CS 470

1
Kerberos

• CS 470
• Introduction to Applied Cryptography
• Instructor Ali Aydin Selcuk

2
Kerberos

• Cryptographic authentication for distributed
  systems
• Based on symmetric-key authentication with KDC
• Requirements
• Security
• Reliability
• Transparency
• Scalability

3
Kerberos

• Advantages
• secure authentication
• single sign-on
• secure data flow
• Applications benefiting from Kerberos
• telnet, ftp
• BSD rtools (rlogin, rsh, rcp)
• NFS
• Others (pine, eudora, etc.)

4
Kerberos Keys

• Each principal shares a master key with KDC
• KA Alices master key. Used for initial
  authentication
• SA Alices session key. Created after initial
  authentication, used instead of KA.
• KAB Alice-Bob session key.
• Ticket Granting Tickets (TGT)
• issued to Alice by KDC after login
• contains SA encrypted with KKDC
• used to obtain session key KAB

5
Logging into the Network

• (doesnt protect against dictionary attacks with
  eavesdropping)

Alice, pwd
Alice needs a TGT
Alices terminal
Alice
KDC
KASA, TGT
6
Logging into the Network (contd)

• The workstation,
• converts Alices password into a DES key
• when receives the credentials from the server,
  decrypts them using this DES key
• if decrypts correctly, authentication is
  successful
• discards Alices master key retains the TGT.
• TGT contains all the information KDC needs about
  Alices session hence KDC can work without
  remembering any volatile data.

7
Accessing a Remote Principal
Alice, Bob, TGT, SAtimestamp
rlogin Bob

• Afterwards, the traffic between Alice Bob can
  be
• unprotected
• authenticated
• encrypted authenticated

Alices workstation
KDC
SABob, KAB, KBAlice, KAB
Alice
KBAlice, KAB, KABtimestamp
Bob
KABtimestamp1
8
Replicated KDCs

• A single KDC would be
• a performance bottleneck
• a single point of failure
• Have multiple replicas of the KDC with the
  database and the master key
• Any replica can serve as KDC for authentication
• Only one KDC (the master copy) handles the
  additions deletions of principals (for
  consistency)

9
Multiple Realms

• KDCA KDCB must have registered with each other
• Chains longer than two KDCs not allowed (v.4)

Alice, KDCB
KDCA
ticket to KDCB
Alice
Alice, Bob
KDCB
ticket to Bob
Bob
AP_REQ
10
Kerberos v5

• Platform-independent coding (ASN.1)
• Support for non-IP addresses
• non-DES encryption
• Delegation of rights
• Hierarchy of realms
• Extended ticket lifetime
• Has public-key extensions (e.g., SESAME, Win2000)