Network Security

1. Network Security Chapter 2. Network Security Protocols

2. Objectives • Key Establishment Technique • Key Authentication • Authenticated Key Establishment Protocol • Key generation in SKC • Kerberos : Key generation in SKC • Key Establishment in PKC • Authentication Protocols • Security of password • Authentication using SKC • Authentication using PKC

3. Introduction • The three important aspect of network security: • authentication, encryption, message authentication • Key : Central to the idea of cryptography. • Some definitions related to key. • key establishment : a process or protocol where by a shared secret becomes available to two or more parties, for subsequent cryptographic use.

4. Key establishment technique • Key transport : a key establishment technique where one party creates or otherwise obtains a secret value, and securely transfers it to the other(s). • Key agreement : a key establishment technique in which a shared secret is derived by two (or more) parties as a function of information contributed by, or associated with, each of these, (ideally) such that no part can predetermine the resulting value.

5. Key establishment technique • Key pre-distribution : key establishment protocols whereby the resulting established keys are completely determined apriori by initial keying material. • Dynamic key establishment : the key is established by a fixed pair (or group) of users varies on subsequent executions. Also referred to as session key establishment

6. Key authentication • Key authentication : the property whereby one party is assured that no other party asides from a specifically identified second party( and possibly additional identified trusted parties) may gain access to a particular secret key. • It need not involve any action whatsoever by the second party. For this reason, it is some times referred to more precisely as (implicit) key authentication. • Key conformation : the property whereby one party is assured that a second (possibly unidentified) party actually has possession of particular secret key. • Explicit key authentication : the property obtained when (implicit) key authentication and key conformation hold.

7. Authentication Summary

8. Authenticated key establishment protocol • Authentication protocol : to provide to one party some degree of assurance regarding the identity of another with which it is purportedly communicating • Key establishment protocol : to establish a shared secret. • Authenticated key establishment protocol :to establish a shared secret with a party whose identity has been (or can be) collaborated.

9. Key Generation in SKC • Requirement for a SKC : - random and long enough to deter a brute force attack. - practical key size : AES : 128, 192, 256 bits • Key distribution in SKC - For a network with n nodes, each nodes wish to talk securely to every other node. How many keys would this require? n = 50  1,225 keys n = 250  31,125 keys

10. Key Generation in SKC • Solutions for key distribution in SKC • : Key distribution center (KDC) • KDC stores keys for all nodes in the network • Each node in the network is configured with only one key • How does it work? • 1) Alice  KDC : request session key for Bob. • 2) KDC  B, A : send same session key

11. Kerberos : Key Generation in SKC • Entity authentication and session key generation • Based on Needham-Schroeder protocol • Drawback • Bottleneck • single point of failure

12. Key Establishment in PKC • Public key Cryptography - each entity : (public key, private key) pair. - Certificate Authority(CA) : - trusted third party : certifying the owner of a public key - combine public key with entity’s identity. - issue certificate = “Alice’s public key is Kwa” + SignPCA( hash(“Alice’s public key is Kwa”)) - provide certificate verification service

13. Diffie-Hellman Key Exchange Remember DHP !!!

14. Man-in-the-middle attack against Diffie-Hellman • Alice and Bob think they are talking each other. • Eve impersonate Alice and Bob to Bob and Alice respectively.

15. Enhanced Diffie-Hellman Key Exchange • Static Diffie Hellman - g, n is fixed, - CA  Alice : - CA  Bob : • Dynamic Diffie-Hellman - g, n : ephemeral (established dynamically) - CA  Alice : - CA  Bob :

16. RSA • RSA encryption See the chap. 8 of handbook!!

17. RSA • RSA signing

18. RSA based cryptographic schemes • PKCS#1 • homomorphic property of basic RSA • RSA based cryptographic schemes • see [ this ] for security analysis

19. Authentication Protocol • Authentication : the Process of verifying that a node or users is who they claim to be. • Usage in network : access control • Access control : primary defense mechanisms in network security and computer security.

20. Address-Based Authentication • Use the address of the node in the network. • MAC address or IP address • Allows only a preconfigured set of MAC or IP address to access the network. • Usually implemented in the switch or router • Loop holes : • Simple one-to-one mapping between a node and a user. • So does not really authenticate the user • Weak to MAC spoofing and IP spoofing attack.

