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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

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Kerberos Authentication Systems

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  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

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