CSCE 815 Network Security CSCE 815 Network Security Lecture 10 Lecture 10CSCE 815 Network Security CSCE 815 Network Security Lecture 10 Lecture 10

KerberosKerberos

X.509X.509

February 13, 2003

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Version 4 Authentication DialogueVersion 4 Authentication Dialogue

Authentication Service Exchange, To obtain Ticket-Granting Authentication Service Exchange, To obtain Ticket-Granting TicketTicket

(1) C (1) C AS: AS: ID IDcc || ID || IDtgstgs ||TS ||TS11

(2) AS (2) AS C: C: E EKc Kc [K[Kc,tgsc,tgs|| ID|| IDtgstgs || TS || TS2 2 || Lifetime|| Lifetime2 2 || Ticket|| Tickettgstgs]]

Ticket-Granting Service Exchange: To obtain Service-Granting Ticket

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

(4) TGS C: EKc [Kc,¨v|| IDv || TS4 || Ticketv]

Client/Server Authentication Exchange: To Obtain Service

(5) C V: Ticketv || Authenticatorc

(6) V C: EKc,v[TS5 +1]

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Kerberos 4 Overview Fig 4.1Kerberos 4 Overview Fig 4.1

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Kerberos RealmsKerberos Realms

a Kerberos environment consists of: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 realmthis is termed a realm typically a single administrative domain

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

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Request for Service in Another RealmRequest for Service in Another Realm

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Kerberos Version 5Kerberos Version 5

developed in mid 1990’sdeveloped in mid 1990’s

provides improvements over v4provides improvements over v4 addresses environmental shortcomings

encryption algorithm – v4 DES based, v5 tags ciphertext by type of encryption

IP dependence – v4 requires internet Protocol (IP), v5 general byte order - v5 message defined using Abstract Syntax Notation(ASN.1)

and encoded with Basic Encoding Rules (BER) ticket lifetime – 8 bits (255)x five minutes = 21+ hours; v5 start/stop

times authentication forwarding: In v5 one server can forward credentials to

another e.g., a print server can forward credentials to file server so that a file can be printed

interrealm authorization – In v4 n realms n(n-1)/2 relationships

and technical deficiencies

specified as Internet standard RFC 1510specified as Internet standard RFC 1510

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Kerberos Version 5Kerberos Version 5

V5 solves technical deficiencies of v4V5 solves technical deficiencies of v4 double encryption: tickets provided to clients are encrypted

twice (fig 4.1) PCBC encryption: v4 uses propagating block chaining

(PCBC) - non-std and vulnerable to ciphertext block interchange attack

session keys – a key used by the client to encrypt the AS to the service; however it may be reused to gain access to the service again. V5 allows subsession keys to prevent replays.

password attacks – both versions are susceptible to attacks on the password. The message from the AS to the client is encrypted with a key based on the client’s password. This can be captured and then attempts to decrypt and figure out the password.

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Kerberos Version 5 Message ExchangesKerberos Version 5 Message Exchanges

Table 4.3Table 4.3

Authentication Service Exchange: to obtain the ticket-Authentication Service Exchange: to obtain the ticket-granting ticketgranting ticket

New additionsNew additions Realm of the user Options Times: from, till, renewTime Nonce – random value to be repeated in response to insure

freshness

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Ticket-Granting Service ExchangeTicket-Granting Service Exchange

To obtain a service-granting ticketTo obtain a service-granting ticket

Both versions includeBoth versions include Authenticator a ticket Name of the requested service

In addition v5 includesIn addition v5 includes Requested times for the ticket Options for the ticket And a nonce

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Client/Server Authentication ExchangeClient/Server Authentication Exchange

To obtain serviceTo obtain service

Both versions includeBoth versions include Authenticator Ticket Name of the requested service

In addition v5 includesIn addition v5 includes Options for mutual authentication Subkey – client’s choice of encryption key (default is Kc,v)

Sequence number – used to detect replays

Ticket Flags table 4.4Ticket Flags table 4.4

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X.509 Authentication Service X.509 Authentication Service

part of CCITT X.500 directory service standardspart of CCITT X.500 directory service standards distributed servers maintaining some user info database

defines framework for authentication services defines framework for authentication services directory may store public-key certificates with public key of user signed with private key by certification authority

also defines authentication protocols also defines authentication protocols

uses public-key cryptogrraphy & digital signatures uses public-key cryptogrraphy & digital signatures algorithms not standardised, but RSA recommended

