CMSC 414 Computer and Network Security Lecture 20

1. CMSC 414Computer and Network SecurityLecture 20 Jonathan Katz

2. HW3 • Some students have reported problems when using different grace machines • Logging in to scary.umd.edu should work

3. Network Security

4. Authentication: an Overview

5. Authentication • Verifying the identity of another entity • Computer authenticating to another computer • Person authenticating to a local/remote computer • Important to be clear about what is being authenticated • The user? • The machine? A specific application on the machine? • The data? • What assumptions are being made? • E.g., login from untrusted terminal

6. Authentication • Mutual authentication vs. unidirectional authentication • Authentication -- two main issues: • How authentication information is stored (at both ends) • Authentication protocol itself

7. Authentication • Authentication may be based on • What you know • What you have • What you are • Examples? Tradeoffs? • Others? • Can also consider two-factor authentication

8. Address-based authentication • Is sometimes used • Generally not very secure • Relatively easy to forge source addresses of network packets • But can be useful if the adversary does not know what IP address to forge • E.g., IP address of a user’s home computer

9. Location-based authentication • More interest lately, as computation becomes more ubiquitous • Re-authentication if laptop moves

10. Attack taxonomy • Passive attacks • Active attacks • Impersonation • Client impersonation • Server impersonation • Man-in-the-middle • Server compromise • Different attacks may be easier/more difficult in different settings

11. Password-based protocols • Password-based authentication • Any system based on low-entropy shared secret • Distinguish on-line attacks vs. off-line attacks

12. Password selection • User selection of passwords is typically very poor • Lower entropy password makes dictionary attacks easier • Typical passwords: • Derived from account names or usernames • Dictionary words, reversed dictionary words, or small modifications of dictionary words • Users typically use the same password for multiple accounts • Weakest account determines the security! • Can use program like pwdHash to correct this

13. Better password selection • Non-alphanumeric characters • Longer phrases • Can try to enforce good password selection… • …but these types of passwords are difficult for people to memorize and type!

14. From passwords to keys? • Can potentially use passwords to derive symmetric or public keys • What is the entropy of the resulting key? • Often allows off-line dictionary attacks on the password

15. Password-based protocols • Any password-based protocol is potentially vulnerable to an “on-line” dictionary attack • On-line attacks can be detected and limited • How? • “Three strikes” • Ratio of successful to failed logins • Gradually slow login response time • Potential DoS • Cache IP address of last successful login

16. Password-based protocols • Off-line attacks can never be ‘prevented’, but protocols can be made secure against such attacks • Any password-based protocol is vulnerable to off-line attack if the server is compromised • Once the server is compromised, why do we care?

17. Password-based protocols • Best: Use a password-based protocol which is secure against off-line attacks when server is not compromised • Unfortunately, this has not been the case in practice (e.g., telnet, cell phones, etc.) • This is a difficult problem!

18. Password storage • In the clear… • Hash of password (done correctly) • Doesn’t always achieve anything! • Makes adversary’s job harder • Potentially protects users who choose good passwords • “Salt”-ed hash of password • Makes bulk dictionary attacks harder, but no harder to attack a particular password • Prevents using ‘rainbow tables’ • Encrypted passwords? (What attack is this defending against?) • Centralized server stores password…