1 / 31

Kerberos Authentication Systems

Kerberos Authentication Systems. KERBEROS. In Greek mythology, a many headed dog, the guardian of the entrance of Hades (Hell). Outline. Authentication in Campus Kerberos 4 Realms (Domains) under Kerberos 4. Authentication in Campus. Workstations, Servers are distributed

perdy
Download Presentation

Kerberos Authentication Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Kerberos Authentication Systems Prof. R. Aviv: Kerberos

  2. KERBEROS In Greek mythology, a many headed dog, the guardian of the entrance of Hades (Hell) Prof. R. Aviv: Kerberos

  3. Outline • Authentication in Campus • Kerberos 4 • Realms (Domains) under Kerberos 4 Prof. R. Aviv: Kerberos

  4. Authentication in Campus • Workstations, Servers are distributed • Users/Clients: anyone, log in at any Wstn • Servers software: • need to authenticate and authorize users Can we trust Workstation software to authenticate users on behalf of servers What are the threats? Prof. R. Aviv: Kerberos

  5. Authentication in Campus • Threats: actions enabling unauthorized users to gain access to services and data • User pretend to be another user. • User alter the network address of a workstation. • User listens to exchanges and use a replay attack. How Users and Servers authenticate each other? Prof. R. Aviv: Kerberos

  6. Approaches to Authentication • Need Mutual Authentication • Workstation cannot hold policy for all servers all users why not? • Use a trusted Authentication Server - KERBEROS Server • Kerberos Server holds policy – which users can access which servers, and keys for all principals Should we use Symmetric or A-symmetric keys Prof. R. Aviv: Kerberos

  7. KERBEROS • Centralized Authentication Server • authenticating users to servers and v,v • Relies on conventional encryption • no use of public-key encryption – unlike PKI • Long term shared secrets between Kerberos and Servers and Users (not with client wstn) What are the requirements from the Kerberos Protocol? Prof. R. Aviv: Kerberos

  8. Kerberos Protocol Requirements • Partners are authenticated continuously • Partners: Client, K Servers, Server • Messages between Kerberos Server and others encrypted by secret short lived keys • Little user involvement Prof. R. Aviv: Kerberos

  9. Kerberos Protocol: 3 phases Server Ticket Granting Server (TGS) Server Server Print Server 2. Authorization 3. Start service Kerberos Database Kerberos Database Workstation: K Client 1. Authentication Authentication Server (AS) Assume user wants to print Prof. R. Aviv: Kerberos

  10. Kerberos Version 4: Items/Notation • C = Kerberos Client module (on the workstation) • AS = Kerberos Authentication Server • TGS = Ticket Granting Server • tgt= Ticket Granting Ticket • V = server (e.g. mail server, ftp server, print server) • IDU = identifier of user on C (e.g. name, email address) • IDv = identifier of server V (e.g. Server IP address+port) • PU = password of user, known to AS • ADc= network address of Client (user’s workstation) • Kv= secret encryption key shared by TGS and V • TS = timestamp Prof. R. Aviv: Kerberos

  11. Basic Kerberos Protocol • 1.K Client sends (User ID: IDU) and PU ? • 2. PU is known to AS. AS sends back a packet with tgt encrypted (DES); key derived from PU • Kerberos Client requests password from user • K Client decrypts packet, finding User’s ID and its own address (ADc) inside, in correct format • ensuring user knows PU; user authenticated What will be the usage of tgt ? Prof. R. Aviv: Kerberos

  12. Basic Kerberos Protocol • tgt: User authenticated, allowed to get ticket • 3. K client sends tgt to the Ticket Granting Server, (TGS) is it now encrypted? • 4. K. client receives a Service Ticket,ticketv • 5. ticketv (client credentials) sent to server V • ticketv encrypted by secret known to V and TGS • tgt, ticketv have some lifetime Prof. R. Aviv: Kerberos

  13. Authenticating Phase: Once per logon • tgt is ”sealed” cannot be read by anyone but TGS (not even by User or Client); Why? • Could the ticket sent directly to TGS? • Timestamp - against replay (1) C  AS: IDU|| Idtgs (2) AS  C: EKc [tgt|| IDU] tgt = EKtgs[IDU || ADc || IDtgs ||TS1 || Lifetime1] What’s the meaning of the tgt Prof. R. Aviv: Kerberos

  14. Client request and get ticketv: Once per service • IDUappears twice where? • ADc appears twice where Why?Problem? • C  TGS: IDV||tgt||IDU • can attacker forge IDU? • can attacker impersonate the User? • (4) TGS  C: ticketv • ticketv = EKv[IDU || ADc || IDv ||TS2 || Lifetime2] Prof. R. Aviv: Kerberos

  15. Client getting Service: Once per service session • Server V sees its own ID  ticketv is genuine • V compares client address in IP header & ticketv • V compares user ID in packet and ticketv Problem? (5) C  V: ticketV|| IDU ticketV = EKv[IDU|| ADC || IDV ||TS2 || Lifetime2] Prof. R. Aviv: Kerberos

