Many remote server terminal authentication protocols use random numbers sent by the server as a challenge that the user will encrypt and return to the server. In this way, an opponent can not initiate a replay attack because the number encrypted and sent to the server (with other settings) is random and unknown in advance.
But what happens if the opponent always saves the line and maps each random number he / she has ever encountered to the encrypted traffic that follows it?
If the opponent encounters a random number previously sent by the server, he will be able to replay the traffic.
So why not use the timestamp as a challenge? A timestamp always grows and will always be different from any previous timestamp?
Why is the use of random numbers preferable to timestamping?