1. CSCE 815 Network Security Lecture 9 Digital Signatures & Authentication Applications Kerberos February 13, 2003

2. – 2 – CSCE 815 Sp 03 Resources Stallings Web Site: http://williamstallings.com/ http://williamstallings.com/ http://web.mit.edu/kerberos/www/ http://web.mit.edu/kerberos/www/dialogue.html http://web.mit.edu/kerberos/www/papers.html Kohl’s paper “ The Evolution of Kerberos…”

3. – 3 – CSCE 815 Sp 03 http://web.mit.edu/kerberos/www/ What is Kerberos? What is Kerberos? Security Advisories Security Advisories Kerberos Releases Kerberos V5 Release 1.2 Historical releases of MIT krb5 Getting Kerberos Sources and Binaries from MIT (US and Canada only) Getting Kerberos Sources from the Crypto Publishing Project The krb5-current Snapshots (for developers; US, Canada only) The krb5-current Snapshots Documentation for the most recent release Documentation for the most recent release Papers about the Kerberos protocol Papers about the Kerberos protocol Frequently Asked Questions The comp.protocols.kerberos FAQ (at NRL; maintained by Ken Hornstein)comp.protocols.kerberos FAQ How do the new US export regulations affect Kerberos? Other Resources Mailing lists comp.protocols.kerberos newsgroup comp.protocols.kerberos USC/ISI Kerberos Page Oak Ridge National Laboratory's "How to Kerberize your Site"

4. – 4 – CSCE 815 Sp 03 Digital Signatures have looked at message authentication but does not address issues of lack of trust digital signatures provide the ability to: verify author, date & time of signature authenticate message contents be verified by third parties to resolve disputes hence include authentication function with additional capabilities

5. – 5 – CSCE 815 Sp 03 Digital Signature Properties must depend on the message signed must use information unique to sender to prevent both forgery and denial must be relatively easy to produce must be relatively easy to recognize & verify be computationally infeasible to forge with new message for existing digital signature with fraudulent digital signature for given message be practical save digital signature in storage

6. – 6 – CSCE 815 Sp 03 Direct Digital Signatures involve only sender & receiver assumed receiver has sender’s public-key digital signature made by sender signing entire message or hash with private-key can encrypt using receivers public-key important that sign first then encrypt message & signature security depends on sender’s private-key

7. – 7 – CSCE 815 Sp 03 Replay Attacks where a valid signed message is copied and later resent simple replay repetition that can be logged repetition that cannot be detected backward replay without modification countermeasures include use of sequence numbers (generally impractical) timestamps (needs synchronized clocks) challenge/response (using unique nonce)

8. – 8 – CSCE 815 Sp 03 Digital Signature Standard (DSS) US Govt approved signature scheme FIPS 186 uses the SHA hash algorithm designed by NIST & NSA in early 90's DSS is the standard, DSA is the algorithm a variant on ElGamal and Schnorr schemes creates a 320 bit signature, but with 512-1024 bit security security depends on difficulty of computing discrete logarithms

9. – 9 – CSCE 815 Sp 03 DSA Key Generation have shared global public key values (p, q, g): a large prime p, with 2 L-1 < p < 2 L where L= 512 to 1024 bits and is a multiple of 64 choose q, a 160 bit prime factor of p-1 choose g = h (p-1)/q where h is any integer 1 1 users choose private & compute public key: choose a random number x with x<q Compute public key y = g x (mod p)

10. – 10 – CSCE 815 Sp 03 DSA Signature Creation to sign a message M the sender: generates a random signature key k, k<q nb. k must be random, be destroyed after use, and never be reused then computes signature pair: r = (g k (mod p))(mod q) s = (k -1.(SHA(M)+ x.r))(mod q) sends signature (r,s) with message M

11. – 11 – CSCE 815 Sp 03 DSA Signature Verification having received M & signature (r,s) to verify a signature, recipient computes: w = s -1 (mod q) u1= (SHA(M).w)(mod q) u2= (r.w)(mod q) v = (g u1.y u2 (mod p)) (mod q) if v=r then signature is verified see C&NS book web site for details of proof why

12. – 12 – CSCE 815 Sp 03 DSS Signing And Verifying

13. – 13 – CSCE 815 Sp 03 Authentication Protocols We cannot enter into alliance with neighboring princes until we are acquainted with their designs. —The Art of War, Sun Tzu used to convince parties of each others identity and to exchange session keys may be one-way or mutual key issues are confidentiality – to protect session keys timeliness – to prevent replay attacks

14. – 14 – CSCE 815 Sp 03 Replay Attacks where a valid signed message is copied and later resent simple replay repetition that can be logged repetition that cannot be detected backward replay without modification countermeasures include use of sequence numbers (generally impractical) timestamps (needs synchronized clocks) challenge/response (using unique nonce)

15. – 15 – CSCE 815 Sp 03 Authentication Applications will consider authentication functions developed to support application-level authentication & digital signatures will consider Kerberos – a private-key authentication service then X.509 directory authentication service

16. – 16 – CSCE 815 Sp 03 KERBEROS In Greek mythology, a many headed dog, the guardian of the entrance of Hades In Greek mythology, a many headed dog, the guardian of the entrance of Hades

17. – 17 – CSCE 815 Sp 03 Threats In a Shared Network Kerberos developed as part of Project Athena at MIT Athena – a network of workstations and distributed or centralized servers Threats in any network sharing resouces  a user may pretend to be another user  A user may change an IP address of a workstation  A user may eavesdrop and use a replay attack for …

18. – 18 – CSCE 815 Sp 03 Approaches to Security in a Network  Rely on workstation to assure the identity of users. Rely on server to enforce security policy on users.  Require workstations authenticate themselves to servers, trust workstations authenticate users.  Require the user prove identity for each service requested. Also servers prove identity to clients.

