Netprog: Kerberos 1

KERBEROS

Contents:IntroductionHistoryComponentsAuthentication ProcessStrengthsWeaknesses and SolutionsApplicationsReferences

IntroductionIt is a secure, single-sign-on, trusted

third-party authentication serviceMakes 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 HadesHades => Underworld (where hackers apparently live).

HistoryDeveloped at MIT as a part of Project

Athena in mid 1980sCurrently, Kerberos is up to Version

5Version 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

ComponentsPrincipals

Realms

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

ComponentsPrincipals: 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

ComponentsKey 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

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

Susan’sDesktop

Computer

Think “Kerberos Server”

Authentication Process

Susan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

Susan’sDesktop

Computer

Represents something requiring Kerberos authentication (web server, ftp server, ssh server, etc…)

Susan’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

“I’d like to be allowed to get tickets from the Ticket Granting Server, please.

Susan’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

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’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

myPassword

XYZ Service

TGT

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’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

“Let me prove I am Susan to XYZ Service.

Here’s a copy of my TGT!”

use XYZ

TGTTGT

Susan’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

TGT

Hey XYZ: Susan is Susan.

CONFIRMED: TGS

You’re Susan.Here, take this.

Susan’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

TGTHey 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’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

TGTHey 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’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

TGTHey 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’sDesktop

ComputerSusan

KeyDistribution

Center

TicketGrantingService

Authen-TicationService

XYZ Service

TGTHey 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 lifetime4. Authentication through the AS only

has to happen once5. 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

THANK YOU