Kerberos, a widely-used authentication protocol, plays a crucial role in network security by providing secure authentication in distributed environments. This paper delves into the intricacies of Kerberos, exploring its architecture, components, authentication process, strengths, weaknesses, and best practices for implementation. By understanding Kerberos thoroughly, network administrators can enhance the security posture of their systems and protect sensitive data from unauthorized access. ChatGPT 3.5 User Description for network security lecture kerberos should be about 3000 words ChatGPT Title: Securing Networks with Kerberos: An In-Depth Analysis Abstract: Kerberos, a widely-used authentication protocol, plays a crucial role in network security by providing secure authentication in distributed environments. This paper delves into the intricacies of Kerberos, exploring its architecture, components, authentication process, strengths, weaknesses, and best practices for implementation. By understanding Kerberos thoroughly, network administrators can enhance the security posture of their systems and protect sensitive data from unauthorized access. Introduction to Network Security: Overview of network security challenges Importance of authentication in securing network communications Evolution of Authentication Protocols: Early authentication methods and their limitations Introduction to Kerberos as a solution to authentication challenges Kerberos Architecture: Components of the Kerberos system: Key Distribution Center (KDC), Authentication Server (AS), Ticket Granting Server (TGS), and clients Detailed explanation of each component's role in the authentication process Authentication Process in Kerberos: Step-by-step explanation of Kerberos authentication: Client authentication request Ticket granting process Service authentication Discussion on the use of tickets and session keys for secure communication ChatGPT 3.5 User Description for network security lecture kerberos should be about 3000 words ChatGPT Title: Securing Networks with Kerberos: An In-Depth Analysis Abstract: Kerberos, a widely-used authentication protocol, plays a crucial role in network security by providing secure authentication in distributed environments. This paper delves into the intricacies of Kerberos, exploring its architecture, components, authentication process, strengths, weaknesses, and best practices for implementation. By understanding Kerberos thoroughly, network administrators can enhance the security posture of their systems and protect sensitive data from unauthorized access. Introduction to Network Security: Overview of network security challenges Importance of authentication in securing network communications Evolution of Authentication Protocols: Early authentication methods and their limitations Introduction to Kerberos as a solution to authentication challenges Kerberos Architecture: Components of the Kerberos system: Key Distribution Center

1. CSCE 815 Network Security
Lecture 10
Kerberos
X.509
February 13, 2003

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Version 4 Authentication Dialogue
Authentication Service Exchange, To obtain Ticket-Granting
Ticket
(1) C  AS: IDc || IDtgs ||TS1
(2) AS  C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || Tickettgs]
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 4.1

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

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Request for Service in Another
Realm

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Kerberos Version 5
developed in mid 1990’s
provides 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 1510

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Kerberos Version 5
V5 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 Exchanges
Table 4.3
Authentication Service Exchange: to obtain the ticket-
granting ticket
New 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 Exchange
To obtain a service-granting ticket
Both versions include
 Authenticator
 a ticket
 Name of the requested service
In addition v5 includes
 Requested times for the ticket
 Options for the ticket
 And a nonce

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Client/Server Authentication Exchange
To obtain service
Both versions include
 Authenticator
 Ticket
 Name of the requested service
In 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 4.4

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X.509 Authentication Service
part of CCITT X.500 directory service standards
 distributed servers maintaining some user info database
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
uses public-key cryptogrraphy & digital signatures
 algorithms not standardised, but RSA recommended
Used in a variety of contexts
 S/MIME chapter 5
 IP secuirty chapter 6
 SSL/TLS, SET chapter 7

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X.509 Certificates
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 CA<<A>> denotes certificate for A signed by CA

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X.509 Certificates

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Obtaining a Certificate
any user with access to CA can get any certificate from
it
only the CA can modify a certificate
because cannot be forged, certificates can be placed in
a public directory

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CA Hierarchy
if both users share a common CA then they are assumed to know
its public key
otherwise CA's must form a hierarchy
use certificates linking members of hierarchy to validate other CA's
 each CA has certificates for clients (forward) and parent (backward)
each client trusts parents certificates
enable verification of any certificate from one CA by users of all
other CAs in hierarchy

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CA Hierarchy Use

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Certificate Revocation
certificates have a period of validity
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 certificates
 the Certificate Revocation List (CRL)
users should check certs with CA’s CRL

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Authentication Procedures
X.509 includes three alternative authentication
procedures:
One-Way Authentication
Two-Way Authentication
Three-Way Authentication
all use public-key signatures

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One-Way Authentication
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
and is signed by A

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Two-Way Authentication
2 messages (A->B, B->A) which also establishes in
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
and nonce from B

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Three-Way Authentication
3 messages (A->B, B->A, A->B) which enables above
authentication without synchronized clocks
has reply from A back to B containing signed copy of
nonce from B
means that timestamps need not be checked or relied
upon

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X.509 Version 3
has been recognised that additional information is
needed in a certificate
 email/URL, policy details, usage constraints
rather than explicitly naming new fields defined a
general extension method
extensions consist of:
 extension identifier
 criticality indicator
 extension value

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Certificate Extensions
key and policy information
 convey info about subject & issuer keys, plus indicators of
certificate policy
certificate subject and issuer attributes
 support alternative names, in alternative formats for
certificate subject and/or issuer
certificate path constraints
 allow constraints on use of certificates by other CA’s

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