On 12/3/12 6:34 PM, Hal Murray wrote:
li...@lazygranch.com said:
I have one of those key fobs. Does the code somehow inform the power the be
about the drift in the built in clock? Or is the time element of the code so
sloppy that the drift is acceptable?
The magic number changes every second or so.
Every 30 seconds or every minute.. I've seen both. My fob is once a
minute, the iPhone "soft fob" is 30 seconds.
You only have to scan a few
seconds either side of the correct time to find a valid match. Every time
the server gets a match it can update its memory of the fob time to reduce
its searching in the future.
Exactly, the maximum time difference is a settable parameter.
You could measure/compute the drift too. I don't know if that's worth the
effort. It would probably change with temperature so seasonal or lifestyle
changes could throw the prediction way off.
I don't think they do that.. I think it's a "reset when validated"...
[I have no inside knowledge. I could be totally wrong, but that seems
reasonable to me. They may have a better approach.]
It's all described on the RSA website..
Hmm.. I suspect I could time my fob once a day, and see how many
seconds a day it drifts.. without a timed camera it would be hard to get
tighter than 1 second resolution..
the iPhone one almost certainly uses the internal clock in the phone.
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