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Learning outcomes

Learning outcomes. At the end of this session, you should be able to: Identify applications of symmetric and public key cryptography Describe the operation and uses of Kerberos 4 Explain the operation and uses of X.509 Describe the operation and uses of Pretty-Good-Privacy (PGP).

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Learning outcomes

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  1. Learning outcomes At the end of this session, you should be able to: • Identify applications of symmetric and public key cryptography • Describe the operation and uses of Kerberos 4 • Explain the operation and uses of X.509 • Describe the operation and uses of Pretty-Good-Privacy (PGP) Internet Management & Security 06

  2. Kerberos Authentication Protocol Kerberos is a distributed authentication protocol using a key distribution centre (i.e., similar to Needham-Schroeder). Kerberos is designed for a distributed client-server network where users may access any client PC and demand service from any server. • For authenticating users to servers • For authenticating servers to users • Heavily uses symmetric encryption (e.g., DES) Internet Management & Security 06

  3. Why KERBEROS-security concerns • Among users who wish to access services on servers, • three threats exist: • User pretends to be someone else • User alters network address of a client • User eavesdrops on exchanges between others and attack by “replay” later Internet Management & Security 06

  4. Abbreviations • C: Client (workstation, computer, etc) • AS: Authentication Server (knows all the passwords) • V: Server providing services such as email, printing • IDc: ID of user on C • IDv: ID of serVer • Pc: Password of user on C • ADc: Network Address of C • Kv: encryption key shared by AS an V • TS: Time Stamp • ||: Concatenation Internet Management & Security 06

  5. Simple Dialogue Problem: a ticket is required for each service ! Problem: password is sent in plaintext • C AS: IDc ||Pc || IDv • AS  C: Ticket • C  V: IDc || Ticket Ticket = EKv[IDc||ADc || IDv] The ticket assures the server that the client has been authenticated by the authentication server (AS). Internet Management & Security 06

  6. Abbreviations • C: Client (workstation, computer, etc) • AS: Authentication Server • TGS: Ticket Granting Server • V: Server providing services such as printing • IDc: ID of user on C • IDv: ID of serVer • IDTGS: ID of Ticket Granting Server • Pc: Password of user on C • ADc: Network Address of C • KC: encryption key shared by AS and C (derived from user’s password Pc) • Kv: encryption key shared by the TGS an the service server • KTGS: encryption key shared by AS and the TGS server • TS: Time Stamp • LifeTime: validity for the ticket • ||: Concatenation Internet Management & Security 06

  7. Improved dialogue Once per user logon session (1) C  AS: IDc|| IDtgs (2) AS  C: EKc [Tickettgs] Tickettgs=EKtgs[IDc|| ADc||IDtgs||TS1||Lifetime1] Once per type of service (3) C  TGS: IDc|| IDv|| Tickettgs (4) TGS  C : Ticketv Ticketv=EKv[IDc|| ADc||IDv||TS2||Lifetime2] Once for service session (5) C  V: IDc|| Ticketv (long) (short) Internet Management & Security 06

  8. Problems Problems: • Lifetime related to ticket-granting ticket • Too short  user repeatedly asked for password • Too long  greater opportunity to replay • An opponent could steal the tickets and use them before they expire • Need to authenticate servers Internet Management & Security 06

  9. Overview of Kerberos Dialogue I Dialogue II Dialogue III Internet Management & Security 06

  10. Kerberos 4 Dialogue (I) Authentication Service Exhange: To obtain Ticket-Granting Ticket • C AS: IDc|| IDtgs ||TS1 (2) AS  C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || Tickettgs] Tickettgs= EKtgs [Kc,tgs|| IDc||ADc||IDtgs||TS2||Lifetime2] Kc,tgs: a session key between C and TGS Internet Management & Security 06

  11. Kerberos 4 Dialogue (II) • Ticket-Granting Service Echange: • To obtain Service-Granting Ticket • (3) C  TGS: IDv||Tickettgs ||Authenticatorc • TGS  C: EKc [Kc,v|| IDv || TS4 || Ticketv] • Tickettgs= EKtgs [Kc,tgs|| IDc||ADc||IDtgs||TS2||Lifetime2] • Ticketv= EKv [Kc,v|| IDc||ADc||IDv||TS4||Lifetime4] • Authenticatorc = Ectgs [IDc||ADc||TS3] Kc,v: a session key between C and server Internet Management & Security 06

  12. Kerberos 4 Dialogue (III) Client/Server Authentication Exhange: To Obtain Service (5) C  V: Ticketv || Authenticatorc • V  C: EKc,v[TS5 +1] Ticketv = EKv [Kc,v|| IDc||ADc||IDv||TS4||Lifetime4] Authenticatorc=EKc,v [IDc||ADc||TS5] Internet Management & Security 06

  13. Recommended Reading • William Stallings, “Network Security Essentials”, chapter 4 for Kerberos, chapter 3.4 for Diffie-Hellman Key Exchange Algorithm. • A. S. Tanenbaum, “Computer Networks”, chapter 8.7 for Authentication Protocols. • Bryant, W. Designing an Authentication System: A Dialogue in Four Scenes. http://web.mit.edu/kerberos/www/dialogue.html • Kohl, J.; Neuman, B. “The Evolution of the Kerberos Authentication Service” http://web.mit.edu/kerberos/www/papers.html • http://www.isi.edu/gost/info/kerberos/ Internet Management & Security 06

