A synchronous password token has to operate at the same pace as the receiver. That is, the token has to advance to the next random number at the same time the receiver advances. Because of clock imprecision, the two units will not always be perfectly together; for example, the token’s clock might run 1 second per day slower than the receiver’s. Over time, the accumulated difference can be significant. Suggest a means by which the receiver can detect and compensate for clock drift on the part of the token.
What will be an ideal response?
The algorithm for SecurID is a good example of maintaining synchronization. Essentially, if the token presents a value of X, the receiver compares X to the current expected value, as well as several earlier and several later values. If any of these matches, the receiver resets its expected value to the matched point. The receiver also checks when the token’s andreceiver’s times were last synchronized. If the times have not been synchronized recently (that is, if the token has not recently been used for authentication), the receiver will allow greater possible drift between the token’s and receiver’s clocks.
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