07_kerberos - Kerberos Peter Sjdin psj@kth.se Based on...

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1 Kerberos Peter Sjödin psj@kth.se Based on material by Vitaly Shmatikov, Univ. of Texas, and by the previous course teachers
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2 Kerberos Many-to-many authentication? Kerberos
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3 Many-to-Many Authentication How can we authenticate users when they connect to machines on the network? Separate logins for each machine Distribute login information from one machine to every other machine Insecure, management burden Users Servers & Workstations ?
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4 Many-to-Many: Requirements Scalable – Must cope with large numbers of users and systems Secure – Must withstand both passive and active attacks Transparent to users – Should not require entering password all the time – Should not be noticed by, or hamper, users
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5 Many-to-Many: Threats User impersonation – Malicious user with access to a workstation pretends to be another user from the same workstation • Can’t trust workstations to verify users’ identities Network address impersonation – Malicious user changes network address of his workstation to impersonate another workstation Eavesdropping, tampering and replay – Malicious user eavesdrops on, tampers with or replays other users’ conversations to gain unauthorized access
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6 Many-to-Many: Trusted Third Party Users Servers & Workstations Authentication server Use ticket to get access Ticket User requests access, providing authentication Trusted authentication server : Manages all passwords; grants access to systems Key Distribution Center, essentially Single point of failure. Require strong physical security
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7 What is a ticket? Ticket is a proof of authentication and authorization . – Should have limited reusability • Should not contain server's password in plain text • Encrypt information in ticket – opaque to user • Server decrypts ticket when information is needed Ticket: gives user access to server User Server
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Ticket content User name Server name User's current network address Ticket lifetime Session key All of which are essential for functionality and security properties! 8 User Server Encrypted ticket Encrypted ticket Creates ticket using server's key User authentication
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9 Authenticating As we've seen – need to protect authentication Repeated authentication! – Once for every (non-local) service we need access to – Inconvenient and increases risk to permenent secret • User typing it or workstation remembering for user – Should only need to authenticate once! “I am Alice” “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I am Alice” R K (R) A-B
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10 Solution: Two-step authentication Authenticate once and receive a special Ticket Granting Ticket (TGT) – System uses TGT to re-authenticate user – Like a login cookie on a web site. User
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07_kerberos - Kerberos Peter Sjdin psj@kth.se Based on...

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