An Introduction to Kerberos

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An Introduction to Kerberos

• Shumon Huque
• ISC Networking Telecommunications
• University of Pennsylvania
• March 19th 2003

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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?

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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

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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)

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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, fromHarry Potter and
the Sorcerers Stone
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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)

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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)

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Password based Authentication

• Transmit password in clear over the network to
  the server
• Main Problem
• Eavesdropping/Interception

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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

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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)

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Symmetric Key Cryptography

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

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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

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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

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Simple shared-secret based cryptographic
authentication
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Add mutual authentication
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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

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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

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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

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Mediated Authentication
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Mediated Authentication
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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

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Kerberos (almost)
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Kerberos (roughly)
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Needham-Schroeder Protocol
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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

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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

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Kerberos without TGS

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

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Combining 2 previous diags

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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

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Ticket Granting Service (TGS)

• Motivation

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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

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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

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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

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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

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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)

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Cross Realm Authentication
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Hierarchy/Chain of Realms
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Kerberos and PubKey Crypto

• Proposed enhancements
• Public key crypto for Initial Authentication
• PKINIT
• Public key crypto for Cross-realm Authentication
• PKCROSS

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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)

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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)

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Advantages of Kerberos (2)

• Easier to recover from host compromises
• Centralized user account administration

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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

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Kerberos caveats (2)

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

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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

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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

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Questions or comments?

• Shumon Huque
• E-mail ltshuque_at_isc.upenn.edugt