| | Speaking of two-factor authentication, can anyone explain how servers
| | validate the code from a SecurID token in the presence of clockskew?
| | Does it look backwards and forwards in time a few minutes?
|
| Yes, it rolls forward and back 3-5 cycles. The server maintains a
| list of what time it thinks the token thinks it is. So its not
| testing what time it is, its testing what time the token thinks it is.
Plus, I believe it maintains and updates an estimate of the skew between its
clock and the token's. So the "what time I think the token thinks it is"
value is pretty accurate. Note that a value read off the token is good for a
minute, so your synchronization doesn't have to be that good to begin with. (I
suppose you could scan over a larger number of cycles to find the entered
value and then use the cycle distance *just to update the skew/token time
calculation*, refusing the value. A legitimate user will just try again with
the next generated number and succeed; but a guesser won't be helped by the
broader scan range. At most, he can screw up the skew/token time estimate
somewhat, but that will "heal" when the legimate user next logs in.)
I think there's some special-case synchronization the first time the token is
used, the details of which I can't recall. (It may be just that the server is
very liberal about the allowed skew if it knows it hasn't talked to a token in
a long time - which is how it would presumably look on the first attempt. I
suppose if you're really worried that this would allow an attacker to get in
before the legitimate user, you can make it a policy to log in with the token
a couple of times after setting up the account but before handing out the
token. Then again, the software could well have a mechanism to do exactly
this synchronization step, so that you don't actually have to log in or even
enable the account until synchronization is complete.)
-- Jerry
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