Which of the following are the best two countermeasures to use against a replay attack?

A.
Private keys and MAC

B.
Timestamps and sequence numbers

C.
Digital certificates and digital signatures

D.
Authentication and integrity

Explanation:
Timestamps and sequence numbers are two countermeasures to the replay
vulnerability. Packets can contain sequence numbers so each machine will be
expecting a specific number on each receiving packet. If a packet has a sequence
number that had been previously used, this is an indication of a replay attack.
Packets can also be timestamped. A threshold can be set on each computer to only
accept packets within a certain time frame. If a packet is received that is past
this threshold, it can help identify a replay attack.