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Kerberos

a single, very secure authentication server AS and ticket-granting server TGS
the server shares a different key K_H with each host H
A ==> AS: A
AS ==> A: K_A(K_S, K_TGS(A, K_S))
A ==> TGS: K_TGS(A, K_S)),B,K_S(t)
TGS ==> A: K_S(B, K_AB),K_B(A, K_AB)
A ==> B: K_B(B, K_AB),K_AB(t)
B ==> A: K_AB(t+1)