1. COEN 350
Kerberos

2. Kerberos
Provide authentication for a user that works on a workstation.
Uses secret key technology
Because public key technology still had patent projection.
Implements authentication by Needham & Schroeder.
On the market in versions 4 and 5.

3. Kerberos Kerberos consists of Key Distribution Center (KDC)
Runs on a physically secure node
Library of Subroutines
Modifies known UNIX libraries such as telnet, rlogin, …

4. Key Distribution Center
KDC:
Database of keys for all users
Invents and hands out keys for each transaction between clients.
Alice KDC Bob
Alice wants Bob
KAlice{ KAB for Bob }
KBob{KAB for Alice}

5. Key Distribution Center
Message from KDC to Bob has some problems.
Timing problem: Alice needs to wait to make sure that Bob got the key.
Change the protocol so that Alice receives a ticket to talk to Bob.

6. Key Distribution Center
Alice KDC Bob
KAlice{Use KAB for Bob}
Ticket for Bob :=
KBob{Use KAB for Alice}
Alice wants Bob
I’m Alice, my ticket is KBob{Use KAB for Alice}

7. Key Distribution Center
Needham Schroeder:
Combines KDC operation with authentication.
Uses nonces instead of timestamps to prevent replay attacks.
A (sequential / random) number used only once.

8. Needham Schroeder N1, Alice, Bob Bob Alice KDC
KAlice{N1, Bob, KAB, ticket to Bob}
Ticket, KAB{N2}
KAB{N2-1, N3}
KAB{N3-1}
Ticket = KBob{KAB, Alice}

9. Needham Schroeder N1, Alice, Bob Bob Alice Trudy (KDC) KDC
Trudy as Bob
KAlice{N1, Bob, KAB, ticket to Bob}
Ticket = KBob{KAB, Alice}, …
But the nonces make all messages unique!
Trudy can now successfully authenticate herself to Alice as Bob.
Trudy waits until Alice makes a request to the KDC.
Trudy impersonates the KDC and replays the old captured message, which looks like a normal message.
Purpose of the nonce is the following scenario:
Assume that Trudy has stolen an old key of Bob’s and stolen the message where Alice previously has requested a key. Bob has in the meantime changed his key.
Trudy now incorporates Bob.

10. Needham Schroeder
Message 2: KAlice{N1, Bob, KAB, ticket} with ticket = KBob{KAB,Alice}
N1 prevents replay attacks.
“Bob” to prevent Trudy from trying to play Bob.
Ticket does not have to be sent encrypted with Alice’s key.

11. Needham Schroeder Message 3: ticket, KAB{N2}
Alice presents a challenge together with her ticket.
Bob decodes ticket to find KAB.
He decodes the latter part of the message to find the challenge.

12. Needham Schroeder Message 4: KAB{N2-1,N3}
Bob solves Alice’s challenge.
Bob sends Alice his own challenge.
Your turn: What is the vulnerability if message 4 were to read: KAB{N2-1}, KAB{N3} ?
Answer on next two slides.

13. Needham Schroeder Answer:
Trudy eavesdrops on an exchange and then splices her own messages to Bob:

14. Needham Schroeder Bob Alice Ticket, KAB{N2} KAB{N2-1}, KAB{N3}
Replays Ticket, KAB{N2}
KAB{N2-1} KAB{N4}
Trudy (later)
Trudy now resumes her first connection: KAB{N4-1} and is authenticated
Ticket, KAB{N4}
KAB{N4-1} KAB{N5}
Trudy (second connection)

15. Needham Schroeder Expanded Needham Schroeder
Prevents replay attacks after Alice’s master key was stolen and Alice changed her master key.

16. Needham Schroeder Vulnerability Scenario
Alice has a previous key JAlice that Trudy captured.
Alice has changed her key to KAlice.
Trudy has captured a previous login request from Alice to KDC:
KDC sent JAlice{N1,Bob,JAB,KBob{JAB,Alice}}

17. Needham Schroeder Vulnerability Scenario
Trudy has JAlice{N1,Bob,JAB,KBob{JAB,Alice}}
Trudy calculates JAB and KBob{JAB,Alice} with JAlice.
Trudy now impersonates Alice to Bob. She sends her round 3 message to Bob:
N2, KBob{JAB,Alice}
She can complete the Needham Schroeder protocol with Bob.
Since the KDC no longer participates, informing the KDC of the change does not prevent Trudy from succeeding impersonating Alice to Bob.

18. Needham Schroeder Vulnerability Scenario
Trudy has
JAlice{N1,Bob,JAB,KBob{JAB,Alice}}, JAB. KBob{JAB,Alice}.
Trudy to Bob: JAB{N2}, KBob{JAB,Alice}
Bob to Trudy: JAB{N2–1, N3}
Trudy to Bob: JAB{N3–1}
Trudy and Bob are mutually authenticated!

