The design of a tutorial The design of a tutorial to illustrate the to illustrate the

Kerberos protocolKerberos protocol

Lindy Carter

Supervisors : Prof Wentworth

John Ebden

ObjectivesObjectives

• To design a teaching approach and tutorial to teach complex protocols

• Kerberos as the chosen example

IntroductionIntroduction

• Authentication protocol used to identify principals on a network using only a single sign-on

• Uses authentication based on cryptography and was developed by MIT to replace authentication based on assertion

• Chosen by Microsoft to replace NTLM as the method for authentication in Window 2000

• Name come from the 3 headed dog in Greek mythology – Cerberus –who used to guard the gates of Hades

The problem with The problem with teaching Kerberosteaching Kerberos

• Not easy to conceptualize

• Formal definitions use a “once over” type of approach and are very technical

• Important concepts are presented in the same step

• Formal definitions use complicated notation

What we have done to What we have done to solve the problemsolve the problem

• We have divided our explanation into 2 passes– The first pass uses a concrete metaphor to explain

the 3 message exchanges in the protocol– The second pass is broken down into 3 phases

and uses another metaphor to explain the message exchanges, encryption and key sharing

• We want start with concrete explanations and move towards more abstract ideas

Pass 1Pass 1

• Uses a club membership example

• The process of joining a club and using its affiliated facilities is similar in Kerberos to authenticating yourself and asking to use a resource.

Pass 2Pass 2

• Uses a coloured envelope metaphor

• We chose the envelope metaphor because it illustrates the 2 level structure of tickets.

• The coloured envelopes show encryption key pairs

• Pass is divided into 3 phases– Phase 1 describes the 3 message

exchanges in a trusted environment– Phase 2 introduces long term key sharing– Phase 3 introduces sessions and session

key generation

Pass 1 – Pass 1 – Club membershipClub membership

Club Membership metaphor Kerberos

1 A person wants a membership card to the umbrella body in order for him to use affiliated facilities. The user presents his ID book or student card to prove his identity, pays for certain facilities, and he is then issued with a membership card containing his membership rights.

The user requests a ticket granting ticket to use the ticket granting service. The user is authenticated and the ticket granting ticket containing his access rights is issued to him.

2 When the member wants to use an affiliated facility, he presents his membership card to the facility office. The office checks to see if the member is allowed to use the facility by looking at the member’s membership rights. If the member is allowed to use the facility he is requesting, the office issues him with a facility ticket.

The user wishes to use a resource. He presents his ticket granting ticket to the ticket granting service to ask for a resource ticket. The ticket granting service checks the access rights in the ticket granting ticket and if the user has rights to the resource that he as requested, a resource ticket it issued to the user.

3 To use the facility, the members presents the facility ticket to the gatekeeper of the facility he is wanting to use.

To use the resource, the user presents the resource ticket to the resource server.

Phase 1Phase 1

Authentication Server

Ticket GrantingService

Resource Server

Authentication Server Exchange

Phase 1Phase 1

Authentication Server

Ticket GrantingService

Resource Server

Ticket Granting Service Exchange

Phase 1Phase 1

Authentication Server

Ticket GrantingService

Resource Server

Resource Server Exchange

Some ObservationsSome Observations

• There is no direct communication between the Authentication Server, Ticket Granting Service of the Resource Server

• Tickets are reusable

Problems with Phase 1Problems with Phase 1

• Principals that receive tickets do not actually know who sent the ticket.

• The access rights in the tickets are in plain text and the user is able to change them

Phase 2Phase 2

• First problem is easy to solve - each time the user sends a ticket to a principal, he sends his name along with the ticket

• Second problem a little more involved…

Phase 2Phase 2

• The user needs to authenticate himself to the Authentication Server and the Authentication Server needs to pass information securely back to the user– The user and AS need to share a long term key

(users password (black key)

• The Authentication Server needs to pass information securely to the Ticket Granting Service– AS and TGS need to share a long term key (red

key)

Phase 2Phase 2

• The Ticket Granting Service needs to pass information securely to the Resource Server– TGS and RS need to share a long term key

(blue key)

Long term key sharingLong term key sharing

AuthenticationServer

Ticket GrantingService

Resource Server

Long term Key

Problems with Phase 2Problems with Phase 2

• Some one listening on the network can intercept the message containing the ticket and the users name.– They will be able to change the name and

use the resource

Phase 3Phase 3

• The user should encrypt his name before he sends it to the TGS or RS (this is called an authenticator)– The user needs a communication channel to

communicate with the TGS and RS

• Sessions keys are generated via a third party. – A copy of the key is given to the user in his reply

message for a ticket. – Another copy is embedded in the ticket that the

user is receiving back

Phase 3Phase 3

• When the TGS or RS receive the ticket and authenticator, it is able to decrypt the ticket and retrieve the session key

• TGS or RS is then able to decrypt the authenticator and see who is requesting service.

Key SharingKey Sharing

AuthenticationServer

Ticket GrantingService Resource

Server

Long term Key

Session Key

Generates session key

Generates session key

Finally…Finally…

Authentication Server

Ticket GrantingService

Resource Server

1 2

3

1. AS exchange

2. TGS exchange

3. RS exchange

QuestionsQuestions