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An Introduction to Kerberos

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If local workstation is compromised, user's password could be stolen by a trojan horse ... draft-ietf-krb-wg-kerberos-clarifications-03.txt. IETF Kerberos ... – PowerPoint PPT presentation

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Title: An Introduction to Kerberos


1
An Introduction to Kerberos
  • Shumon Huque
  • ISC Networking Telecommunications
  • University of Pennsylvania
  • March 19th 2003

2
What this talk is about
  • A high-level view of how Kerberos works
  • How Kerberos differs from some other
    authentication systems
  • SSH password auth, SSH public key auth, SSL
  • Target audience
  • LSPs, computing staff, others?

3
What this talk is not about
  • Details of Penns Kerberos deployment plans
  • How to get PennKeys, which Kerberos enabled
    applications do I need to use
  • Writing Kerberized applications
  • In-depth protocol details and packet formats
  • Number Theory Cryptography

4
What is Kerberos?
  • Developed at M.I.T.
  • A secret key based service for providing
    authentication in open networks
  • Authentication mediated by a trusted 3rd party on
    the network
  • Key Distribution Center (KDC)

5
Kerberos etymology
  • The 3-headed dog that guards the entrance to
    Hades
  • Originally, the 3 heads represented the 3 As
  • But one A was work enough!

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Fluffy, the 3 headed dog, from Harry Potter and
the Sorcerers Stone
8
Some Kerberos benefits
  • Standards based strong authentication system
  • Wide support in various operating systems
  • Make strong authentication readily available for
    use with campus computer systems
  • Prevents transmission of passwords over the
    network
  • Provides single-sign-on capability
  • Only 1 password to remember
  • Only need to enter it once per day (typically)

9
So, what is Authentication?
  • The act of verifying someones identity
  • The process by which users prove their identity
    to a service
  • Doesnt specify what a user is allowed or not
    allowed to do (Authorization)

10
Password based Authentication
  • Transmit password in clear over the network to
    the server
  • Main Problem
  • Eavesdropping/Interception

11
Cryptographic Authentication
  • No password or secret is transferred over the
    network
  • Users prove their identity to a service by
    performing a cryptographic operation,usually on a
    quantity supplied by the server
  • Crypto operation based on users secret key

12
Encryption and Decryption
  • Encryption
  • Process of scrambling data using a cipher and a
    key in such a way, that its intelligible only to
    the recipient
  • Decryption
  • Process of unscambling encrypted data using a
    cipher and key (possibly the same key used to
    encrypt the data)

13
Symmetric Key Cryptography
  • Aka, Secret Key cryptography
  • The same key is used for both encryption and
    decryption operations (symmetry)
  • Examples DES, 3-DES, AES

14
Asymmetric Key Cryptography
  • Aka Public key cryptography
  • A pair of related keys are used
  • Public and Private keys
  • Private key cant be calculated from Public key
  • Data encrypted with one can only be decrypted
    with the other
  • Usually, a user publishes his public key widely
  • Others use it to encrypt data intended for the
    user
  • User decrypts using the private key (known only
    to him)
  • Examples RSA

15
Communicating Parties
  • Alice and Bob
  • Alice initiator of the communication
  • Think of her as the client or user
  • Bob correspondent or 2nd participant
  • Think of him as the server
  • Alice wants to access service Bob
  • Baddies
  • Eve, Trudy, Mallory

16
Simple shared-secret based cryptographic
authentication
17
Add mutual authentication
18
Problems with this scheme
  • Poor scaling properties
  • Generalizing the model for m users and n
    services, requires a priori distribution of m x n
    shared keys
  • Possible improvement
  • Use trusted 3rd party, with which each user and
    service shares a secret key m n keys
  • Also has important security advantages

19
Mediated Authentication
  • A trusted third party mediates the authentication
    process
  • Called the Key Distribution Center (KDC)
  • Each user and service shares a secret key with
    the KDC
  • KDC generates a session key, and securely
    distributes it to communicating parties
  • Communicating parties prove to each other that
    they know the session key

20
Mediated Authentication
  • Nomenclature
  • Ka Master key for alice, shared by alice and
    the KDC
  • Kab Session key shared by alice and bob
  • Tb Ticket to use bob
  • Kdata data encrypted with key K

21
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22
Mediated Authentication
23
Mediated Authentication
24
Kerberos uses timestamps
  • Timestamps as nonces are used in the mutual
    authentication phase of the protocol
  • This reduces the number of total messages in the
    protocol
  • But it means that Kerberos requires reasonably
    synchronized clocks amongst the users of the
    system

25
Kerberos (almost)
26
Kerberos (roughly)
27
Needham-Schroeder Protocol
28
Kerberos (detailed)
  • Each user and service registers a secret key with
    the KDC
  • Everyone trusts the KDC
  • Put all your eggs in one basket, and then watch
    that basket very carefully - Anonymous Mark
    Twain
  • The users key is derived from a password, by
    applying a hash function
  • The service key is a large random number, and
    stored on the server

29
Kerberos principal
  • A client of the Kerberos authentication service
  • A user or a service
  • Format
  • name/instance_at_REALM
  • Examples
  • peggy_at_UPENN.EDU
  • ftp/pobox.upenn.edu_at_UPENN.EDU

30
Kerberos without TGS
  • A simplified description of Kerberos without the
    concept of a TGS (Ticket Granting Service)

