I am having some difficulties in understanding why is the
In case an attacker attempts an attack like a replay attack, it will show immediately that the attacker wasn't able to derive the correct set of keys.
However, this would be evident even without this message as the attacker wouldn't be able to encrypt and decrypt messages.
Is that the reason for this message, or is there another reason? What can we learn from this message that we wouldn't have learned without it?