Network security with authentication application . Kerberos is one of them.

1. Presented By:
Vidula Shukla
M.Tech., Computer Science & Engineering
Dept. of Computer Science & Engineering
Sagar Inst. of Research & Technology
Bhopal

2. Overview
 Introduction
 Motivation
 Requirement
 Kerberos Version 4
 Kerberos Realms
 Kerberos V4 V/s V5
 Kerberos Version 5
 Strength
 Conclusion
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3. Introduction
 Authentication: can be defined as determining an
identity to the required level of assurance.
 Authentication Application : Deals with the
authentication function that have been developed to
support application-level authentication
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4. Introduction to Kerberos
 An authentication service developed for Project Athena at
MIT
 Provides
 strong security on physically insecure network
 a centralized authentication server which authenticates


Users to servers
Servers to users
 Relies on conventional encryption rather than public-key
encryption
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5. Why Kerberos is needed ?
Problem: Not trusted workstation to identify
their users correctly in an open distributed environment
3 Threats:



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Pretending to be another user from the workstation
Sending request from the impersonated workstation
Replay attack to gain service or disrupt operations
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6. Why Kerberos is needed ? Cont.
Solution:
 Building elaborate authentication protocols at
each server
 A centralized authentication server (Kerberos)
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7. Requirements for KERBEROS
 Secure:
 An opponent does not find it to be the weak link
 Reliable:
 The system should be able to back up another
 Transparent:
 An user should not be aware of authentication
 Scalable:
 The system supports large number of clients and
severs
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8. KERBEROS VERSION 4
 Version 4 is most widely used version
 Version 4 uses of DES
 Version 4 build up to the full protocol by
looking at several hypothetical dialogues
 Version 5 corrects some of the security
deficiencies of Version 4
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9.  Problem:
An opponent can pretend to be another client and
obtain unauthorized privileges on server machine.
 Solution :
Server must be able to confirm the identities of client
who request service.
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10. Kerberos Version 4: Dialog 1- Simple
Ticket=E(kv[IDc,ADc,IDv])
10
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11.  Problem:
1. the no. of times the password should be entered
should be minimized.
2. Plaintext transmission of password
 Solution :
1. Ticket-granting Server; Issues ticket to user who have
been authenticated to AS
2. The client can use this ticket to request multiple
service granting ticket.
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12. Kerberos Version 4 : Dialog 2-More Secure
ticketTGS=EKtgs[IDc,ADc,IDtgs,TS1,LifeTime1
]
Once per user logon session
Once per type of service
4-TicketV
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13. Kerberos Version 4 : Dialog 2
- More Secure Cont.
Once per service session
5- TicketV+ IDc
TicketV=EKv[IDc,ADc,IDv,Ts2,Lifetime2]
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14.  Problem:
Lifetime associated with ticket granting ticket
2. Requirement for servers to authenticate themselves to
user.
1.
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15. Kerberos: The Version 4 Authentication Dialog
Once per user logon session
ticketTGS=EKtgs
[Kc.tgs, IDc,ADc,IDtgs,TS2, LifeTi
me2 ]
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16. Kerberos: The Version 4 Authentication Dialog
Cont.
Once per type of service
ticketTGS=EKtgs
[Kc.tgs,IDc,ADc,IDtgs, TS2, LifeTime2 ]
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AuthenticatorC=EKc.tgs[IDc,ADc,TS3]
ticketV=EKV[Kc.v,IDc,ADc,IDv, TS4, LifeTime4 ]
3- TicketTGS + AuthenticatorC +
IDv
4-EKc.tgs[ Kc.v,IDv,Ts4,Ticketv]
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17. Kerberos: The Version 4 Authentication Dialog
Cont.
Once per service session
5- TicketV+ AuthenticatorC
6- EKc.v[TS5+1]
TicketV=EKv [Kv.c, IDc, ADc, IDv, TS4, Lifetime4]
AuthenticatorC=EKc.v [IDc,ADc,TS5]
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18. Tickets:
 Contains information which must be considered
private to the user
 Allows user to use a service or to access TGS
 Reusable for a period of particular time
 Used for distribution of keys securely
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19. Authenticators
 Proves the client’s identity
 Proves that user knows the session key
 Prevents replay attack
 Used only once and has a very short life time
 One authenticator is typically built per session of use
of a service
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20. Kerberos Overview
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21. Kerberos Realms
 A single administrative domain includes:
 a Kerberos server
 a number of clients, all registered with server
 application servers, sharing keys with server
 What will happen when users in one realm need access
to service from other realms?:
 Kerberos provide inter-realm authentication
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22. Inter-realm Authentication:
 Kerberos server in each realm shares a secret key with
other realms.
 It requires
 Kerberos server in one realm should trust the one in
other realm to authenticate its users
 The second also trusts the Kerberos server in the first
realm
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23. Request for Service in another realm:
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24. KERBEROS Version 5 versus Version4
 Environmental shortcomings of Version 4:
 Encryption system dependence: DES
 Message byte ordering
 Internet protocol dependence
 Ticket lifetime
 Authentication forwarding
 Inter-realm authentication
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25. KERBEROS Version 5 versus Version4
 Technical deficiencies of Version 4:
 Double encryption
 Session Keys
 Password attack
 Mode of Encryption
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26. New Elements in Kerberos Version 5
 Realm
 Indicates realm of the user
 Options
 Times
 From: the desired start time for the ticket
 Till: the requested expiration time
 Rtime: requested renew-till time
 Nonce
 A random value to assure the response is fresh
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27. Kerberos Version 5 Message Exchange:1

To obtain ticket-granting ticket:
(1)C  AS : Options || IDc || Realmc || IDtgs ||Times ||
Nonce1
(2) AS  C : Realmc || IDc || Ticket tgs ||
EKc [ Kc,tgs || IDtgs || Times || Nonce1 ||| Realm tgs ]
Ticket tgs= EKtgs [ Flags || Kc,tgs || Realm c ||
IDc || ADc || Times]
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28. Kerberos Version 5 Message Exchange:2
 To obtain service-granting ticket :
(3)C  TGS : Options || IDv || Times || Nonce2 || Ticket tgs ║
Authenticator c
(4)TGS  C : Realmc || IDc || Ticket v || EK c,tgs [ Kc,v ║Times||
Nonce2 || IDv ║ Realm v]
Ticket tgs= EKtgs [ Flags || Kc,tgs || Realm c || IDc || ADc ||
Times]
Ticket v : EK v [Kc,,v ║ Realmc || IDc ║ ADc ║ Times ]
Authenticator c : EK c,tgs [IDc ║ Realmc ║ TS1]
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29. Kerberos Version 5 Message Exchange:3
 To obtain service
(5) C  S : Options || Ticket v|| Authenticator c
(6) S  C : EK c,v [TS2|| Subkey || Seq# ]
 Ticket v : EK v [Flags || Kc,v || Realmc ||
IDc || ADc || Times ]
 Authenticator c : EK c,v [IDc || Realmc ||
TS2 || Subkey|| Seq# ]
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30. Kerberos : Strengths
 User's passwords are never sent across the
network, encrypted or in plain text
 Secret keys are only passed across the network in encrypted
form
 Client and server systems mutually authenticate
 It limits the duration of their users' authentication.
 Authentications are reusable and durable
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31. Conclusion
 Kerberos is an authentication service using convention
encryption
 Kerberos the solution to network security is a protocol
designed to provide centralized authentication whose
function is to authenticate user to server and server to
user.
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32. THANK YOU
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