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Re: replay attacks
> Using e.g. keyed MD5 or whatever one could place a time stamp with sufficent
> granularity into the 'key' part of the authenticated data. So the MAC would
> only be correct if the receiver gets it in the same time-frame.
A design decision to use timestamps would have some annoying consequences:
* the sender & receiver must synchronize their clocks
* all clock code (e.g. NTP) becomes security-critical
* attackers can still replay within the allowed time window
* the time window must be at least the MSL (~ 2 minutes)
References: