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Cryptography and Network Security UNIT IV - NETWORK SECURITY

Dec 27, 2015

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  • Slide 1
  • Cryptography and Network Security UNIT IV - NETWORK SECURITY
  • Slide 2
  • Authentication Functions Message authentication or digital signature mechanism can be viewed as having two levels At lower level: there must be some sort of functions producing an authenticator a value to be used to authenticate a message At lower level: there must be some sort of functions producing an authenticator a value to be used to authenticate a message This lower level functions is used as primitive in a higher level authentication protocol This lower level functions is used as primitive in a higher level authentication protocol Authentication Functions
  • Slide 3
  • Three classes of functions that may be used to produce an authenticator Message encryption Message encryption Ciphertext itself serves as authenticator Message authentication code (MAC) Message authentication code (MAC) A public function of the message and a secret key that produces a fixed-length value that serves as the authenticator Hash function Hash function A public function that maps a message of any length into a fixed-length hash value, which serves as the authenticator Authentication Functions
  • Slide 4
  • KERBEROS In Greek mythology, a many headed dog, the guardian of the entrance of Hades In Greek mythology, a many headed dog, the guardian of the entrance of Hades
  • Slide 5
  • KERBEROS Users wish to access services on servers. Three threats exist: User pretends to be another user. User pretends to be another user. User alters the network address of a workstation. User alters the network address of a workstation. User eavesdrops on exchanges and uses a replay attack. User eavesdrops on exchanges and uses a replay attack.
  • Slide 6
  • KERBEROS Provides a centralized authentication server to authenticate users to servers and servers to users. Relies on conventional encryption, making no use of public-key encryption Two versions: version 4 and 5 Version 4 makes use of DES
  • Slide 7
  • Kerberos Version 4 Terms: C = Client C = Client AS = authentication server AS = authentication server V = server V = server ID c = identifier of user on C ID c = identifier of user on C ID v = identifier of V ID v = identifier of V P c = password of user on C P c = password of user on C ADc = network address of C ADc = network address of C K v = secret encryption key shared by AS and V K v = secret encryption key shared by AS and V TS = timestamp TS = timestamp || = concatenation || = concatenation
  • Slide 8
  • A Simple Authentication Dialogue (1) C AS: ID c || P c || ID v (2) AS C:Ticket (3) C V: ID c || Ticket Ticket = E K v [ ID c || AD c || ID v] Two problems The number of times a user has to enter a password Plaintext transmission of the password
  • Slide 9
  • The Idea towards Solution Introducing a ticket-granting server (TGS) The user first requests a ticket-granting ticket (Ticket tgs ) from the AS; The user first requests a ticket-granting ticket (Ticket tgs ) from the AS; The user then authenticates itself to TGS for a ticket (Ticket v ) for accessing new service; The user then authenticates itself to TGS for a ticket (Ticket v ) for accessing new service; The user finally authenticate itself to V for requesting a particular service. The user finally authenticate itself to V for requesting a particular service.
  • Slide 10
  • Kerberos Version 4 Authentication Dialogue Kerberos Version 4 Authentication Dialogue
  • Slide 11
  • Slide 12
  • Slide 13
  • Overview of Kerberos
  • Slide 14
  • Request for Service in Another Realm
  • Slide 15
  • Difference Between Version 4 and 5 Encryption system dependence (V.4 DES) Internet protocol dependence Message byte ordering Ticket lifetime Authentication forwarding Interrealm authentication
  • Slide 16
  • Kerberos Encryption Techniques
  • Slide 17
  • PCBC Mode
  • Slide 18
  • Kerberos - in practice Kerberos - in practice Currently have two Kerberos versions: 4 : restricted to a single realm 4 : restricted to a single realm 5 : allows inter-realm authentication, in beta test 5 : allows inter-realm authentication, in beta test Kerberos v5 is an Internet standard Kerberos v5 is an Internet standard specified in RFC1510, and used by many utilities specified in RFC1510, and used by many utilities To use Kerberos: need to have a KDC on your network need to have a KDC on your network need to have Kerberised applications running on all participating systems need to have Kerberised applications running on all participating systems major problem - US export restrictions major problem - US export restrictions Kerberos cannot be directly distributed outside the US in source format (& binary versions must obscure crypto routine entry points and have no encryption) Kerberos cannot be directly distributed outside the US in source format (& binary versions must obscure crypto routine entry points and have no encryption) else crypto libraries must be reimplemented locally else crypto libraries must be reimplemented locally
  • Slide 19
  • X.509 Authentication Service Distributed set of servers that maintains a database about users. Each certificate contains the public key of a user and is signed with the private key of a CA. Is used in S/MIME, IP Security, SSL/TLS and SET. RSA is recommended to use.
