Authentication ApplicationsThe Kerberos Protocol Standard

Rabie A. Ramadan

Lecture 7

Outline

I. Introduction

II. Introduction to Kerberos v4

III. Details of Kerberos v4

IV. Kerberos v5

V. Realms and Inter-Realm Authentication

Introduction

Introduction Open distributed environment

• Users at workstations wish to access servers distributed throughout the network

• Servers must restrict access to authorized users

• Servers must authenticate request for service

Workstation

Introduction

Workstation (your computer) can not be trusted to authenticate its user correctly to network services:

Three threats exist:

• User pretends to be another user.

• User alters the network address of a workstation.

• User eavesdrops on exchanges and use a replay attack.

Solutions

Server (V)

Authentication Server (AS)

Client (C)

AS knows all the passwords of all users

AS shares unique secret Keys with each server

A Simple Authentication Dialogue

Security holes? • Password sent as plain ASCII

• No time limits for tickets

• Man-in-the middle can steal the ticket and fake IDC ( simple, because it is sent as clear text).

Server (V)

Authentication Server (AS)

Client (C)

More Secure Dialogue Idea:-Introducing a Ticket Granting Server (TGS)

- Kc’ : A key that is derived from the user password

Problems with the Previous Protocol

Problems ?1. Lifetime associated with the ticket-granting ticket:

If too short → the user is repeatedly asked for the password

If too long → a greater opportunity to replay exists.• The threat is that an opponent will steal the ticket and use it

before it expires.

2. There may be a requirement for servers to authenticate themselves to users. • The false server would then be in a position to act as a real

server and capture any information from the user and deny the true service to the user.

What is Kerberos? Network authentication protocol

Developed at MIT in the mid 1980s

Relies on conventional encryption, making no use of public-key encryption.

Available as open source or in supported commercial software

Two versions: version 4 (passing slowly away) and 5 coexist.

Kerberos 4 Overview

Version 4: Authentication Dialogue

Authenticatorc

First Step : C to AS

Second step: AS to C

Third step: C to TGS

Fourth step: TGS to C

Fifth step: C to V

Sixth step: V to C

Kerberos Realms

A 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 a single administrative domain

Request for Service in Another Realm V.4

Users on one realm may need access to servers in other realms

Please consult the book for more details

Difference Between Version 4 and 5 Encryption system dependence (v.4 DES)

Message byte ordering (v.4 arbitrary; v.5 defined by ASN1 Standard)

Ticket lifetime (v.4 21h max; v.5 arbitrary)

Authentication forwarding to other hosts (v.4 no; v.5 yes),• A client accesses a server. The server can not act on another

server on behalf of the client)

Inter-realm authentication: v.4 (v5. simpler)

Kerberos Version 5

Developed in mid 1990’s Provides improvements over v4

• addresses environmental shortcomings• encryption algoithms, network protocol, byte order,

ticket lifetime, authentication forwarding, interrealm authentication

Specified as Internet standard RFC 1510

New Fields in V5

Realm: Indicates realm of userOptions: Used to request that certain flags be set in the returned ticketTimes: Used by the client to request the following time settings in the ticket:

from: the desired start time for the requested tickettill: the requested expiration time for the requested ticketrtime: requested renew-till time

Nonce: A random value to be repeated in message (2) to assure that the response is fresh and has not been replayed by an opponent

New Fields in V5

Subkey: The client's choice for an encryption key to be used to protect this specific application session. If this field is omitted, the session key from the ticket (Kc,v) is used.

Sequence number: An optional field that specifies the starting sequence number to be used by the server for messages sent to the client during this session. Messages may be sequence numbered to detect replays.

Kerberos Limitations Every network service must be individually modified for use with Kerberos

Doesn’t work well in time sharing environment

Requires a secure Kerberos Server

Requires a continuously available Kerberos Server

Stores all passwords encrypted with a single key

Assumes workstations are secure

Scalability