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Chapter 14 Authentication Applications

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Title: Chapter 14 Authentication Applications


1
Chapter 14 Authentication Applications
  • Fourth Edition
  • by William Stallings
  • Lecture slides by Lawrie Brown
  • (modified by Prof. M. Singhal, U of Kentucky)

2
Authentication Applications
  • developed to support application-level
    authentication digital signatures
  • will discuss Kerberos a private-key
    authentication service
  • discuss X.509 - a public-key directory
    authentication service

3
Kerberos
  • Authentication service developed as a part of
    MITs Athena project
  • provides centralized private-key third-party
    authentication in a distributed network
  • allows users access to services distributed
    through network
  • without needing to trust all workstations
  • rather all trust a central authentication server
  • two versions in use 4 5

4
Athena
  • An open distributed environment
  • Any user can access services from any workstation
  • Several security threats exists in such an
    environment
  • A user impersonate another user
  • A user may change the network address of a w/s
    and may make it look as another w/s
  • A user may eavesdrop on a session and mount a
    replay attak later

5
Kerberos Requirements
  • its first report identified requirements as
  • secure
  • reliable
  • transparent
  • scalable
  • implemented using an authentication protocol
    based on Needham-Schroeder

6
Kerberos v4 Overview
  • a basic third-party authentication scheme
  • have 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

7
Kerberos v4 Dialogue
  • obtain ticket granting ticket from AS
  • once per session
  • obtain service granting ticket from TGT
  • for each distinct service required
  • client/server exchange to obtain service
  • on every service request

8
Kerberos 4 Overview
9
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, Kerberos servers must
    share keys and trust each other

10
Kerberos Realms
11
Kerberos Version 5
  • developed in mid 1990s to address the
    deficiencies of v4
  • provides improvements over v4
  • encryption algorithm DES is weak and vulnerable
    to attacks. V5 allows a suit of encryption
    algorithms.
  • V5 breaks away from IP only networks
  • V4 uses 8bit ticket lifetime.V5 uses start time
    and end time.

12
X.509 Authentication Service
  • part of CCITT X.500 directory service standards
  • distributed servers maintaining user 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 certificates are widely used

13
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 CAltltAgtgt denotes certificate for A signed
    by CA

14
X.509 Certificates
15
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
  • If there are a large number of users, one CA may
    not be able to handle the load
  • Also it is difficult to propagate the public key
    of the CA securely.

16
Certificate Chaining
  • if both users share a common CA then they are
    assumed to know its public key
  • What if both users have their certificates issued
    by two different CAs? (and one does not know the
    public key of the other CA)
  • Suppose As certificate is issued by X1 and Bs
    by X2
  • And A does not know the public key of X2.
  • (A can not verify the public key of B).

17
Certificate chaining
  • Suppose X1 and X2 have securely exchanged their
    public keys.
  • X1 can prepare a certificate for X2 and sends it
    to A.
  • A can request this certificate from X1, obtain
    the public key of X2, and then verify Bs
    certificate.
  • Notationally,
  • X1ltltX2gtgtX2ltltBgtgt
  • --Chain of two certficates.
  • --need not be limited to two certificates.

18
CA Hierarchy
  • CAs can certify each other.
  • CAs are linked by this relation.
  • CAs can be organized in several structures
  • X.509 suggests 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

19
A CA Hierarchy
20
CA Hierarchy
  • A can verify Bs certificate using the following
    certificate chain
  • XltltWgtgtWltltVgtgtVltltYgtgtYltltZgtgtZltltBgtgt
  • -- There is chain of trust also.
  • Likewise, B can verify As public key using the
    following certificate chain
  • ZltltYgtgtYltltVgtgtVltltWgtgtWltltXgtgtXltltAgtgt
  • --can obtain these certificates from the
    directory.

21
Certificate Revocation
  • certificates have a period of validity
  • may need to revoke before expiry, e.g.
  • user's private key is compromised
  • user is no longer certified by this CA
  • CA's certificate is compromised
  • CAs maintain list of revoked certificates
  • the Certificate Revocation List (CRL)
  • CRL is advertised widely through directory.
  • users should check certificates with CAs CRL

22
Authentication Procedures
  • X.509 includes three alternative authentication
    procedures
  • One-Way Authentication
  • Two-Way Authentication
  • Three-Way Authentication
  • all use public-key signatures
  • It is assumed that the two parties know each
    others public key.

23
One-Way Authentication
  • 1 message ( A-gtB) 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
  • may include additional info for B
  • E.g., session key

24
Two-Way Authentication
  • 2 messages (A-gtB, B-gtA) 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
  • may include additional info for A

25
Three-Way Authentication
  • 3 messages (A-gtB, B-gtA, A-gtB) 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

26
Summary
  • have considered
  • Kerberos trusted key server system
  • X.509 authentication and certificates
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