1. Kerberos and X.509 Fourth Edition by William Stallings
Lecture slides by Lawrie Brown
(Changed by Somesh Jha)

2. Authentication Applications
will consider authentication functions
developed to support application-level authentication & digital signatures
will consider Kerberos – a private-key authentication service
then X.509 directory authentication service

3. Kerberos trusted key server system from MIT
provides centralised private-key third-party authentication in a distributed network
allows users access to services distributed through out the network
without needing to trust all workstations
rather all trust a central authentication server
two versions in use: 4 & 5
One of the best known and most widely implemented trusted third party key distribution systems. It was developed as part of Project Athena at MIT.

4. Kerberos Requirements
first published report identified its requirements as:
security
reliability
transparency
scalability
implemented using an authentication protocol based on Needham-Schroeder

5. Kerberos 4 Overview a basic third-party authentication scheme
have an Authentication Server (AS)
users initially negotiate with AS to identify themselves
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
The core of Kerberos is the Authentication and Ticket Granting Servers – these are trusted by all users and servers and must be securely administered.

6. A Simple Authentication Dialogue
(1) C -> AS : IDC || PC || IDV
C = client
AS = authentication server
IDC = identifier of user on C
PC = password of user on C
IDV = identifier of server V
C asks user for the password
AS checks that user supplied the right password

7. Message 2 (2) AS -> C : Ticket Ticket = E K(V) [IDC || ADC || IDV]
K(V) = secret encryption key shared by AS and V
ADC = network address of C
Ticket cannot be altered by C or an adversary

8. Message 3 (3) C -> V: IDC || Ticket
Server V decrypts the ticket and checks various fields
ADC in the ticket binds the ticket to the network address of C
However, this authentication scheme has problems

9. Problems
Each time a user needs to access a different service he/she needs to enter their password
Read several times
Print, mail, or file server
Assume that each ticket can be used only once (otherwise open to replay attacks)
Password sent in the clear

10. Authentication Dialogue II
Once per user logon session
(1) C -> AS: IDC || IDTGS
(2) AS -> C: E K(C) [TicketTGS]
TicketTGS is equal to
E K(TGS) [IDC || ADC || IDTGS
|| TS1 || Lifetime1 ]

11. Explaining the fields TGS = Ticket-granting server
IDTGS = Identifier of the TGS
TicketTGS = Ticket-granting ticket or TGT
TS1 = timestamp
Lifetime1 = lifetime for the TGT
K(C) = key derived from user’s password

12. Messages (3) and (4) Once per type of service
(3) C -> TGS: IDC || IDV || TicketTGS
(4) TGS -> C : TicketV
TicketV is equal to
E K(V) [ IDC || ADC || IDV ||
TS2 || Lifetime2 ]
K(V): key shared between V and TGS
Is called the service-granting ticket (SGT)

13. Message 5 Once per service session (5) C -> V: IDC || TicketV
C says to V “I am IDC and have a ticket from the TGS” . Let me in!
Seems secure, but..
There are problems

14. Problems Lifetime of the TGT Same problem with SGT
Short : user is repeatedly asked for their password
Long : open to replay attack
Oscar captures TGT and waits for the user to logoff
Sends message (3) with network address IDC (network address is easy to forge)
Same problem with SGT

15. What should we do?
A network service (TGS or server) should be able to verify that
person using the ticket is the same as the person that the ticket was issued to
Remedy : use an authenticator
Server should also authenticate to user
Otherwise can setup a “fake” server
A “fake” tuition payment server and capture the student’s credit card
Remedy : use a challenge-response protocol

16. 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

17. Kerberos Version 5 developed in mid 1990’s
provides improvements over v4
addresses environmental shortcomings
encryption algorithm, network protocol, byte order, ticket lifetime, authentication forwarding, inter-realm authentication
and technical deficiencies
double encryption, non-standard mode of use, session keys, password attacks
specified as Internet standard RFC 1510

18. Reading assignment Inter-realm authentication in version 4 Version 5
Pages
Version 5
Fixes some shortcomings of version 4
Page

19. X.509 Authentication Service
part of CCITT X.500 directory service standards
distributed servers maintaining some info database
defines framework for authentication services
directory may store public-key certificates
with public key of user
signed by certification authority
also defines authentication protocols
uses public-key crypto & digital signatures
algorithms not standardised, but RSA recommended
X.509 is the Internationally accepted standard for how to construct a public key certificate, and is becoming widely used. It has gone through several versions. It is used by S/MIME secure , SSL/TLS secure Internet links (eg for secure web).

20. X.509 Certificates
issued by a Certification Authority (CA), containing:
version (1, 2, or 3)
serial number (unique within CA) identifying certificate
signature algorithm identifier
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
The X.509 certificate is the heart of the standard. There are 3 versions, with successively more info in the certificate - must be v2 if either unique identifier field exists, must be v3 if any extensions are used.

21. 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

22. 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
All very easy is both parties use the same CA. If not, then there has to be some means to form a chain of certifications between the CA's used by the two parties. And that raises issues about whether the CA's are equivalent, whether they used the same policies to generate their certificates, and how much you're going to trust a CA at some remove from your own. These are all open issues.

23. CA Hierarchy Use Stallings Fig 14-4. Track chains of certificates:
A acquires B certificate using chain: X<<W>>W<<V>>V<<Y>>Y<<Z>>Z<<B>>
B acquires A certificate using chain: Z<<Y>>Y<<V>>V<<W>>W<<X>>X<<A>>

24. Certificate Revocation
certificates have a period of validity
may need to revoke before expiry
user's private key is compromised
user is no longer certified by this CA
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

25. Authentication Procedures
X.509 includes three alternative authentication procedures:
One-Way Authentication
Two-Way Authentication
Three-Way Authentication
all use public-key signatures
The X.509 standard specifies the authentication protocols that can be used when obtaining and using certificates. 1-way for unidirectional messages (like ), 2-way for interactive sessions when timestamps are used, 3-way for interactive sessions with no need for timestamps (and hence synchronised clocks).

26. 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

27. 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

28. 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
Reading assignment: pages

29. X.509 Version 3
has been recognised that additional information is needed in a certificate
/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

30. 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

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