  14. X.509 Authentication Service What should be the exact format of a public key certificate? Who will issue certificates? • X.509 answers these questions. X.509 is a standard • that defines • the certificate format, and • the hierarchical structure to distribute certificates. Internet Management & Security 06

  15. X.509 Certificate Format Internet Management & Security 06

  16. X.509 Certificate Format The basic fields of an X.509 certificate. Internet Management & Security 06

  17. X.509 Certificate Revocation • Certificates can be revoked (cancelled) by the CA. • Certificate Revocation Lists (CRLs) are disseminated in the network periodically by the CA. Internet Management & Security 06

  18. X.509 Hierarchy Who is the Certificate Authority anyway ??? Imagine we have one CA for whole world. It doesn’t work. The CA servers would collapse under the global load. Imagine we have many CAs that are run by the same organisation, with the same private and public key. It doesn’t work. If one set of private key is used by all CAs all around the world, eventually someone will leak the private key out. Besides who will be that central organisation that runs all CAs? Internet Management & Security 06

  19. X.509 Hierarchy Regional Authority • Many CAs exist, each with its own private & public keys and registered users. • Each CAs is certified by a Regional Authority. • Each Regional Authorithy is certified by Root. • The overall hierarchy including users, certificates, CAs and directories is also called Public Key Infrastructure (PKI). Internet Management & Security 06

  20. X.509 Hierarchy • PKI is an evolving idea and a current research field. The terms Root and Regional Authority are not standard. • Today there is no one Root. And that’s normal. Noone wants to trust one root. • Each browser comes with public keys for about 100 roots, such as VeriSign, GTE, GlobalSign etc. Internet Management & Security 06

  21. X.509 Authentication Procedures Notation: Y{I}: the signing of I by Y. This indicates a packet that consists of I with an encrypted hash code appended by Y. It is assumed that the two parties, A and B, know each other’s public key, either by obtaining each other’s certificates from the directory or because the certificate is included in the initial message from each side. Internet Management & Security 06

  22. X.509 Authentication Procedures One-Way Authentication: • One-way authentication to establish • A’s ID and message’s authenticity • Message’s intended destination • Message’s integrity & originality Internet Management & Security 06

  23. X.509 Authentication Procedures Two-Way Authentication: Two-way authentication establish 1) + 2) + 3) + 4) B’s ID and reply’s authenticity 5) Intended recipient of reply 6) Reply’s integrity and originality Internet Management & Security 06

  24. X.509 Authentication Procedures Three-Way Authentication: Three-way authentication establish 1) + 2) + 3) + 4) + 5) + 6) + 7) Signed nonce if without synchronised clocks Internet Management & Security 06

  25. Pretty Good Privacy • Philip R. Zimmerman is the creator of PGP. • PGP provides a confidentiality and authentication service that can be used for electronic mail and file storage applications. Internet Management & Security 06

  26. Why Is PGP Popular? • It is availiable free on a variety of platforms. • Based on well known algorithms. • Wide range of applicability. • Not developed or controlled by governmental or standards organisations. Internet Management & Security 06

  27. Operational Description • Consist of five services: • Authentication • Confidentiality • Compression • E-mail compatibility • Segmentation Internet Management & Security 06

  28. Abbreviations Ks: session key used in symmetric encryption KRa: private key of user A, used in public-key encryption KUa: public key of user A, used in public-key encryption EP: public-key encryption DP: public-key decryption EC: symmetric encryption DC: symmetric decryption H: Hash function ||: concatenation Z: compression using ZIP algorithm R64: conversion to radix 64 ASCII format Internet Management & Security 06

  29. PGP-Authentication Only Internet Management & Security 06

  30. PGP-Confidentiality Only Source A Destination B Internet Management & Security 06

  31. PGP-Authentication and Confidentiality Source A Destination B Internet Management & Security 06

  32. Compression • PGP compresses the message after applying the signature but before encryption • The placement of the compression algorithm is critical. • The compression algorithm used is ZIP Internet Management & Security 06

  33. E-mail Compatibility • The scheme used is radix-64. • The use of radix-64 expands the message by 33%. Internet Management & Security 06

  34. Segmentation and Reassembly • Email systems are often restricted to a maximum message length of 50,000 octets. • Longer messages must be broken up into segments. • PGP automatically subdivides a message that is too large. • Segmentation is done after all other processing, including Radix 64 conversion. • The receiver strip of all e-mail headers and reassemble the block. Internet Management & Security 06

  35. PGP Generic Operation K←DKRb[EKUb[Ks]]; X←DK[X] Internet Management & Security 06

  36. Format of PGP Message Internet Management & Security 06

  37. PGP Operation and Key Rings Internet Management & Security 06

  38. PGP Operation and Key Rings Internet Management & Security 06

  39. PGP Public Key Management • Alice and Bob can physically exchange public keys. • Alice and Bob may rely on a phone conversation, if they can recognise each other’s voices, to exchange public keys. • Alice and Bob may both trust David, and David can send Alice and Bob each other’s public keys. • Alice and Bob may rely on trusted Certification Authorities. PGP supports all four options. More specifically, PGP is recognises X.509 certificates (i.e., option 4) and it recognises certificates signed by trusted individuals (i.e., option 3). Internet Management & Security 06

  40. Recommended Reading • William Stallings, “Network Security Essentials” • www.pgp.com • http://www.rubin.ch/pgp/weboftrust.en.html • https://digitalid.verisign.com/client/help/tutorial.htm • RFC 822, 2045, 2046, 2630, 2632, and 2633 Internet Management & Security 06

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