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Enhancing Authentication Security: A Comprehensive Overview of Kerberos and X.509

This document explores the security concerns surrounding authentication frameworks, specifically Kerberos and X.509. It discusses the critical aspects of confidentiality, timeliness, and the prevention of replay attacks. The Kerberos protocol is analyzed, detailing its functioning, problems, and solutions involving ticket granting servers. X.509 authentication and its use in various security protocols like SSL/TLS are also covered. The evolution from Kerberos version 4 to version 5 highlights improvements in encryption systems, inter-realm authentication, and certificate authority operations.

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Enhancing Authentication Security: A Comprehensive Overview of Kerberos and X.509

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  1. Authentication Applications Ola Flygt Växjö University, Sweden http://w3.msi.vxu.se/users/ofl/ Ola.Flygt@msi.vxu.se +46 470 70 86 49

  2. Outline • Security Concerns • Kerberos • X.509 Authentication Service

  3. Security Concerns • key concerns are confidentiality and timeliness • to provide confidentiality one must encrypt identification and session key info which requires the use of previously shared private or public keys • need timeliness to prevent replay attacks.provided by using sequence numbers or timestamps or challenge/response

  4. KERBEROS In Greek mythology, a many headed dog, the guardian of the entrance of Hades

  5. KERBEROS • Users wish to access services on servers. • Three threats exist: • User pretend to be another user. • User alter the network address of a workstation. • User eavesdrop on exchanges and use a replay attack.

  6. KERBEROS • Provides a centralized authentication server to authenticate users to servers and servers to users. • Relies on conventional encryption, making no use of public-key encryption • Two versions: version 4 and 5 • Version 4 makes use of DES

  7. Kerberos terminology • Terms: • C = client • AS = authentication server • V = server • IDc = identifier of user on C • IDv = identifier of V • Pc = password of user on C • ADc= network address of C • Kv= secret encryption key shared by AS an V • TS = timestamp • || = concatenation

  8. A simple Authentication Dialogue • C  AS: IDc ||Pc || IDv • AS  C: Ticket • C  V: IDc || Ticket Ticket = EKv[IDc ||Pc || IDv]

  9. Problems with the simple dialogue • Password in clear text • Solution: Encrypt the password • Need to authenticate on each request • Solution: Let a ticket have a lifetime • Need to authenticate to each new server • Solution: Split the Kerberos server up in two parts, one Authentication Server (AS) and one Ticket Granting Server (TGS).

  10. A better Authentication Dialogue Once per user logon session: (1) C AS: IDc || IDtgs (2) AS  C: E[Kc , Tickettgs] Tickettgs=E(Ktgs, [IDc || ADc || IDtgs || TS1 || Lifetime1]) Once per type of service: (3) C  TGS: IDc || IDv ||Tickettgs TGS  C: Ticketv Ticketv=E(Kv, [IDc || ADc || IDv || TS2 || Lifetime2]) Once per server session: (5) C  V: IDc || Ticketv

  11. Problems with the better dialogue • Problem 1, Lifetime associated with the ticket-granting ticket • If too short  repeatedly asked for password • If too long  greater opportunity to replay The threat is that an opponent will steal the ticket and use it before it expires. • Problem 2, a rouge server can give incorrect credentials

  12. Version 4 Authentication Dialogue Authentication Service Exchange: To obtain Ticket-Granting Ticket (1) C AS: IDc|| IDtgs ||TS1 (2) AS  C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || Tickettgs] Tickettgs=E(Ktgs, [Kc,tgs|| IDc|| ADc || IDtgs || TS2 || Lifetime2]) Ticket-Granting Service Echange: To obtain Service-Granting Ticket (3) C  TGS: IDv||Tickettgs ||Authenticatorc TGS  C: E(Kc,tgs,[Kc,v || IDv || TS4 || Ticketv]) Tickettgs=E(Ktgs, [Kc,tgs|| IDc|| ADc || IDtgs || TS2 || Lifetime2]) Ticketv=E(Kv, [Kc,v|| IDc|| ADc || IDv || TS4 || Lifetime4]) Authenticatorc=E(Kc,tgs , [IDc|| ADc || TS3])

  13. Version 4 Authentication Dialoguecont. Client/Server Authentication Exchange to obtain Service (5) C  V: Ticketv || Authenticatorc V  C: EKc,v[TS5 +1] Ticketv=E(Kv, [Kc,v|| IDc|| ADc || IDv || TS4 || Lifetime4]) Authenticatorc=E(Kc,v , [IDc|| ADc || TS5])

  14. Overview of Kerberos

  15. Request for Service in Another Realm

  16. Difference Between Version 4 and 5 • Encryption system independence (v4 DES) • Internet protocol independence (v4 IP) • Message byte ordering (v5 ASN.1) • Ticket lifetime (v4 max 21 hours) • Authentication forwarding • Inter realm authentication

  17. Kerberos Encryption Techniques

  18. PCBC Mode

  19. Kerberos - in practice • Currently have two Kerberos versions: • 4 : restricted to a single realm • 5 : allows inter-realm authentication, in beta test • Kerberos v5 is an Internet standard • specified in RFC1510, and used by many utilities • To use Kerberos: • need to have a KDC on your network • need to have Kerberized applications running on all participating systems

  20. X.509 Authentication Service • Distributed set of servers that maintains a database about users. • Each certificate contains the public key of a user and is signed with the private key of a CA. • Is used in S/MIME, IP Security, SSL/TLS and SET. • RSA is recommended to use.

  21. X.509 Formats

  22. Typical Digital Signature Approach

  23. Obtaining a User’s Certificate • Characteristics of certificates generated by CA: • Any user with access to the public key of the CA can recover the user public key that was certified. • No part other than the CA can modify the certificate without this being detected.

  24. X.509 CA Hierarchy

  25. Revocation of Certificates • Reasons for revocation: • The users secret key is assumed to be compromised. • The user is no longer certified by this CA. • The CA’s certificate is assumed to be compromised.

  26. Authentication Procedures

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