Computer Security: Principles and Practice

1. Computer Security: Principles and Practice Chapter 3 – User Authentication by William Stallings and Lawrie Brown

2. User Authentication • fundamental security building block • basis of access control & user accountability • is the process of verifying an identity claimed by or for a system entity • has two steps: • identification - specify identifier • verification - bind entity (person) and identifier • distinct from message authentication

3. Means of User Authentication • four means of authenticating user's identity • based on • something you have - e.g. key, token, smartcard • something you know - e.g. password, PIN • something you are • static biometrics - e.g. fingerprint, retina, face • dynamic biometrics - e.g. voice, typing • somebody you know - the social network of the user, CCS’06 • can use alone or combined • all can provide user authentication • all have issues

4. Password Authentication • widely used user authentication method • user provides name/login and password • system compares password with that saved for specified login • authenticates ID of user logging and • that the user is authorized to access system • determines the user’s privileges • is used in discretionary access control

5. Password Vulnerabilities • offline dictionary attack • specific account attack • popular password attack • workstation hijacking • exploiting user mistakes • exploiting multiple password use • electronic monitoring

6. Countermeasures • stop unauthorized access to password file • intrusion detection measures • account lockout mechanisms • policies against using common passwords but rather hard to guess passwords • training & enforcement of policies • automatic workstation logout • encrypted network links

7. Use of Hashed Passwords

8. UNIX Implementation • original scheme • 8 character password form 56-bit key • 12-bit salt used to modify DES encryption into a one-way hash function • 64-bit zeros as initial input, output is further encrypted, … repeated for 25 times • final output translated to 11 character sequence • now regarded as woefully insecure • e.g. supercomputer, 50 million tests, 80 min • sometimes still used for compatibility

9. Improved Implementations • have other, stronger, hash/salt variants • many systems (Solaris, Linux) now use MD5 • with 48-bit salt • password length is unlimited • is hashed with 1000 times inner loop • produces 128-bit hash • OpenBSD uses Blowfish block cipher based hash algorithm called Bcrypt • uses 128-bit salt to create 192-bit hash value

10. Password Cracking Approaches • dictionary attacks • try each possible password then obvious variants in large dictionary against hash in password file • rainbow table attacks • precompute tables of hash values of all possible passwords for all possible salts • a mammoth table of hash values • e.g. 1.4GB table cracks 99.9% of alphanumeric Windows passwords in 13.8 secs • not feasible if larger salt values used

11. Password Choices • users may pick short passwords • e.g. 3% were 3 chars or less, easily guessed • system can reject choices that are too short • users may pick guessable passwords • so crackers use lists of likely passwords • e.g. one study of 14000 encrypted passwords guessed nearly 1/4 of them • would take about 1 hour on fastest systems to compute all variants, and only need 1 break!

12. Password File Access Control • can block offline guessing attacks by denying access to encrypted passwords • make available only to privileged users • often using a separate (from user IDs) shadow password file • still have vulnerabilities • exploit O/S bug • accident with permissions making it readable • users with same password on other systems • access from unprotected backup media • sniff passwords in unprotected network traffic

13. Using Better Passwords • clearly have problems with passwords • goal to eliminate guessable passwords • whilst still easy for user to remember • techniques: • user education • computer-generated passwords • reactive password checking • proactive password checking

14. User Education • A good technique: using the first letter of each word of a phrase; however, don’t pick a well-known phrase • An apple a day keeps the doctor away (aaadktda) • My sister peg is 24 years old (mspi24yo) • Guidelines may be ignored …

15. Computer-generated Passwords • FIPS PUB 181 defines one of the best-designed automated password generators • Generate words by forming pronounceable syllables • In general, computer-generated password schemes have a poor acceptance by users.

16. Reactive Password Checking • System periodically runs its own password cracker to find guessable passwords. • John the Ripper password cracker • Resource intensive, vulnerable until identification

17. Proactive Password Checking • rule enforcement plus user advice, e.g. • 8+ chars, upper/lower/numeric/punctuation • may not suffice • use password cracker to reject bad passwords • time and space issues • Markov Model • generates guessable passwords • hence reject any password it might generate • Bloom Filter • use to build table based on dictionary using hashes • check desired password against this table

18. Biometric Authentication • authenticate user based on one of their physical characteristics

19. Operation of a Biometric System

20. Biometric Accuracy • never get identical templates • problems of false match / false non-match

21. Biometric Accuracy • can plot characteristic curve • pick threshold balancing error rates

22. Authentication Protocols • used to convince communication parties of each other’s identity and to exchange session keys • may be one-way or mutual • key issues are • confidentiality – to prevent masquerade and to protect session keys • timeliness – to prevent message replay attacks

23. Remote User-Authentication using Symmetric Encryption • using KDC and hierarchy of keys (Needham-Schroeder Protocol) vulnerable to a replay attack if an old session key Ks has been compromised, then attacker X can • impersonate A and trick B to use old Ks by replaying msg. 3 • intercept msg. 4, impersonate A’s response in msg. 5 • impersonate A for further secure communication

24. Kerberos • trusted authentication service from MIT • provides centralized mutual authentication in a distributed network • allows users access to distributed services in the network • a workstation cannot be trusted to identify its user • rather all trust a central authentication server • relies exclusively on symmetric encryption • requires a user to prove his or her identity for each service invoked, also requires servers to prove their identity its user • two versions in use: version 4 (1988) & 5 (1994)

25. Kerberos Requirements • requirements in its first published report: • secure • reliable - distributed server architecture • transparent - users only need to enter passwords • scalable - distributed architecture • thus, a trusted third-party authentication service • clients and servers trust Kerberos • implemented using an authentication protocol based on Needham-Schroeder

26. Kerberos v4 Overview • a basic third-party authentication scheme • have an Authentication Server (AS) • knows the passwords of all users • users initially negotiate with AS to identify themselves • AS provides a non-corruptible authentication credential (ticket granting ticket TGT) • have a Ticket Granting Server (TGS) • users subsequently present TGT to TGS to acquire Service Granting Ticket (SGT) to access specific services • using a complex protocol based on DES

27. Actions in Kerberos v4

28. Authentication Security Issues • client attacks • host attacks • eavesdropping • replay • trojan horse • denial-of-service • phishing

29. PwdHash(Ross, B., Jackson, C., Miyake, N., Boneh, D., Mitchell, J.C.: Stronger passwordauthentication using browser extensions. In: Proc. of the USENIX Security Symposium, 2005) Bank A Hash (PwdA, BankA) Spoofed Bank A Hash (PwdA, SpoofedBankA) PwdHash (Browser Extension) Plain-text password: PwdA Site-password:Hash (PwdA, BankA) • Unique password per site (domain name is the salt) • Focuses on protecting against phishing attacks

30. Summary • introduced user authentication • using passwords • using tokens • using biometrics • remote user authentication • Kerberos