Netprog: Kerberos1 KERBEROS

Contents: Introduction History Components Authentication Process Strengths Weaknesses and Solutions Applications References

Introduction It is a secure, single-sign-on, trusted third- party authentication service  Makes assumption that the connection between a client and service is insecure  Passwords are encrypted to prevent others from reading them  Clients only have to authenticate once during a pre-defined lifetime  Provides a way to authenticate clients to services to each other through a trusted third party

How did Kerberos get it’s name? The name "Kerberos" comes from a mythological three-headed dog that guarded the entrance to Hades Hades => Underworld (where hackers apparently live).

History Developed at MIT as a part of Project Athena in mid 1980s Currently, Kerberos is up to Version 5 Version 4 being the first version to be released outside of MIT. Adopted by several private companies as well as added to several operating systems. Its creation was inspired by client-server model

Components Principals Realms Key Distribution Centers (KDC’s) ◦ Authentication Service ◦ Ticket Granting Service

Components Principals: Each entity, such as clients or application servers, is represented as a principal Realms: Companies and organizations are composed of different departments, each with a different service named realm

Components Key Distribution Centers (KDC’s) ◦ composed of an Authentication Service and Ticket Granting Server ◦ has a database that houses all principals and their keys for a given realm ◦ at least one KDC per realm

Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service Susan’s Desktop Computer Think “Kerberos Server” Authentication Process

Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service Susan’s Desktop Computer Represents something requiring Kerberos authentication (web server, ftp server, ssh server, etc…)

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service “I’d like to be allowed to get tickets from the Ticket Granting Server, please.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service “Okay. I locked this box with your secret password. If you can unlock it, you can use its contents to access my Ticket Granting Service.”

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service myPassword XYZ Service TGT

Because Susan was able to open the box (decrypt a message) from the Authentication Service, she is now the owner of a “Ticket- Granting Ticket”. The Ticket-Granting Ticket (TGT) must be presented to the Ticket Granting Service in order to acquire “service tickets” for use with services requiring Kerberos authentication. The TGT contains no password information.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service “Let me prove I am Susan to XYZ Service. Here’s a copy of my TGT!” use XYZ TGT

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS You’re Susan. Here, take this.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS I’m Susan. I’ll prove it. Here’s a copy of my legit service ticket for XYZ. Hey XYZ: Susan is Susan. CONFIRMED: TGS

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS Hey XYZ: Susan is Susan. CONFIRMED: TGS That’s Susan alright. Let me determine if she is authorized to use me.

Authorization checks are performed by the XYZ service… Just because Susan has authenticated herself does not inherently mean she is authorized to make use of the XYZ service.

One remaining note: Tickets (your TGT as well as service-specific tickets) have expiration dates configured by your local system administrator(s). An expired ticket is unusable. Until a ticket’s expiration, it may be used repeatedly.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS ME AGAIN! I’ll prove it. Here’s another copy of my legit service ticket for XYZ. Hey XYZ: Susan is Susan. CONFIRMED: TGS use XYZ

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS Hey XYZ: Susan is Susan. CONFIRMED: TGS That’s Susan… again. Let me determine if she is authorized to use me.

Strengths 1.Passwords are never sent across the network unencrypted 2.Clients and applications services mutually authenticated 3.Tickets have a limited lifetime 4.Authentication through the AS only has to happen once 5.Shared secret keys between clients and services are more efficient than public-keys

Weaknesses and Solutions If TGT stolen, can be used to access network services. Only a problem until ticket expires in a few hours. Very bad if Authentication Server compromised. Physical protection for the server.

Applications :  Kerberos-aware applications are called Kerberized  Some kerberized applications are – Berkeley R-commands Telnet POP USC’s Win2000 network FTP

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