Kerberos Part 1

CNS 4650Fall 2004

Rev. 2

The Name

• Greek Mythology• Cerberus

• Gatekeeper of Hates• Only allowed in dead• Prevented dead from

leaving

• Spelling different so there is no confusion

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Time Share Computing

• One large computer

• Account information in one location

• NO encryption (dumb terminals)

• No shared media for communication• Dedicated serial lines

• No need to trust (since admin owns everything)

Time Share Computing

Client/Server

• Shared medium (network)

• Nodes can be unknown

• Power shifted from administrators to users

• Trust no one, admin know only controls half

Client/Server

Project Athena

• May 1983• 5 year charter of a consortium of

computer vendors• Notable technologies from Athena

• Kerberos• X Windows• Hesiod name service• Moira distributed network administration

Basic of Kerberos

• Secure

• Single-sign-on

• Trusted third party

• Mutual authentication

What happened to version 1, 2, 3?

• Used internally by MIT

• Never released into the wild

• Various limitations

• Mostly for testing

Three A’s

• Authentication

• Authorization

• Auditing

Authentication

• Process of verifying the identity of a user.• User is required to give information

• Factors of authentication• What the user knows• What the users has• What the user is

Example: Drivers License

• Authentication is that it is issued from an authoritative source• State• Country

• Your Picture

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What the User Knows

• Most common

• Secret password• User defined password• Random password

What the User Has

• Less common

• Some type of device• RSA SecurID

• Randomly generates key• Key matches key on authentication server

• Smart Cards

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What the User Is

• Less common

• Biometrics• Fingerprint scanning• Retina scanning• Voiceprint recognition• Face recognition

Authorization

• Granting or denying access to specific resources based on identity

• Access Control Lists• Authorization is dependent on solid

authentication!• NFS

• Server trusts client, user “authenticated” by UID• Easy to spoof, ACLs are almost worthless

Example: Drivers License

• Authorization is what you have rights to drive• Standard• Commercial • Motorcycle• Etc.

Auditing

• Records authentication and authorization

• Reactive system (does not stop attacks, just records them ;-)

Privacy and Integrity

• Encryption• Protect data from unwanted parties

• Message Integrity• Ensure the message was not tampered

• MD5• SHA1• CRC-32

Terminology

• Realms, Principles, Instances

• Keys, Salts, Passwords

• Key Distribution Center

• Tickets

Realms, Principles, Instances

• Realms• Administrative control unique to each

Kerberos installation• Convention is DNS domain in uppercase

• REALM.ORG• EXAMPLE.COM

• Realm names are case sensititive

Realms, Principles, Instances

• Principles• Every user and service has a principle• Every principle has a long term key associated

with it• Password or passphrase

• Global unique name• User or service name combined with realm name• Three components

• [username].[optional instance]@[realm] (Kerberos 4)• [component]/[component]/[component]/…@[realm]

(Kerberos 5)

Realms, Principles, Instances

• Principles• Kerberos 4 Examples

• dsinema@REALM.ORG• dsinema.admin@REALM.ORG• smb.server@REALM.ORG

• Kerberos 5 Examples• dsinema@REALM.ORG• dsinema/admin@REALM.ORG• smb/server.realm.org@REALM.ORG• smb/server.differentrealm.org/REALM.ORG

Realms, Principles, Instances

• Instances• Used in two situations

• Service principles• Special principles for administrative purposes

• Example• Admins can have two principles

• One for day to day (dsinema@REALM.ORG)• One for administrative tasks

(dsinema/admin@REALM.ORG)

Keys, Salts, Passwords

• Keys• Are shared between at least two parties

• End user, service, or KDC

• String2key converts password to encryption key

• Salt is added before password is hashed• Kerberos 5 default “salt” is realm name

The Key Distribution Center (KDC)

• Three components• Principle database• Authentication server• Ticket Granting Server

The Key Distribution Center (KDC)

• Principle Database• Stores principles and associated keys• Stores other information

• Password lifetimes• Last password change

• MIT stores in lightweight database

The Key Distribution Center (KDC)

• Authentication Server• Issues Ticket Granting Ticket (TGT)

• Passwords never cross wire• TGT encrypted with users password• TGT can then be used to request service

tickets• TGT provides “single-sign-on”

The Key Distribution Center (KDC)

The Key Distribution Center (KDC)

• Ticket Granting Server (TGS)• Takes two pieces of data

• Principle name of service requested• Users Ticket Granting Ticket (TGT)

• TGS verifies TGT, then issues a service ticket to the user

Tickets

• Encrypted data structure• Requesting principle name• Service principle name• Ticket lifetime• IP Addresses the ticket can be used from• Session key (shared secret) for

user/service communication

Tickets

• Service Tickets• User requests from TGS• Session Key for communication• Data encrypted with service key, which

contains the Session key• All is encrypted with user key

Tickets