes AP and STAs
inject bogus BMFs using keys from valid Beacon
Protected synchronization is needed
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

8. Proposed Synchronization
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Proposed Synchronization
Add new timer, which is used for security purposes, i.e. for controlling of
waiting periods
assignment between keys and beacon interval
Add MIC of Beacon calculated using current key from hash chain into the Maintenance Beacon
Adjust security timer only when MIC of Beacon is verified
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

9. Disclosing of Previous Key
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Disclosing of Previous Key
We need to include previous key into the Maintenance Beacon
To synchronize security timer, when no BMFs are sent
To decrease complexity of verification of disclosed key, if BMFs were not received during long time
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

10. Maintenance Beacon Format
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Maintenance Beacon Format
Order
Information 1
Previous Key 2
Beacon MIC 3
BMFs Number 4
BMFs MIC 5
Management MIC IE
Beacon MIC and BMFs MIC are calculated on the current key from the hash chain
MIC from Management MIC IE is calculated on the IGTK
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company

11. Broadcast Management Frame
Month Year
doc.: IEEE yy/xxxxr0
January 2006
Broadcast Management Frame
Header
Management Frame Body
Management MIC IE
FCS
Element ID
Length
Key SN
Key ID
MIC
Octets 1
0 or 16 6 2 8
Management MIC IE
Key is the current key from the hash chain
MIC from Management MIC IE is calculated on the IGTK
S. Bezzateev, A. Fomin, M. Wong
John Doe, Some Company