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

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

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

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 = E KR A i.e. encrypting H with Alice’s private key 5 Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

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

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? ? private key public key AliceBob Mallory 7 Based on slides by Dr. Lawrie Brown of the Australian Defence Force Academy, University College, UNSW

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

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

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

Key Management Public-Key Certificate Use 11 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 –ID c = identifier of user on C –ID v = identifier of V –P c = password of user on C –ADc = network address of C – K v = 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 (1)C  AS: ID c || P c || ID v (2)AS  C:Ticket (3)C  V: ID c || Ticket Ticket = E K v [ ID c || AD c || ID v] 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 use it 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 (1)C  AS: IDc || IDtgs ||TS 1 (2)AS  C: E Kc [K c,tgs || ID tgs || TS 2 || Lifetime 2 || Ticket tgs ] Ticket-Granting Service Echange: To obtain Service-Granting Ticket (3) C  TGS: IDv ||Ticket tgs ||Authenticatorc (4) TGS  C: E Kc [K c,¨v || IDv || TS 4 || Ticket v ] Client/Server Authentication Exhange: To Obtain Service (5) C  V: Ticket v || Authenticator c (6) V  C: E Kc,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 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