  16. The Lifetime of tgt • If too short user will repeatedly enter password. • If too long, an attacker might reuse message 3 (with forged IDU, ADc) before tgt expires • Hence: TGS must authenticate Client again • Client adds secret authenticator to message 3 • For this Client and TGS Need a shared secret How would they get it? • AS to send a shared secret to both in message 2 Prof. R. Aviv: Kerberos

  17. New Message 2: AS sends to client a tgt, AND a shared TGS-session key to be used in message 3 tgt: login name (IDU) TGS name net address (ADc) TGS session key TGS session key Encrypted with user key Encrypted with TGS key Who knows the session key? Prof. R. Aviv: Kerberos

  18. New Message 3: Client authenticates itself to TGS, requests ticket • 3 parts message • tgt (previously obtained from AS), Encrypted by key known only to TGS (and AS) • authenticator: encrypted with TGS session key • Server (V) ID encrypted with TGS key tgt authenticator encrypted with TGS session key Server V Name Prof. R. Aviv: Kerberos

  19. The Lifetime of ticketv • If lifetime is long, attacker might reuse it How? • V must authenticate the Client again • Solution: AS gives in message 4 to both V and the K client a one time, shared session key, Kc,v • Client attach authenticator to its message 5 Prof. R. Aviv: Kerberos

  20. New Message 4: TGS sends to client a ticketvAND a shared V session key to be used in message 5 ticketv: login name IDU V name net address ADc V session key V session key Encrypted with V key Encrypted with user key Prof. R. Aviv: Kerberos

  21. New Message 5: Client authenticates itself to V, requests service sealed with V key ticketv authenticator sealed with V session key Server Name • 3 parts message • ticketv(previously obtained from TGS), Encrypted by key known only to V • authenticator: encrypted with V session key • Server (V) ID Prof. R. Aviv: Kerberos

  22. Phase 1: User gets ticket granting ticket, and Client Authenticates the User • Authentication Service Exchange: 1. C  AS: IDU|| IDtgs ||TS1 2. AS  C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || tgt] Shared Session key Prof. R. Aviv: Kerberos

  23. Phase 2: TGS authenticates User, User gets ticketv • Authenticator: Info about the Client (User name, IP Address, Timestamp) encrypted with shared secret. Expires immediately • 3. C  TGS: IDv || tgt || authenticatorc • 4. TGS  C: EKc [Kc,v|| IDv || TS4 || ticketv] • tgt = EKtgs[Kc,tgs || IDU|| ADC || IDtgs || TS2 || Lifetime2] • ticketv = EKv[Kc,v || IDU || ADC || IDV || TS4 || Lifetime4] • authenticatorc= EKc,tgs[IDU || ADc || TS3] Prof. R. Aviv: Kerberos

  24. Phase 3: V Server, User authenticate each other, User gets service • 6. Client Authenticate the Server: • Server reply: TS5+1, encrypted by the shared session key (Kc,v) • 5. C  V: ticketv|| authenticatorc • V  C: EKc,v[TS5 +1] • ticketv = EKv[Kc,v || IDU || ADC || IDV || TS4 || Lifetime4] • authenticatorc = EKc,v[IDU || ADC || TS3] Prof. R. Aviv: Kerberos

  25. Summary of Kerberos 4 Protocol Prof. R. Aviv: Kerberos

  26. Realm (Domain) • Organization is organized in Realms (Domains) • A Realm (e.g. faculty) under a single admin • Includes: AS, TGS, Clients, service Servers • The TGS must share a secret key with each Server in its Realm • All Servers in a Realm register with the TGS Prof. R. Aviv: Kerberos

  27. Inter-operating Realms • Users in one realm might need access to Servers in another, interoperating realm example? • TGS in realms register with all other AS • AS in a realm trust other AS to authenticate its users • Servers in one realm trust TGS of the other realm Prof. R. Aviv: Kerberos

  28. User access Server RV in a remote Realm • Client applies to local AS for a Tickettgs for local TGS • 1. CAS: IDU || IDtgs || TS1 • 2. AS C: EKc[Kc,tgs || IDtgs || TS2 ||LT2||tgt] Prof. R. Aviv: Kerberos

  29. User access Server RV in a remote Realm • Client applies to local TGS for a tgtr for (remote) R-TGS • 3. C TGS: IDR-TGS || tgt || authenticatorc • 4. TGS  C: EKc,tgs[Kc,r-tgs || IDR-TGS || TS4 || tgtr] Prof. R. Aviv: Kerberos

  30. User access Server RV in a remote Realm • Client applies to RTGS for a ticketrv for RV Server • 5. C  R-TGS: IDRV || tgtr || authenticatorc • 6. R-TGSC: EKc,rtgs[Kc,rv || IDrv || TS6 || ticketrv] • Client connect to remote server • 7. C  RV: ticketrv|| authenticatorc Prof. R. Aviv: Kerberos

  31. Prof. R. Aviv: Kerberos

More Related