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Outline

This article discusses the differences between authentication and authorization in local networks compared to general networks, with a focus on Kerberos authentication and authorization. It also explores the security implications of LANs and WANs, and the role of Kerberos in providing authentication and access control in large local networks.

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Outline

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  1. Authentication and Authorization in Local NetworksAdvanced Network Security Peter ReiherAugust, 2014

  2. Outline • Authorization vs. authentication • Differences between local networks and the general case • Kerberos

  3. Authorization and Authentication • Authentication is determining who someone is • Authorization is determining if someone is allowed to do something • Authorization usually depends on good authentication • But authentication isn’t sufficient

  4. LANs and WANs • LANs – Local area networks • WANs – Wide area networks • What are the differences? • What are the security implications? • With regards to authentication and authorization

  5. Local Area Networks • Local area networks are under a single entity’s control • Such as a company or university • They have a limited geographical scope • A building, a campus, etc. • They usually have a closed user community

  6. Wide Area Networks The Internet is the biggest example Very large number of sites No single entity is in charge Almost unlimited geographical scope Broad, maybe totally unrestricted, membership

  7. Security Implications Smaller is easier than big One manager is easier than many Closed is easier than open Less geography covered is easier than more

  8. Security Implications for Authentication and Authorization • In LANs, you know everyone who uses it • Easier to prearrange authentication • You can configure everything to use a common mechanism • You can more readily specify who should be able to do what • The vital authorization issue • More confidence in physical security

  9. Kerberos • Provides authentication and authorization for large local networks • E.g., a university or corporation • Originally developed at MIT • Uses trusted third parties • And symmetric cryptography • Extensions use PK • Goal: authenticate parties and provide them access to approved services

  10. The Kerberos Model • Clients and servers sit on the (usually local) network • Clients want to interact securely with servers • Using a fresh key for each session • Servers want assurances that clients are authorized • Kerberos handles both concerns • Scalability is an issue

  11. Obtaining Keys and Services Through Kerberos • The client needs to get a key to give to the server and use himself • Key will prove authorization, too • Obtained from a ticket-granting server • Essentially, a server who hands out keys to talk to other servers • But the ticket-granting server needs authentication of the client • Which is obtained from the Kerberos server

  12. What’s the Point of the Ticket-Granting Server? • Scalability • Most requests for keys for servers go to ticket-granting server • There can be lots of them • And issues of trust • Different ticket-granting servers can work with different servers and clients • So not everyone needs to trust one ticket-granting server

  13. Players in the Kerberos Protocol • The client • The server • The Ticket-Granting Service - someone the server trusts to authenticate the clients • The Kerberos Server - someone everyone trusts

  14. Client Kerberos Ticket-Granting Server Server Kerberos Participants

  15. Client Kerberos Ticket-Granting Server Server Client Requests a Ticket-Granting Ticket From Kerberos I need to talk to the Ticket-Granting Server

  16. Client Kerberos Ticket-Granting Server Server Kerberos Sends the Client a Ticket-Granting Ticket

  17. Client Kerberos Ticket-Granting Server Server Client Asks TGS for a Server Ticket Ticket-Granting Server checks ticket validity

  18. Client Kerberos Ticket-Granting Server Server TGS Sends Ticket to Client

  19. Client Kerberos Ticket-Granting Server Server Client Requests Service Server checks ticket

  20. Tickets and Authenticators • A Kerberos ticket is used to pass information to a server securely • An authenticator is an additional credential passed along with the ticket • Used to pass timestamp information about lifetime of a key

  21. What’s In a Ticket • TC,S = s, {c,a,v,KC,S}KS • s is the server • c is the client • a is the client’s network address • v is a timestamp • KC,S is a session key • KS is the server’s key

  22. Client Kerberos Ticket-Granting Server Server Tracy Kerberos in More Detail: Step 1 Alice, Tracy Alice Sidney

  23. {KAlice,Tracy}KAlice, Alice Kerberos Tracy Sidney Kerberos Sends Client Ticket-Granting Ticket TAlice,Tracy = Tracy, {Alice,xxx.xxx.xxx.xxx,TNow, KAlice,Tracy}KTracy What’s in the ticket?

  24. So What Has the Client Got? • KAlice is derived from her password • Which gets a session key allowing her to communicate securely with the TGS • KAlice,Tracy • And she has a ticket for the TGS • Not directly usable by Alice • But the TGS (Tracy) can use it to authenticate Alice

  25. An authenticator Alice {AAlice,Tracy}KAlice,Tracy Tracy, Kerberos Tracy Sidney Client Asks TGS for a Server Ticket

  26. What Has the TGS Got? • It can decrypt the ticket created by the Kerberos server • Obtaining KAlice,Tracy and other information • Authenticating that the transmission went through Kerberos server • And it’s got the authenticator

  27. Why the Authenticator? • We want to avoid involving the Kerberos server every time a client needs a ticket • So the ticket-granting ticket will be used multiple times • Authenticator protects against replay attacks involving the multi-use ticket-granting ticket

  28. {KAlice,Sidney}KAlice,Tracy Alice Kerberos Tracy Sidney TGS Sends Ticket to Client TAlice,Sidney = Sidney, {Alice,xxx.xxx.xxx.xxx,TNow1, KAlice,Sidney}KSidney What’s in the ticket?

  29. Now What Has the Client Got? • She can decrypt the part of the message containing the new session key • So she’s ready to communicate • She can’t decrypt the ticket • That’s in a key only the server Sidney knows • But Sidney can use it

  30. {AAlice,Sidney}KAlice,Sidney Alice Kerberos Tracy Sidney Client Requests Service Alice creates a new authenticator to show freshness

  31. What Does the Server Have? • He can decrypt the ticket from the TGS • Since it’s in his key • The ticket contains the session key • And authentication information • He can then decrypt the authenticator • Which ensures a session isn’t being replayed (by timestamp) • He can then determine authorization

  32. Why Is There Both a Kerberos Server and a TGS? • The TGS handles normal interactions between clients and servers • The Kerberos server bootstraps interactions with the TGS • A ticket-granting ticket can be reused with a TGS over some time • Compromise of the TGS has limited effects

  33. Why Is There Both a Ticket and An Authenticator? • The ticket is reusable • It has a timespan • Typically 8 hours • The authenticator is one-use-only • Supposedly • And its timestamp must be within the ticket’s timespan

  34. Potential Weaknesses in Kerberos • Timestamp-based attacks • Password-guessing attacks • Replacement of Kerberos software • The server is probably well protected • But are the clients? • Not unique to Kerberos

  35. Practicalities of Kerberos • Open source software • Available for Windows, Linux, Macs • Widely used • Still actively supported by MIT • Originally used DES • AES supported • DES will be removed eventually

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