Used in a variety of contextsUsed in a variety of contexts S/MIME chapter 5 IP secuirty chapter 6 SSL/TLS, SET chapter 7

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X.509 CertificatesX.509 Certificates

issued by a Certification Authority (CA), containing (fig 4.3): issued by a Certification Authority (CA), containing (fig 4.3): version (1, 2, or 3) serial number (unique within CA) identifying certificate signature algorithm identifier and parameters issuer X.500 name (CA) period of validity (from - to dates) subject X.500 name (name of owner) subject public-key info (algorithm, parameters, key) issuer unique identifier (v2+) subject unique identifier (v2+) extension fields (v3) signature (of hash of all fields in certificate)

notation notation CA<<A>>CA<<A>> denotes certificate for A signed by CA denotes certificate for A signed by CA

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X.509 CertificatesX.509 Certificates

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Obtaining a Certificate Obtaining a Certificate

any user with access to CA can get any certificate from any user with access to CA can get any certificate from it it

only the CA can modify a certificate only the CA can modify a certificate

because cannot be forged, certificates can be placed in because cannot be forged, certificates can be placed in a public directory a public directory

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CA Hierarchy CA Hierarchy

if both users share a common CA then they are assumed to know if both users share a common CA then they are assumed to know its public key its public key

otherwise CA's must form a hierarchy otherwise CA's must form a hierarchy

use certificates linking members of hierarchy to validate other use certificates linking members of hierarchy to validate other CA's CA's each CA has certificates for clients (forward) and parent (backward)

each client trusts parents certificates each client trusts parents certificates

enable verification of any certificate from one CA by users of all enable verification of any certificate from one CA by users of all other CAs in hierarchy other CAs in hierarchy

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CA Hierarchy UseCA Hierarchy Use

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Certificate RevocationCertificate Revocation

certificates have a period of validitycertificates have a period of validity

may need to revoke before expiry, eg:may need to revoke before expiry, eg:1. user's private key is compromised

2. user is no longer certified by this CA

3. CA's certificate is compromised

CA’s maintain list of revoked certificatesCA’s maintain list of revoked certificates the Certificate Revocation List (CRL)

users should check certs with CA’s CRLusers should check certs with CA’s CRL

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Authentication ProceduresAuthentication Procedures

X.509 includes three alternative authentication X.509 includes three alternative authentication procedures: procedures:

One-Way Authentication One-Way Authentication

Two-Way Authentication Two-Way Authentication

Three-Way Authentication Three-Way Authentication

all use public-key signaturesall use public-key signatures

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One-Way AuthenticationOne-Way Authentication

1 message ( A->B) used to establish 1 message ( A->B) used to establish the identity of A and that message is from A message was intended for B integrity & originality of message

message must include timestamp, nonce, B's identity message must include timestamp, nonce, B's identity and is signed by A and is signed by A

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Two-Way AuthenticationTwo-Way Authentication

2 messages (A->B, B->A) which also establishes in 2 messages (A->B, B->A) which also establishes in addition:addition: the identity of B and that reply is from B that reply is intended for A integrity & originality of reply

reply includes original nonce from A, also timestamp reply includes original nonce from A, also timestamp and nonce from Band nonce from B

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Three-Way AuthenticationThree-Way Authentication

3 messages (A->B, B->A, A->B) which enables above 3 messages (A->B, B->A, A->B) which enables above authentication without synchronized clocks authentication without synchronized clocks

has reply from A back to B containing signed copy of has reply from A back to B containing signed copy of nonce from B nonce from B

means that timestamps need not be checked or relied means that timestamps need not be checked or relied upon upon

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X.509 Version 3X.509 Version 3

has been recognised that additional information is has been recognised that additional information is needed in a certificate needed in a certificate email/URL, policy details, usage constraints

rather than explicitly naming new fields defined a rather than explicitly naming new fields defined a general extension methodgeneral extension method

extensions consist of:extensions consist of: extension identifier criticality indicator extension value

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Certificate ExtensionsCertificate Extensions

key and policy informationkey and policy information convey info about subject & issuer keys, plus indicators of

certificate policy

certificate subject and issuer attributescertificate subject and issuer attributes support alternative names, in alternative formats for

certificate subject and/or issuer

certificate path constraintscertificate path constraints allow constraints on use of certificates by other CA’s

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SummarySummary

have considered:have considered: Kerberos trusted key server system X.509 authentication and certificates