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Authentication Applications

Authentication Applications. Digital Signatures Security Concerns X.509 Authentication Service Kerberos. Security Concerns. key concerns are confidentiality and timeliness to provide confidentiality must encrypt identification and session key info

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Authentication Applications

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  1. Authentication Applications Digital Signatures Security Concerns X.509 Authentication Service Kerberos Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  2. Security Concerns • key concerns are confidentiality and timeliness • to provide confidentiality must encrypt identification and session key info • which requires the use of previously shared private or public keys • need to prove ownership of public keys • need timeliness to prevent replay attacks • provided by using sequence numbers or timestamps or challenge/response Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  3. Digital signatures • A digital signature is an encryption of a document with the creator’s private key • It is attached to a document that validates the creator of the document • Any one can validate it by decrypting the signature with the claimed creator’s public key Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  4. Some Misconceptions • Some misconceptions, corrected • Public key cryptography is NOT more secure than symmetric key • Public key cryptography does NOT Makes symmetric key obsolete • Central agent is needed for both public key and conventional cryptography Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  5. Alternatively…. Digital signatures on hashes • A more efficient way for a digital signature is by creating an authenticator of the document first (a hash) • Then sign the hash (i.e. encrypt the hash using private key) • If M is the message (or document) and Alice wants Bob to be certain that M is from her. • M is not confidential • H = Hash(M) • Authenticator = EKRA • i.e. encrypting H with Alice’s private key Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  6. Digital Signatures: The basic idea public key ? public key private key Alice Bob Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  7. Key management • Distribution of public keys • Well, what’s the issue? • Can’t we just trust Mallory if she claims a key as her public key? Mallory publickey ? publickey private key Alice Bob Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  8. Public keys to exchange secret keys • Using public-keys to exchange secret keys • why exchange secret keys? • aren’t public keys sufficient? Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  9. Authenticity of public keys Bob’s key ? private key Bob Alice public key Problem: How does Alice know that the public key she received is really Bob’s public key? Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  10. Public-key certificates • Anyone can forge public-keys • Therefore, use public-key certificates • A public-key certificate is a public-key that was signed by a trusted third party (called a certificate authority or CA) • See figure on next slide Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  11. Key Management Public-Key Certificate Use Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  12. 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. Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  13. X.509 Formats Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  14. Typical Digital Signature Approach Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  15. 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 party other than the CA can modify the certificate without this being detected. Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  16. X.509 CA Hierarchy Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  17. 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. Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  18. Authentication Procedures Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  19. 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. Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  20. 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 Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  21. Kerberos Version 4 • 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 Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  22. A Simple Authentication Dialogue • C AS: IDc ||Pc || IDv • AS  C: Ticket • C  V: IDc || Ticket Ticket = EKv[IDc ||ADc || IDv] Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  23. Version 4 Authentication Dialogue • Problems: • 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 useit before it expires Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  24. Version 4 Authentication Dialogue Authentication Service Exhange: To obtain Ticket-Granting Ticket • C AS: IDc || IDtgs ||TS1 • AS  C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || Tickettgs] Ticket-Granting Service Echange: To obtain Service-Granting Ticket (3) C  TGS: IDv ||Tickettgs ||Authenticatorc (4) TGS  C: EKc [Kc,¨v|| IDv || TS4 || Ticketv] Client/Server Authentication Exhange: To Obtain Service (5) C  V: Ticketv || Authenticatorc (6) V  C: EKc,v[TS5 +1] Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  25. Overview of Kerberos Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  26. Request for Service in Another Realm Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  27. Difference Between Version 4 and 5 • Encryption system dependence (V.4 DES) • Internet protocol dependence • Message byte ordering • Ticket lifetime • Authentication forwarding • Interrealm authentication Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

  28. 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 Kerberised applications running on all participating systems • major problem - US export restrictions though these have been relaxed in recent years Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

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