A Private Key System KERBEROS.

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Presentation transcript:

A Private Key System KERBEROS

Kerberos: Structure Ticket User (U) Kerberos Granting Server (K) Client (C) User (U) Kerberos Server (K) Server Ticket Granting Server (tgs) user secret key database server secret key database Requirements: each user has a private password known only to the user a user’s secret key can be computed by a one-way function from the user’s password the Kerberos server knows the secret key of each user and the tgs each server has a secret key know by itself and tgs CS 5204 – Fall, 2008

Kerberos: Steps authentication authorization Client (C) User (U) Server (K) Server Ticket Granting Server (tgs) user secret key database server secret key database CS 5204 – Fall, 2008

Protocol Overview 2. Tu,tgs User (U) 3. (Tu,tgs, S) Ticket Kerberos Server (K) Ticket Granting Server (tgs) 2. Tu,tgs User (U) 3. (Tu,tgs, S) Client (C) 1. U: user id 4. TC,S 5. (TC,S, request) ( 6. T' ) Server Ticket Structure: EK(S) {C, S, KC,S , timestamp, lifetime} CS 5204 – Fall, 2008

Kerberos Phase 1 1. The user logs on to the client and the client asks for credentials for the user from Kerberos U > C : U (user id) C > K: (U, tgs) 2. Kerberos constructs a ticket for U and tgs and a credential for the user and returns them to the client Tu,tgs = EK(tgs) { U, tgs, KU,tgs , ts, lt} K > C: EK(U) {TU,tgs , KU,tgs , ts, lt} The client obtains the user's password, P, and computes: K'(U) = f(P) The user is authenticated to the client if and only if K'(U) decrypts the credential. CS 5204 – Fall, 2008

Kerberos Phase 2 3. The client constructs an “authenticator” for user U and requests from TGS a ticket for server, S: AU = E K(U,tgs) {C, ts } C > TGS : (S, TU,tgs , AU ) 4. The server authenticates the request as coming from C and constructs a ticket with which C may use S: TC,S = EK(S) { C, S, KC,S , ts, lt} TGS > C: EK(U,tgs) {TC,S , KC,S , ts, lt } CS 5204 – Fall, 2008

Kerberos Phase 3 5. The client builds an authenticator and send it together with the ticket for the server to S: AC = EK(C,S) { C, ts } C > S : (TC,S , AC ) 6. The server (optionally) authenticates itself to the client by replying: S > C: E K(C,S) {ts + 1 } CS 5204 – Fall, 2008