The following figures show two scenarios. In the first, Alice has already obtained a TGT$,$ and uses this TGT to ask KDC for a key to talk to Bob. In the second, Alice obtains KAB and the ticket to Bob, and talks to Bob by sending him the ticket obtained from KDC$.$ Explain the reason why the $$authenticator$$ does not have security value in the first figure, and why the $$authenticator$$ has security value in the second figure $($when Alice talks to Bob$).$

Remember that the purpose of the authenticator, as its name indicates, is used for the other party, i$.$e$.,$ KDC in the first figure and Bob in the second figure, to authenticate Alice.

When answering this question, you want to explain: why the authenticator is not necessary in the first figure; and what can happen if the authenticator is not used in the second figure.

Note that we only discuss authentication in this question. When the authentication is done, Alice$-$KDC and Alice$-$Bob may or may not encrypt the following messages. Do not assume the keys shared between Alice$-$KDC and Alice$-$Bob will be used to encrypt any message not displayed in these figures. Only discuss the significance of the authenticator based on what is shown in these figures.