Network Security - G. Steffen

User Authenticationfundamental security building block

basis of access control & user accountabilityis the process of verifying an identity claimed

by or for a system entityhas two steps:

identification - specify identifierverification - bind entity (person) and identifier

distinct from message authentication

Network Security - G. Steffen

Means of User Authentication four means of authenticating user's identitybased one something the individual

knows - e.g. password, PINpossesses - e.g. key, token, smartcardis (static biometrics) - e.g. fingerprint, retinadoes (dynamic biometrics) - e.g. voice, sign

can use alone or combinedall can provide user authenticationall have issues

Network Security - G. Steffen

Authentication Protocolsused to convince parties of each others

identity and to exchange session keysmay be one-way or mutualkey issues are

confidentiality – to protect session keystimeliness – to prevent replay attacks

Network Security - G. Steffen

Replay Attackswhere a valid signed message is copied

and later resentsimple replayrepetition that can be loggedrepetition that cannot be detectedbackward replay without modification

countermeasures includeuse of sequence numbers (generally

impractical)timestamps (needs synchronized clocks)challenge/response (using unique nonce)

Network Security - G. Steffen

One-Way Authenticationrequired when sender & receiver are not in

communications at same time (eg. email)have header in clear so can be delivered by

email systemmay want contents of body protected &

sender authenticated

Network Security - G. Steffen

Using Symmetric Encryptionas discussed previously can use a two-level

hierarchy of keysusually with a trusted Key Distribution

Center (KDC)each party shares own master key with KDCKDC generates session keys used for

connections between partiesmaster keys used to distribute these to them

Network Security - G. Steffen

Needham-Schroeder Protocoloriginal third-party key distribution protocolfor session between A B mediated by KDCprotocol overview is:

1. A->KDC: IDA || IDB || N1

2. KDC -> A: E(Ka,[Ks||IDB||N1|| E(Kb,[Ks||IDA])])

3. A -> B: E(Kb, [Ks||IDA])

4. B -> A: E(Ks, [N2])

5. A -> B: E(Ks, [f(N2)])

Network Security - G. Steffen

Needham-Schroeder Protocolused to securely distribute a new session key

for communications between A & Bbut is vulnerable to a replay attack if an old

session key has been compromisedthen message 3 can be resent convincing B

that is communicating with Amodifications to address this require:

timestamps in steps 2 & 3 (Denning 81)using an extra nonce (Neuman 93)

Network Security - G. Steffen

One-Way Authenticationuse refinement of KDC to secure email

since B no online, drop steps 4 & 5protocol becomes:

1. A->KDC: IDA || IDB || N1

2. KDC -> A: E(Ka, [Ks||IDB||N1 || E(Kb,[Ks||IDA])])

3. A -> B: E(Kb, [Ks||IDA]) || E(Ks, M)

provides encryption & some authenticationdoes not protect from replay attack

Network Security - G. Steffen

Kerberostrusted key server system from MIT provides centralised private-key third-party

authentication in a distributed networkallows users access to services distributed

through networkwithout needing to trust all workstationsrather all trust a central authentication server

two versions in use: 4 & 5

Network Security - G. Steffen

Kerberos Requirementsits first report identified requirements as:

securereliabletransparentscalable

implemented using an authentication protocol based on Needham-Schroeder

Network Security - G. Steffen

Kerberos v4 Overviewa basic third-party authentication schemehave 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

using a complex protocol using DES

Network Security - G. Steffen

Kerberos v4 Dialogue

Network Security - G. Steffen

Kerberos 4 Overview

Network Security - G. Steffen

Kerberos Realmsa Kerberos environment consists of:

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

this is termed a realmtypically a single administrative domain

if have multiple realms, their Kerberos servers must share keys and trust

Network Security - G. Steffen

Kerberos Realms

Network Security - G. Steffen

Kerberos Version 5developed in mid 1990’sspecified as Internet standard RFC 1510provides improvements over v4

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

lifetime, authentication forwarding, interrealm authand technical deficiencies

double encryption, non-std mode of use, session keys, password attacks

Network Security - G. Steffen

Kerberos v5 Dialogue

Network Security - G. Steffen

Remote User Authenticationin Ch 14 saw use of public-key encryption for

session key distributionassumes both parties have other’s public keysmay not be practical

have Denning protocol using timestampsuses central authentication server (AS) to

provide public-key certificatesrequires synchronized clocks

have Woo and Lam protocol using noncescare needed to ensure no protocol flaws

Network Security - G. Steffen

One-Way Authenticationhave public-key approaches for email

encryption of message for confidentiality, authentication, or both

must now public keysusing costly public-key alg on long message

for confidentiality encrypt message with one-time secret key, public-key encrypted

for authentication use a digital signaturemay need to protect by encrypting signature

use digital certificate to supply public key

Network Security - G. Steffen

Federated Identity Managementuse of common identity management scheme

across multiple enterprises & numerous applications

supporting many thousands, even millions of users principal elements are:

authentication, authorization, accounting, provisioning, workflow automation, delegated administration, password synchronization, self-service password reset, federation

Kerberos contains many of these elements

Network Security - G. Steffen

Identity Management

Network Security - G. Steffen

Identity Federation

Network Security - G. Steffen

Standards UsedSecurity Assertion Markup Language (SAML)

XML-based language for exchange of security information between online business partners

part of OASIS (Organization for the Advancement of Structured Information Standards) standards for federated identity managemente.g. WS-Federation for browser-based federation

need a few mature industry standards

Network Security - G. Steffen

Federated Identity Examples

Network Security - G. Steffen

Summaryhave considered:

remote user authentication issuesauthentication using symmetric encryptionthe Kerberos trusted key server systemauthentication using asymmetric encryptionfederated identity management

Network Security - G. Steffen