21. Password for Local Authentication (Login) • Storing <username, password> pair list in a file on the server machine. • If the password file is compromised, all user passwords are compromised. • Machine store <username, hash(password)> pair • Even though the file is compromised, the passwords are still secure. • But still open to dictionary attack.

22. Insecurity of Passwords • Human generated passwords • Come from a small domain • Easy to guess – dictionary attacks • Stronger passwords • Computer generated or verified • Not user friendly • Hard to remember

23. Possible attacks on passwords • Eavesdropping. (Solution: encrypt the channel, e.g. using SSL or SSH.) • Offline dictionary attacks. • Attacker compute < word, hash(word)> pair list • Attacker get password file and search hash(password) in his stored list. • (Solution: limit access to password file, use salt.) < word, hash(word+salt), salt> • Online dictionary attacks: Attacker guesses a username/password pair and tries to login. Real time. • Case study : e-Bay user account hacking [ link ]

24. Username / pwd-1 Username / pwd-2 Username / pwd-5 Answer 2 (No) Answer 1 (No) Answer 5 (No) Countermeasures against online dictionary attacks Delayed answer Account locked

25. Risks of locking accounts • eBay experiences dictionary attacks, but does not implement account locking. • Denial of service attacks:To lock a user, try to login into his account with random passwords. (auctions, corporates…) • Customer service costs: Users whose accounts are locked call a customer service center – impose call cost

26. Password for Network Authentication • Password for network authentication differ from local login. • Hashed password can not be sent over the network. • Captured hashed password can be used for offline dictionary attack • Using Salt (transmit in plain text)  still weak to offline attack.

27. Authentication using SKC • In a network authentication, use password for deriving shared keys to be used in challenge response system. • Key = part of hash(password) • One-way authentication using SKC

28. One-way Authentication using SKC-variation

29. One-way Authentication using SKC-variation • Bob : state-less  prevent Denial of Service(DoS) attack • timestamp : require time synchronization, not trivial in a large network. • if stream cipher is used, 1 bit flip in the cipher text flips 1 bit in the plain text. • Eve may get an approximate time stamp by flipping the millisecond bits.

30. Mutual Authentication using SKC • Authenticate each other. • (Reduced Massages)

31. Mutual authentication using SKC-Reflection Attack • How to prevent the reflection attack • Unique format for each direction – even and odd challenge • Different symmetric key for each direction

32. Lamport’s Hash • Bob(server) saves (username, ) • After one authentication, Bob sets raise to (m-1) • When m=1, reconfigure new password. • How to avoid new password reconfiguration when m=1  use salt with password : (m-1) times hash of R1.

33. Authentication using PKC One-way authentication Mutual Authentication • Key Database compromise  does not compromise the security of the System.

34. What to use for authentication : SKC or PKC? • SKC (Advantages) • less computation intensive • more resilient to DoS Attacks. (Disadvantages) • Key database compromise  security of whole system is compromised. • Eve can collect < plaintext, ciphertext> pairs launch dictionary attack. How to: Eve claims to be Bob and send challenge to A, then collect the cipher text for the challenge.

35. What to use for authentication: SKC or PKC? • PKC (advantages) • Key database compromise  does not compromise the security of whole system • dictionary attack is not applicable. (Disadvantages) • computation intensive • weak to DoS Attacks.

36. Session Hijacking • Instead of trying to break the authentication protocol, it circumvent it completely. • Cause : authentication result is not linked to the rest of the session. • Solution : Use authenticated key agreement protocol.

37. Needham Schroeder • SKC-based mutual authentication and key establishment

38. Kerberos

39. Resources • [B. Pinkas] Securing Passwords against Dictionary attack http://www.pinkas.net/PAPERS/pwdweb.pdf • [e-bay case] http://news.com.com/2100-1017-868278.html?tag=yt • Collin Boyd, Anish Mathuria, Protocols for Authentication and Key Establishment, Springer-Verlag • Evaluation of RSA cryptographic Schemes, http://www.ipa.go.jp/security/enc/CRYPTREC/fy15/doc/1011_rsa.pdf