CSCE 715:Network Systems Security

Chin-Tser Huanghuangct@cse.sc.edu

University of South Carolina

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Authentication Applications

Developed to support application-level authentication and digital signatures

A famous example is Kerberos – a password authentication service

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Kerberos Trusted key server system from MIT Provide centralized password third-party

authentication in a distributed network allow users access to services distributed

through network without needing to trust all workstations instead all trust a central authentication

server Two versions in use: 4 & 5

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Kerberos Requirements First published report identified its

requirements as security reliability transparency scalability

Implemented using an authentication protocol based on Needham-Schroeder

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Kerberos 4 Overview A basic third-party authentication scheme Have an Authentication Server (AS)

users initially negotiate with AS to identify self AS provides a non-corruptible authentication

credential (ticket granting ticket, TGT) Have a Ticket-Granting Server (TGS)

users subsequently request access to other services from TGS on basis of users TGT

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First Design

(1) C AS: IDc||Pc||IDv

(2) AS C: Ticket(3) C V: IDc||Ticket

Ticket = EKv [IDc||ADc||IDv]

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Problems with First Design

User may have to submit password many times in the same logon session

Password is transmitted in clear

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Second DesignOnce per user logon session:

(1) C AS: IDc||IDtgs

(2) AS C: EKc [Tickettgs]Once per type of service:

(3) C TGS: IDc||IDv||Tickettgs

(4) TGS C: Ticketv

Once per service session:(5) C V: IDc||Ticketv

Tickettgs = EKtgs [IDc||ADc||IDtgs||TS1||Lifetime1]

Ticketv = EKv [IDc||ADc||IDv||TS2||Lifetime2]

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Problems with Second Design

Requirement for server (TGS or application server) to verify that the person using a ticket is the same person to whom ticket was issued

Requirement for server to authenticate themselves to users

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Kerberos 4 Message Exchange

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Kerberos 4 Message Exchange

Kc,tgs: a session key created by AS to permit secure exchange between client and TGS without requiring them to share a permanent key

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Kerberos 4 Message Exchange

Authenticatorc: generated by client to assure TGS that the ticket presenter is the same as the client for whom Tickettgs was issued. Has very short lifetime to prevent replay

EKc,tgs

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Kerberos 4 Message Exchange

Message (6) ensures C that it is really talking to the legitimate server V and this message is not a replayed message

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Kerberos 4 Overview

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Kerberos Realms Kerberos environment consists of

a Kerberos server a number of clients, all registered with server application servers, sharing keys with server

This is termed a “realm” typically within a single administrative domain

If have multiple realms, their Kerberos servers must share keys and trust each other

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Request Service in Another Realm

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Kerberos Version 5 Developed in mid 1990’s Provide improvements over Version 4

addresses environmental shortcomings encryption alg, network protocol, byte order,

ticket lifetime, authentication forwarding, interrealm auth

and technical deficiencies double encryption, non-std mode of use, session

keys, password attacks

Specified as Internet standard RFC 1510

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Kerberos 5 Message Exchange

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Next Class

X.509 certificate and authorization