19. Needham Schroeder Solution: Prevent replays after long duration:
Clock and date.
Certificate from Bob.
Extended Needham Schroeder picks the latter.

20. Extended Needham Schroeder
Alice to Bob: I want to talk to you.
Bob to Alice: KBob{NB}
Alice to KDC: N1, “Alice wants Bob”, KBob{NB}
KDC to Alice: KAlice{N1,“Bob”,KAB, KBob{KAB, “Alice”, NB}}
Alice to Bob: KBob{KAB, “Alice”, NB}, KAB{N2}
Bob to Alice: KAB{N2-1,N3}
Alice to Bob: KAB{N3-1}.
NB prevents the previous attack. Bob can determine whether Alice is using the key that the KDC has.

21. Extended Needham Schroeder
Alice now needs to receive a certificate from Bob before starting standard Needham Schroeder.

22. Otway Rees Replaces extended Needham Schroeder Uses only 5 messages
Speed-up results from the “suspicious party” (Bob) going to the KDC.

23. Otway Rees Alice to Bob: NC, Alice Bob KAlice{NA,NC,“A.”,“B.”}
Bob to KDC: KAlice{NA,NC, Alice, Bob, KBob{NB,NC,“A.”,“B.”}
KDC to Bob NC, KAlice{NA,KAB}, KBob{NB,KAB}
Bob to Alice: KAlice{NA,KAB}
Alice to Bob: KAB{NC}

24. Kerberos Based on Needham Schroeder, but uses time instead of nonces.
Approximate time is easy in distributed systems.

25. Kerberos Kerberos Authentication Service:
Alice to KDC N1 “Alice wants Bob”
KDC to Alice KAlice{N1, “Bob”, KAB, KBob{KAB, Alice, expir. Time}}
Alice to Bob KBob{KAB, “Alice”, expir. Time}, KAB{cur. Time}
Bob to Alice KAB{cur. Time +1}

26. Kerberos Kerberos Setup Master key shared by KDC with each principal.
When Alice logs into her machine, her station asks the KDC for a session key for Alice. The KDC also gives her a Ticket Granting Ticket. (TGT)
Alice’s workstation retains only the session key and the TGT.
Alice’s workstation uses the TGT to receive other tickets from the Ticket Granting Service (TGS).

27. Kerberos Two entities: Key distribution center.
Authentication Server (AS)
Ticket granting server (TGS).
Both need the same database, so they are usually on the same machine.

28. Kerberos Kerberos V. 4 Logging in: AS Alice Workstation AS_REQ{Alice}
KAlice
AS_REP{KAlice{SAlice,TGT}}
TGT = KKDC{Alice, SA}
Workstation calculates session key SAlice and TGT, throws KAlice away.

29. Kerberos Why wait for the password?
Workstation should know Alice’s password for minimum time.
Kerberos v. 5 changes this.
The workstation would contain data on which a password cracker could be run.

30. Kerberos Kerberos V. 5 Logging in: AS Alice Workstation AS_REQ{Alice}
AS_REP{KAlice{SAlice,TGT}}
Password?
KAlice
TGT = KKDC{Alice, SA}
Workstation calculates session key SAlice and TGT, throws KAlice away.

31. Kerberos Purpose of TGT AS, TGS does not need to retain session state.
Can recuperate quickly from a crash.

32. Kerberos Remote Login Step 1: Get a ticket for Bob.
Step 2: Use the ticket to log into Bob.

33. Kerberos Alice Workstation TGS
TGS_REQ{ Alice to Bob, TGT, SA{timestamp}}
rlogin Bob
Gets SA from TGT, verifies timestamp, creates ticket to Bob
KBob{ Alice, KAB }
TGS_REP{ SA{“Bob”, KAB, KBob{Alice, KAB}}

34. Kerberos A’s Workstation Bob AP_REQ{ KBob{Alice, KAB}, KAB{timestamp}}
Bob decrypts the ticket to find KAB.
He then checks the timestamp.
AP_REP{ KAB {timestamp + 1}}
Workstation authenticates Bob because Bob has proven he knows KAB.

35. Kerberos
After the successful rlogin, Alice and Bob are not forced to use KAB
But they can.

36. Kerberos Replicated KDC To remedy single point of failure.
To remedy bottleneck.
Critical design point is the master key database.
Can be made read-only at replicated KDC and updated by a single master.
Updates of the master key database need to be protected against substitution attacks.

37. Kerberos
Realms
Every entity in a Kerberos realm trusts the Kerberos TGS & AS.
Each realm has its own master key database.
Principals in one realm can be authenticated to principals in another realm.

