TOPIC HIERARCHY
Distributed Environment
Security
Privacy Authentication Authorization Non Repudiation
ORIGIN
MIT developed Kerberos to protect network services.
Developed under the Project Athena.
Developed at MIT in the mid 1980s.
Network authentication protocol.
Available as open source or in supported commercial
software.
This form of security has been evolving in Unix world
for over a decade and is now becoming a standard.
KERBEROS FROM CERBURUS
( THE GREEK MYTHOLOGICAL THREE
HEADED DOG)
Kerberos was named after its name.
It is the keeper of Hades (The God of the underworld) .
Originally, the 3 heads represented the 3 A’s
Authentication, Authorization, Access Control.
WHAT IS KERBEROS ?
A secret key based service for providing authentication in
open networks.
Authentication is mediated by a trusted third party on the
network (KDC : Key Distribution Center).
Kerberos is “secure,
single
-sign-on, trusted service”.
Trusted 3rd party authentication scheme.
WHY KERBEROS?
Sending usernames and passwords in the clear jeopardizes (harm)
the security of the network.
Each time a password is sent in the clear , there is a chance of an
security attack.
TYPES OF SECURITY
ATTACKS
Interception: an attack on confidentiality.
Interruption : an attack of availability.
Modification: an attack on integrity.
Fabrication: an attack on authenticity.
COMPUTER/NETWORK SECURITY
NEEDS:
Authentication
•
Who is requesting access
Authorization
•
What user is allowed to do
Auditing (Accessing Control)
•
What has user done
Kerberos addresses all of these needs. Mostly it provides
authentication service.
AUTHENTICATION
Three ways to prove identity
Something you know Something you have Something you are
WHAT IS KERBEROS GOOD
FOR?
Verify identity of users and servers.
Encrypt communication if desired.
Centralized repository of accounts.
(Kerberos uses ‘realm’ to group accounts)
Local authentication.
Enforce ‘good’ password policy.
Provide an audit trail of usage.
KERBEROS VERSIONS
Three versions: version 1,2 and 3
Used for internal work in MIT.
Two versions: version 4 and 5
Version 4 makes use of DES (Data Encryption
Standard).
COMPONENTS
Principals
Realms
Key Distribution Centers (KDC’s).
•Authentication Service (AS). •Ticket Granting Server (TGS).
Tickets
Principals
Each entity, such as clients or application servers (Service), is represented as a principal
Coupled with a key.
Keys are stored in a database on KDC. Keys must be unique.
Realms
Companies and Organizations are composed of different departments, each with a different function.
To make things less complex, system administrators represent each department with a realm.
Key Distribution Centers (KDC’s)
Composed of an AS and TGS.
Has a database that houses all principals and their keys for a given realm At least one KDC per realm
Authentication Service (AS)
• Piece of software that accepts requests from clients.
• Creates TGT’s based on the information in the request (principal names for client and service).
• Establishes the secret session key used for communication between clients and services.
Ticket Granting Server (TGS)
• Clients authenticate themselves once to the AS to get TGT’s, which are presented to the TGS.
• Responsible for accepting Ticket Granting Tickets. • Verifies that the TGT’s are correct.
Tickets
Tickets serve following purposes:
• To confirm identity of the end participants.
• To establish a short-lived encryption key that both parties can share for secure communication (called the session key).
• Contains valuable data to check the authentication is real. Includes data such as:
• Requesting client’s principal name. • A list of valid IP addresses.
• A secret session key.
Authenticators
Consist of timestamps encrypted with the secret session key. Can only be used once.
Timestamp can’t exceed maximum time frame. Used to prevent replay attacks.
KERBEROS DESIGN
User must identify itself once at the beginning of a workstation
session (login session).
Passwords are never sent across the network in clear text (or stored
in memory)
Every user has a password.
Every service has a password.
The only entity that knows all the passwords is the Authentication
Server
Server
Server
Server
Server
Server
Server
Server
Kerberos
Kerberos
Database
Database
Ticket Granting
Ticket Granting
Server
Server
Authentication
Authentication
Server
Server
Workstation
Workstation
COMPLETE FUNCTIONALITY
OF KERBEROS
WEAKNESSES AND SOLUTIONS
If TGT stolen, can be used to
access network services.
Only a problem until
ticket expires in a
few hours.
Subject to dictionary attack.
Timestamps require
hacker to guess in 5
minutes.
Very bad if Authentication
Server compromised.
Physical protection
for the server.
THE COMPETITION: SSL
SSL Kerberos
Uses public key encryption Uses private key encryption Is certificate based (asynchronous) Relies on a trusted third party
(synchronous)
Ideal for the WWW Ideal for networked environments Key revocation requires Revocation
Server to keep track of bad certificates
Key revocation can be accomplished by disabling a user at the Authentication Server
Certificates sit on a users hard drive (even if they are encrypted) where they are subject to being cracked.
Passwords reside in users' minds where they are usually not subject to secret attack.
Uses patented material, so the service is not free. Netscape has a profit motive in wide acceptance of the standard.
Kerberos has always been open source and freely available.
ADVANTAGES
More efficient authentication to servers
Mutual authentication
Delegated authentication
Simplified trust management
Interoperability
LIMITATIONS
Scalability.
Doesn’t explicitly protect against Trojan attacks.
Is mainly intended for single-user workstations.
KDC can be a single point of failure.
APPLICATIONS OF
KERBEROS:
The Kerberos V5 applications are versions of existing UNIX
network programs with the Kerberos features added.
•telnet •rlogin •FTP •rsh •rcp •Ksu
