At 5:55 AM +0900 2/10/2001, [EMAIL PROTECTED] wrote:
> >WF1
>>In WF1 the 802.11 WEP keys would be changed many times each hour, say
>>every 10 minutes. A parameter, P , determines how many time per hour
>>the key is to be changed, where P must divide 3600 evenly. The WEP
>>keys are  derived from a master key, M,  by taking the low order N
>>bits (N = 40, 104, whatever) of the SHA1 hash of the master key with
>>the date and time (UTC) of the key change appended.
>>      WEPkey = Bits[0-N](SHA1(M | yyyymmddhhmmss))
>>Clearly good synchronization of the time-of-day clock on each node is
>>essential in WF1,  but protocols already exist that can do this over
>>a network. Small synchronization discrepancies can be handled by the
>>802 retry mechanism and should look very much like a short RF outage.
>       i see chicken and egg loop here - for instance, if I've got a laptop
>       with 802.11 card only, I need to use the 802.11 network to synchronize
>       clock.  i'm not sure if WF1 is workable (if you have other secure
>       channel for synchronizing clock, you are okay - but then why bother
>       using 802.11?).
That is one of the reasons I suggested a key change interval of every 
10 minutes. Most PCs internal clocks will keep time to within a few 
seconds from day to day, so re-synchronization should not be a 
problem. If necessary the PC's time can be manually set well enough 
using any number of time sources:
    Most phone companies in the US have a number you can call.
    "News" radio stations announce the time frequently.
    Many cell phones have clocks.
    GPS receivers give accurate time.
    For about $60 you can buy a clock that synchronizes itself to WWVB.
    802.11 has a short range, so there are likely other PCs nearby 
that you can get the time from.

I don't know how tolerant actual 802.11 systems are to a delayed key 
change (experiments are welcome), but if a user sets their PC time in 
the middle of a ten minute interval, there will be no delay at all.

Actually there is a highly accurate time synchronism mechanism built 
into 802.11. The transceivers must be sync'd way before they get to 
worry about encryption.  But I don't know if that time value is 
accessible by the client computer.  If so, more frequent key changes 
should be workable.

Arnold Reinhold

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