38. Kerberos Realm 1 Alice Realm 2 Realm 3
Request and ticket for KDC in Realm 2
Realm 2
Request and ticket for KDC in Realm 3
Realm 3
Request

39. Kerberos Realms V4: Principal names have three strings:
service
INSTANCE
typically server on which service runs
REALM
Version 5: Uses ASN.1
now arbitrary number of fields
Realm

40. Kerberos
A single rogue KDC cannot subvert this process and grant tickets for things in other realms.

41. Kerberos Tickets contain Newly minted authentication key KAB
Name of requestor
Expiration Time
At most 23 hours

42. Kerberos Keys contain version numbers.
Network resources (incl. KDC) remember several versions of their master keys.
This allows a key change without invalidating all pending requests.
Important for batch jobs when additional authentication is not possible.

43. Kerberos Encryption is used to prevent disclosure and modification.
Key: Cannot be disclosed nor modified
Name: cannot be modified.
Version 4: Proprietary system with a few flaws
Plaintext Cipher Block Chaining

44. Kerberos: PCBC
Modifying any Ci results in modifying all subsequent Ci.
At end of message, put in something recognizable.
Flaw: Swapping to adjacent blocks, subsequent blocks are back in order.

45. Kerberos Kerberos V. 4 uses a checksum mechanism for integrity.
Algorithm is suspect, but not proven broken.
Kerberos V. 5 uses a suite of possible checksum algorithms

46. Kerberos Kerberos messages contain network addresses in the TGT.
The TGS checks for the network address when granting tickets.
This is not much of a protection
It is easy to fake network addresses
But together with a firewall might be useful to thwart attackers from outside.

47. Kerberos Kerberos puts 4B IPv4 address inside a ticket.
Recipient of ticket checks whether the source IP address is the same as in the ticket.
Prevents use of a stolen session key and TGT.
Probably not worth the trouble, since it is easy to spoof IP addresses.
Generates problems with NAT.
Makes delegation of rights difficult / impossible.

48. Kerberos Version 5 updates ASN.1 data representation language
No fixed message formats.
Adds considerable overhead.
ASN.1 is presented in COEN 351.

49. Kerberos Optional delegation of Rights:
Delegation of rights allows someone to give them their access rights for a limited scope and limited time.
Important to allow access to resources by a long-lasting batch-job.
Cannot be done by handing out the master key, or there would be no limitation to the delegation.
Handing tickets to the batch-job will not work if they are used after they expire.

50. Kerberos Optional delegation.
Kerberos v. 5 allows Alice to ask for a TGT with a network address different from her address.
This TGT is not usable by Alice, but can be used by some entity to act on Alice’s behalf.

51. Kerberos Optional delegation. Limited Delegation
Alice can give Bob tickets to the specific service that he will need acting on her behalf.
Instead of giving Bob a TGT.
Alice can give Bob a TGT with the AUTHORIZATION-DATA field specified.
This field is interpreted by the application, not Kerberos.
Application reads the field to determine what Bob can do.
OSF/DCE and Windows 2000 use this field extensively.

52. Kerberos Optional Delegation
Flag in TGT indicates whether delegation is allowed:
Forwardable Flag
TGT can be exchanged for a TGT with a different network layer address.
Alice decides whether the new TGT still has the forwardable flag set. In this way, Bob can ask Carol to act for him on behalf of Alice, …
Proxiable Flag
TGT can be used to request tickets (but not TGTs) with a different network address.

53. Kerberos Ticket Lifetimes
There is a need for longer lived tickets, but granting them in general poses security risks.
K v. 5 allows
Specifying a start time.
An end time.
Authorization time.
Renew till times.

54. Kerberos Alice can: Get a renewable ticket.
Ticket is valid for 100 years.
But Alice needs to renew it daily.
Renewing a ticket is done by
Giving the ticket to the KDC and have the KDC reissue it.
If there is something wrong, the KDC can be told to not renew the ticket.
KDC only needs to retain revocation data for the ticket lifetime.
Uses the renewable flag.

55. Kerberos Alice can: Get a postdated ticket.
Used to run a batch-job sometimes in the future.
Kerberos uses the Start-Time field to indicate the future moment when the ticket becomes valid.
Original post-dated ticket is marked invalid.
If Bob wants to use the ticket, Bob has to present it to the KDC, which clears the invalid field.
This allows revocation of postdated tickets.

56. Kerberos Key Versions KDC maintains versions of keys. Needed for
Stored as
key (encrypted version of Alice’s key)
p_kvno (Alice’s key version number)
k_kvno (Version of KDC key used to obtain key)
Needed for
Post-dated tickets
Renewable tickets

57. Kerberos Making Master Keys Different
Master keys in different realms should be different, when generated with the same password.
Kerberos v.5 uses a password to key hash function that has the realm name as an additional parameter.
Keys are different in different realms in an unpredictable way.