19. – 19 – CSCE 815 Sp 03 Kerberos trusted key server system from MIT provides centralised private-key third-party authentication in a distributed network allows users access to services distributed through network without needing to trust all workstations rather all trust a central authentication server two versions in use: 4 & 5

20. – 20 – CSCE 815 Sp 03 Kerberos Requirements first published report identified its requirements as: security reliability transparency scalability

21. – 21 – CSCE 815 Sp 03 Kerberos 4 Overview a basic third-party authentication scheme have an Authentication Server (AS) users initially negotiate with AS to identify self AS provides a non-corruptible authentication credential (ticket granting ticket TGT) have a Ticket Granting server (TGS) users subsequently request access to other services from TGS on basis of users TGT

22. – 22 – CSCE 815 Sp 03 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

23. – 23 – CSCE 815 Sp 03 A Simple Authentication Dialogue C  AS: ID c || P c || ID v AS  C:Ticket C  V: ID c || Ticket Ticket = E K v [ ID c || P c || ID v]

24. – 24 – CSCE 815 Sp 03 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

25. – 25 – CSCE 815 Sp 03 Version 4 Authentication Dialogue Authentication Service Exhange: To obtain Ticket-Granting Ticket C  AS: IDc || IDtgs ||TS 1 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: EKc,v[TS5 +1]

26. – 26 – CSCE 815 Sp 03 Kerberos 4 Overview Fig 4.1

27. – 27 – CSCE 815 Sp 03 Kerberos Realms a Kerberos environment consists of: a Kerberos server a number of clients, all registered with server application servers, sharing keys with server this is termed a realm typically a single administrative domain if have multiple realms, their Kerberos servers must share keys and trust

28. – 28 – CSCE 815 Sp 03 Request for Service in Another Realm

29. – 29 – CSCE 815 Sp 03 Kerberos Version 5 developed in mid 1990’s provides improvements over v4 addresses environmental shortcomings encryption algorithm, network protocol, byte order, ticket lifetime, authentication forwarding, interrealm authorization and technical deficiencies double encryption, non-std mode of use, session keys, password attacks specified as Internet standard RFC 1510

30. – 30 – CSCE 815 Sp 03 X.509 Authentication Service part of CCITT X.500 directory service standards distributed servers maintaining some info database defines framework for authentication services directory may store public-key certificates with public key of user signed by certification authority also defines authentication protocols uses public-key crypto & digital signatures algorithms not standardised, but RSA recommended

31. – 31 – CSCE 815 Sp 03 X.509 Certificates issued by a Certification Authority (CA), containing: version (1, 2, or 3) serial number (unique within CA) identifying certificate signature algorithm identifier issuer X.500 name (CA) period of validity (from - to dates) subject X.500 name (name of owner) subject public-key info (algorithm, parameters, key) issuer unique identifier (v2+) subject unique identifier (v2+) extension fields (v3) signature (of hash of all fields in certificate) notation CA > denotes certificate for A signed by CA

32. – 32 – CSCE 815 Sp 03 X.509 Certificates

33. – 33 – CSCE 815 Sp 03 Obtaining a Certificate any user with access to CA can get any certificate from it only the CA can modify a certificate because cannot be forged, certificates can be placed in a public directory

34. – 34 – CSCE 815 Sp 03 CA Hierarchy if both users share a common CA then they are assumed to know its public key otherwise CA's must form a hierarchy use certificates linking members of hierarchy to validate other CA's each CA has certificates for clients (forward) and parent (backward) each client trusts parents certificates enable verification of any certificate from one CA by users of all other CAs in hierarchy

35. – 35 – CSCE 815 Sp 03 CA Hierarchy Use

36. – 36 – CSCE 815 Sp 03 Certificate Revocation certificates have a period of validity may need to revoke before expiry, eg: 1. user's private key is compromised 2. user is no longer certified by this CA 3. CA's certificate is compromised CA’s maintain list of revoked certificates the Certificate Revocation List (CRL) users should check certs with CA’s CRL

37. – 37 – CSCE 815 Sp 03 Authentication Procedures X.509 includes three alternative authentication procedures: One-Way Authentication Two-Way Authentication Three-Way Authentication all use public-key signatures

38. – 38 – CSCE 815 Sp 03 One-Way Authentication 1 message ( A->B) used to establish the identity of A and that message is from A message was intended for B integrity & originality of message message must include timestamp, nonce, B's identity and is signed by A

39. – 39 – CSCE 815 Sp 03 Two-Way Authentication 2 messages (A->B, B->A) which also establishes in addition: the identity of B and that reply is from B that reply is intended for A integrity & originality of reply reply includes original nonce from A, also timestamp and nonce from B

40. – 40 – CSCE 815 Sp 03 Three-Way Authentication 3 messages (A->B, B->A, A->B) which enables above authentication without synchronized clocks has reply from A back to B containing signed copy of nonce from B means that timestamps need not be checked or relied upon

41. – 41 – CSCE 815 Sp 03 X.509 Version 3 has been recognised that additional information is needed in a certificate email/URL, policy details, usage constraints rather than explicitly naming new fields defined a general extension method extensions consist of: extension identifier criticality indicator extension value

42. – 42 – CSCE 815 Sp 03 Certificate Extensions key and policy information convey info about subject & issuer keys, plus indicators of certificate policy certificate subject and issuer attributes support alternative names, in alternative formats for certificate subject and/or issuer certificate path constraints allow constraints on use of certificates by other CA’s

43. – 43 – CSCE 815 Sp 03 Summary have considered: Kerberos trusted key server system X.509 authentication and certificates