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34
Combining 2 previous diags

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36
Review Kerberos Credentials
  • Ticket
  • Allows user to use a service (actually
    authenticate to it)
  • Used to securely pass the identity of the user to
    which the ticket is issued between the KDC and
    the application server
  • Kbalice, Kab, lifetime
  • Authenticator
  • Proves that the user presenting the ticket is the
    user to which the ticket was issued
  • Proof that user knows the session key
  • Prevents ticket theft from being useful
  • Prevents replay attacks (timestamp encrypted with
    the session key) Kabtimestamp, in combination
    with a replay cache on the server

37
Ticket Granting Service (TGS)
  • Motivation

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39
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40
Kerberos with TGS
  • Ticket Granting Service (TGS)
  • A Kerberos authenticated service, that allows
    user to obtain tickets for other services
  • Co-located at the KDC
  • Ticket Granting Ticket (TGT)
  • Ticket used to access the TGS and obtain service
    tickets
  • Limited-lifetime session key TGS sessionkey
  • Shared by user and the TGS
  • TGT and TGS session-key cached on Alices
    workstation

41
TGS Benefits
  • Single Sign-on (SSO) capability
  • Limits exposure of users password
  • Alices workstation can forget the password
    immediately after using it in the early stages of
    the protocol
  • Less data encrypted with the users secret key
    travels over the network, limiting attackers
    access to data that could be used in an offline
    dictionary attack

42
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46
Levels of Session Protection
  • Initial Authentication only
  • Safe messages
  • Authentication of every message
  • Keyed hashing with session key
  • Private messages
  • Encryption of every message
  • With session key, or mutually negotiated
    subsession keys
  • Note Application can choose other methods

47
Pre-authentication
  • Kerberos 5 added pre-authentication
  • Client is required to prove its identity to the
    Kerberos AS in the first step
  • By supplying an encrypted timestamp (encrypted
    with users secret key)
  • This prevents an active attacker being able to
    easily obtain data from the KDC encrypted with
    any users key
  • Then able to mount an offline dictionary attack

48
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49
Kerberos Two-factor auth
  • In addition to a secret password, user is
    required to present a physical item
  • A small electronic device h/w authentication
    token
  • Generates non-reusable numeric responses
  • Called 2-factor authentication, because it
    requires 2 things
  • Something the user knows (password)
  • Something the user has (hardware token)

50
Cross Realm Authentication
51
Hierarchy/Chain of Realms
52
Kerberos and PubKey Crypto
  • Proposed enhancements
  • Public key crypto for Initial Authentication
  • PKINIT
  • Public key crypto for Cross-realm Authentication
  • PKCROSS

53
Kerberos summary
  • Authentication method
  • Users enter password on local machine only
  • Authenticated via central KDC once per day
  • No passwords travel over the network
  • Single Sign-on (via TGS)
  • KDC gives you a special ticket, the TGT,
    usually good for rest of the day
  • TGT can be used to get other service tickets
    allowing user to access them (when presented
    along with authenticators)

54
Advantages of Kerberos (1)
  • Passwords arent exposed to eavesdropping
  • Password is only typed to the local workstation
  • It never travels over the network
  • It is never transmitted to a remote server
  • Password guessing more difficult
  • Single Sign-on
  • More convenient only one password, entered once
  • Users may be less likely to store passwords
  • Stolen tickets hard to reuse
  • Need authenticator as well, which cant be reused
  • Much easier to effectively secure a small set of
    limited access machines (the KDCs)

55
Advantages of Kerberos (2)
  • Easier to recover from host compromises
  • Centralized user account administration

56
Kerberos caveats
  • Kerberos server can impersonate anyone
  • KDC is a single point of failure
  • Can have replicated KDCs
  • KDC could be a performance bottleneck
  • Everyone needs to communicate with it frequently
  • Not a practical concern these days
  • Having multiple KDCs alleviates the problem
  • If local workstation is compromised, users
    password could be stolen by a trojan horse
  • Only use a desktop machine or laptop that you
    trust
  • Use hardware token pre-authentication

57
Kerberos caveats (2)
  • Kerberos vulnerable to password guessing attacks
  • Choose good passwords!
  • Use hardware pre-authentication
  • Hardware tokens, Smart cards etc

58
References
  • Kerberos An Authentication Service for Open
    Network Systems
  • Steiner, Neuman, Schiller, 1988, Winter USENIX
  • Kerberos An Authentication Service for Computer
    Networks
  • Neuman and Tso, IEEE Communications, Sep 1994
  • A Morons guide to Kerberos - Brian Tung
  • http//www.isi.edu/gost/brian/security/kerberos.ht
    ml
  • Designing an Authentication System A Dialogue in
    Four Scenes
  • Bill Bryant, 1988
  • http//web.mit.edu/kerberos/www/dialogue.html

59
References (cont)
  • RFC 1510 The Kerberos Network Authentication
    Service (v5)
  • Kohl and Neuman, September 1993
  • draft-ietf-krb-wg-kerberos-clarifications-03.txt
  • IETF Kerberos Working Group rfc1510 revision
  • Using Encryption for Authentication in Large
    Networks of Computers
  • Roger Needham, Michael D. Schroeder
  • CACM, Volume 21, December 1978, pp 993-999

60
Questions or comments?
  • Shumon Huque
  • E-mail ltshuque_at_isc.upenn.edugt
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