what is its purpose?

Most Kerberos implementations use an authenticator. What is an authenticator andwhat is its purpose?

A.
Principal identification and a time stamp encrypted with a shared secret key. Itis used to authenticate the requesting principal and is a countermeasure againstreplay attacks.

B.
Principal identification and a time stamp encrypted with a shared session key.It is used to authenticate the requesting principal and is a countermeasure againstdictionary attacks.

C.
TGS identification and a time stamp encrypted with a shared session key. It isused to authenticate the requesting principal and is a countermeasure against replayattacks.

D.
Principal identification and a time stamp encrypted with a shared session key.It is used to authenticate the requesting principal and is a countermeasure against replay attacks.

Explanation:
If a Kerberos implementation is configured to use an authenticator, the user willsend the network resource her identification information and a time stamp encryptedwith the session key they share. The resource will decrypt this information andcompare it with the identification data the KDC sent to it about this requestinguser. If the data is the same, the resource allows the user to communicate with it.The time stamp is used to help fight against replay attacks. The resource willcompare the sent time stamp with its own internal time. This will help determine ifthe ticket had been sniffed and copied by an attacker and submitted at a later timein hope of impersonating the legitimate user and gain unauthorized access.