  • Slide 20
  • X.509 Authentication Service Distributed set of servers that maintains a database about users. Each certificate contains the public key of a user and is signed with the private key of a CA. Is used in S/MIME, IP Security, SSL/TLS and SET. RSA is recommended to use.
  • Slide 21
  • X.509 Formats
  • Slide 22
  • Obtaining a User s Certificate Characteristics of certificates generated by CA: Any user with access to the public key of the CA can recover the user public key that was certified. Any user with access to the public key of the CA can recover the user public key that was certified. No part other than the CA can modify the certificate without this being detected. No part other than the CA can modify the certificate without this being detected.
  • Slide 23
  • X.509 CA Hierarchy
  • Slide 24
  • Revocation of Certificates Reasons for revocation: The users secret key is assumed to be compromised. The users secret key is assumed to be compromised. The user is no longer certified by this CA. The user is no longer certified by this CA. The CA s certificate is assumed to be compromised. The CA s certificate is assumed to be compromised.
  • Slide 25
  • Authentication Procedures
  • Slide 26
  • Summary have considered: message authentication using message authentication using message encryption MACs hash functions Kerberos Kerberos X.509 Authentication Service X.509 Authentication Service
  • Slide 27
  • Slide 28
  • Authentication Applications Developed to support application-level authentication and digital signatures Most widely used services: Kerberos Kerberos X.509 X.509 Kerberos a private-key authentication service X.509 a public-key directory authentication service
  • Slide 29
  • Kerberos
  • Slide 30
  • Kerberos Developed as part of Project Athena at MIT Symmetric encryption using no public keys using no public keys Provides centralised private-key third-party authentication in a distributed network Version 4 and 5
  • Slide 31
  • Kerberos Motivation Provide security in a distributed architecture consisting of dedicated user workstations (clients), and distributed or centralized servers Require the user to prove his identity for each service invoked Require that servers prove their identity to clients Secure, Reliable, Transparent, and Scalable
  • Slide 32
  • Kerberos Scheme Trusted third party authentication service Uses a protocol based on Needham and Schroeder [NEED78], see Chapter 7 Clients and servers trust Kerberos to mediate their mutual authentication
  • Slide 33
  • Kerberos Version 4 Uses DES, in a rather elaborate protocol, to provide authentication Uses an Authentication Server (AS) Knows all user passwords, and stores in a DB Knows all user passwords, and stores in a DB Shares a unique secret key with each server Shares a unique secret key with each server Send an encrypted ticket granting ticket Send an encrypted ticket granting ticket TGT contains a lifetime and timestamp TGT contains a lifetime and timestamp
  • Slide 34
  • Kerberos Version 4 Uses a Ticket Granting Server (TGS) Issues tickets to users authenticated by AS Issues tickets to users authenticated by AS Encrypted with a key only known by AS and TGS Encrypted with a key only known by AS and TGS Returns a service granting ticket Returns a service granting ticket Service granting ticket contains timestamp and lifetime
  • Slide 35
  • Kerberos Dialog Problem: lifetime and no server authenticate to user Uses a session key Message Exchanges (see table 14.1) AS exchange to obtain ticket-granting ticket AS exchange to obtain ticket-granting ticket TGS exchange to obtain service granting ticket TGS exchange to obtain serv
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