Dr. Nermi hamza

 A user may gain access to a particular workstation and pretend to be another user operating from that workstation.  A user may eavesdrop on exchanges and use a reply attack to gain entrance to a server or to disrupt operations.  A user may alter the network address of a workstation so that the requests sent from the altered workstation appear to come from the impersonated workstation. 2

3 ◦ Computer Network Authentication Protocol ◦ Individuals communicating over an unsecured network can prove their identity to one another in a secure manner ◦ Prevents eavesdropping or replay attacks

 provide authentication between any pair of entities  primarily used to authenticate user-at- workstation to server  servers can build authorization and access control services on top of Kerberos

5  A secret key based service for providing authentication in open networks  Authentication mediated by a trusted 3rd party on the network: ◦ Key Distribution Center (KDC)

6  Developed in 1988 at MIT as part of Project Athena. ◦ Athena was started in 1983 with the goal to provide campus-wide access to networked computing services. ◦ Security problems (eavesdropping, forging identities, and so on) made it clear that a security solution was needed; Kerberos was the solution, based on Needham and Schroeder’s secret key protocol, with DES to handle encryption and decryption.  Since then, Kerberos has been used to provide secure access for various networked file systems and computing environments.  There are two commonly available versions of Kerberos, version 4 and version 5.

* AS = Authentication Server * TGS = Ticket Granting Server * SS = Service Server * TGT = Ticket Granting Ticket The client authenticates to AS using a long-term shared secret and receives a ticket from the AS. Later the client can use this ticket to get additional tickets for SS without resorting to using the shared secret. These tickets can be used to prove authentication to SS.

15.8 Key-Distribution Center: KDC Key-distribution center (KDC)

9 ASTGS Client Server KDC

10 ASTGS Client Server KDC TGT Authenticator SK (ticket) Server id.

11 ASTGS Client Server KDC Authenticator SK

12  Alice logs into a computer workstation  Workstation forwards the network ID to the Authentication Server (AS) unencrypted  AS sends a message which is encrypted with Alice’s key K(A) ◦ Contains a session key K(S) and a ticket for the TGS

13  Alice sends the following to the TGS: ◦ Ticket received from the AS ◦ Name of server she wishes to access (Bob) ◦ Timestamp which has been encrypted with K(S)  The TGS returns two tickets to Alice ◦ Each key has the session key K(AB) which allows Alice and Bob to communicate

14  Alice sends Bob’s ticket together with an time-stamp encrypted with K(AB) to Bob  Bob confirms receipt by adding 1 to the time-stamp, which is encrypted with K(AB) and sent to Alice  Alice and Bob can now freely conduct transaction using K(AB) as the symmetric shared key

15  Kerberos caches tickets and encryption keys (collectively called credentials)  Have a limited life  Allows a user to obtain tickets and encryption keys without requiring the re- entry of the user's password

16  Suppose a client C wants to communicate with a server S in a Kerberos version 5 realm. Step 1:  The first step is for the client to authenticate itself with the Kerberos Authentication Service and request a Ticket Granting Ticket.  This requests a ticket for client (C) for the Ticket Granting Service (TGS), with N 1 as a timestamp nonce. C AS C, K C-AS (TGS, N 1 )

17 Step 2:  When the Authentication Service receives the request, it decrypts the request and verifies the client’s identity. It then generates a session key for the client and Ticket Granting Service to use, as well as a ticket. It sends back: where TGT = (C, TGS, T 1, L 1, K C-TGS ) T 1, L 1 are the ticket’s timestamp and lifespan  Since this is encrypted with C’s secret key, only the client can make use of it, and only the Authentication Service could have sent it. C AS E[K C-AS, (K C-TGS, K AS-TGS (TGT),N 1 )]

20 Step 3:  With the client authenticated, it decrypts the response to get a session key for the Ticket Granting Service and a ticket- granting ticket.  When the client needs to contact the server S, it creates a fresh authenticator (AUTH) and requests a ticket from the Ticket Granting Service: where AUTH=(C, ADc, N 3 ) C TGS K C-TGS (AUTH), K AS-TGS (TGT), S, N 2

21 Step 4:  The Ticket Granting Service decrypts the ticket-granting ticket and obtains the session key within it.  The service decrypts the authenticator and compares client identifiers in the authenticator and ticket.  The service generates a new session key for the client and server, as well as a service ticket, and sends: where STK = (C, S, T 2, L 2, K C-S ) T 2, L 2 are the ticket’s timestamp and lifespan  Since this is encrypted with the session key, only the client can make use of it, and only the Ticket Granting Service could have sent it. C TGS K C-TGS (K C-S, K S-AS (STK),N 2 )

25 Step 5:  The client decrypts the response from the Ticket Granting Service to get the session key for the server and a service ticket to use with it.  When the client needs to contact the server S, it creates a fresh authenticator (AUTH) and sends this, along with the service ticket, and its request R: where AUTH=(C, N 5 ) C S K C-S (AUTH), K S-AS (STK), R, N 4

26 Step 6:  The server decrypts the service ticket and obtains the session key within it.  The server decrypts the authenticator and compares client identifiers in the authenticator and ticket.  The server executes request R and replies with answer A, and the nonce in the client’s request.  Since this is encrypted with the session key, only the client can make use of it, and only server could have sent it. The session key can be used for additional requests in this session, and then destroyed. C S K C-S (A, N 4 )

29 C ASTGSS C, K C-AS (TGS, N 1 ) K C-AS (K C-TGS, K AS-TGS (TGT),N 1 ) K C-TGS (AUTH), K AS-TGS (TGT), S, N 2 K C-TGS (K C-S, K S-AS (STK),N 2 ) K C-S (AUTH), K S-AS (STK), R, N 4 K C-S (A, N 4 )

30  Kerberos Encryption ◦ User's encryption key is derived from their password ◦ Uses the data encryption standard ◦ Symmetric cryptography  The Kerberos Ticket  Ticket Granting Server

31  A = user requesting the service  B = the service requested  TGS = Ticket Granting Server issues “proof of identity tickets”  AS = Authentication Server verifies users during login  S = session key  t = timestamp  K = key (encryption, decryption)

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 Authentication  Authorization  Confidentiality  Within networks and small sets of networks

15.34 The minor differences between version 4 and version 5 are briefly listed below: Kerberos Version 5 1)Version 5 has a longer ticket lifetime. 2)Version 5 allows tickets to be renewed. 3)Version 5 can accept any symmetric-key algorithm. 4)Version 5 uses a different protocol for describing data types. 5)Version 5 has more overhead than version 4.

15.35 Kerberos allows the global distribution of ASs and TGSs, with each system called a realm. A user may get a ticket for a local server or a remote server. The concept of realm can be : is set of managed nodes that share the same Kerberos database Realms

 The Kerberos protocol operates across organizational boundaries. If an organization wants to have local control of authentication of its users, it can run its own Kerberos server. All the users and applications that use a Kerberos server compose a realm. The name of the realm in which a client is registered is part of the client's name and can be used by the application server to decide whether to honor a request.