Kerberos
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Akshat Sharma
Samarth Shah
Outline
Introduction
What is Kerberos?
Importance
Why Kerberos?
Functionality
About
How Kerberos works?
Kerberos Model, Functionality, Benefits,
Drawbacks
Conclusion
Why it is important?
References
Sources of Information
What is Kerberos
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Network authentication protocol.
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Developed at MIT in the mid 1980s.
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A secret key based service for providing
authentication in open networks.
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Provides strong authentication for clientserver applications.
Why Kerberos
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Authentication is a key feature in multi-user
system.
• divide up resources with capabilities
between many users.
• restrict user’s access to resources.
• typical authentication mechanism –
passwords
Why Kerberos
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Sending usernames and passwords in the
clear text jeopardizes the security of the
network.
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Each time a password is sent in the clear
text, there is a chance for interception.
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Firewalls make a risky assumption: that
attackers are coming from the outside. In
reality, attacks frequently come from
within.
Kerberos model
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Kerberos is based on the Secret-Key
Distribution Model that was originally
developed by Needham & Schroeder.
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keys are the basis of authentication in Kerberos.
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typically a short sequence of bytes.
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used to both encrypt & decrypt
Kerberos model
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Encryption => plaintext + Encryption key = ciphertext
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Decryption => ciphertext + Decryption key = plaintext
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Encryption Key – identical to – Decryption Key (in
Conventional Crypto).
Kerberos Basics
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Three basic functions (message exchanges)
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1.
The Authentication Service Exchange
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2.
(Logon)
The Ticket-Granting Service Exchange
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3.
a request and a reply
(Getting a Ticket to Ride)
The Client/Server Authentication Exchange
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(Accessing a Resource)
Functions
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Authentication
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Integrity – Is the assurance that the data received is
the same as generated.
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Confidentiality – is the protection of info from
disclosure to those not intended to receive it.
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Authorization – is the process by which one
determines whether a principal is allowed to perform
an operation. Authorization is done usually after
principal has been authenticated or based on
authenticated statements by others.
Kerberos Functionality
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Instead of client sending password to application
server:
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Request ticket from authentication server
Ticket and encrypted request sent to application
server
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Steps :
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An user requests use of a network service
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Service wants assurance that user is who he says he is.
(Continued…….)
Kerberos Functionality
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User presents a ticket that is issued to it by a
Kerberos Authentication Server(AS).
If the ticket is valid, service is granted.
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The tickets must be unequivocally linked to the user
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Ticket demonstrates that the bearer knows
something that only its intended user would know.
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Ticket must obviously be safeguarded against all
attacks.
Benefits of Kerberos
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More efficient authentication to servers.
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Mutual authentication.
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Server can authenticate the client by examining
credentials. Clients can obtain credentials for a
particular server once and reuse them throughout a
network logon session.
Parties at both ends of a network connection can know
that the party on the other end is who it claims to be.
Delegated authentication.
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Kerberos protocol has a proxy mechanism that allows
a service to impersonate its client when connecting to
other services.
Drawbacks of Kerberos
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Single point of failure: It requires continuous availability of a
central server. When the Kerberos server is down, no one can
log in.
Kerberos has strict time requirements, which means the
clocks of the involved hosts must be synchronized within
configured limits.
The administration protocol is not standardized and differs
between server implementations.
Since all authentication is controlled by a centralized KDC,
compromise of this authentication infrastructure will allow
an attacker to impersonate any user.
Each network service which requires a different host name
will need its own set of Kerberos keys. This complicates
virtual hosting and clusters.
Conclusion
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Authentication is critical for the security of computer systems.
Without knowledge of the identity of a principal requesting an
operation, it's difficult to decide whether the operation should
be allowed.
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Traditional authentication methods are not suitable for use in
computer networks where attackers monitor network traffic to
intercept passwords.
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The use of strong authentication methods that do not disclose
passwords is imperative. The Kerberos authentication system is
well suited for authentication of users in such environments.
References
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Kerberos: An Authentication Service for Open
Network Systems
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Steiner, Neuman, Schiller, 1988, Winter USENIX
http://en.wikipedia.org/wiki/Kerberos_(protocol)
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