CMSC 414 Computer and Network Security Lecture 21

1. CMSC 414Computer and Network SecurityLecture 21 Jonathan Katz

2. Revocation • Revocation is a key component of a PKI • Secret keys stolen/compromised, user leaves organization, etc. • This is in addition to expiration dates included in certificates • Certificate might need to be revoked before expiration date • Expiration dates improve efficiency

3. Cert. revocation lists (CRLs) • CA issues signed list of (un-expired) revoked keys • Must be updated and released periodically • Must include timestamp • Verifier must obtain most recent CRLs before verifying a certificate • Using “delta CRLs” improves efficiency

4. OLRS • “On-line revocation server” • Verifier queries an OLRS to find out if a certificate is still valid • If OLRS has its own key, it can certify that a certificate is valid at a certain time

5. “Good lists” • The previous approaches basically use lists of “bad” certificates • Also possible to use a list containing only “good” certificates • Likely to be less efficient

6. Directories • PKIs do not require directories • Users can store and present their own certificate chains to a trust anchor • Directories can make things easier, and enable non-interactive setup

7. Finding certificate chains • Two approaches: work “forward” from target toward a trust anchor, or “backward” from trust anchor to target • The better approach depends on implementation details • For example, in system with name constraints, easier to work “backward”

8. Anonymity

9. Anonymity vs. pseudonymity • Anonymity • No one can identify the source of any messages • Can be achieved via the use of “persona” certificates (with “meaningless” DNs) • Pseudonymity • No one can identify the source of a set of messages… • …but they can tell that they all came from the same person

10. Levels of anonymity • There is a scale of anonymity • Ranges from no anonymity (complete identification), to partial anonymity (e.g., crowds),to complete anonymity • Pseudonymity is an orthogonal issue…

11. Anonymizers • Proxies that clients can connect to, and use to forward their communication • Primarily used for email, http • Can also provide pseudonymity • This may lead to potential security flaws if mapping is compromised • Must trust the anonymizer… • Can limit this by using multiple anonymizers

12. Traffic analysis • If messages sent to remailers are not encrypted, it is easy to trace the sender • Even if encrypted, may be possible to perform traffic analysis • Timing • Message sizes • Replay attacks

13. Http anonymizers • Two approaches • Centralized proxy/proxies • “Crowds…”

14. Implications of anonymity? • Is anonymity good or bad? • Unclear… • Can pseudonymity help?

15. “Cookies” • Cookies are tokens containing state information about a transaction • May contain (for example): • Name/value; expiration time • Intended domain (cookie is sent to any server in that domain) • No requirement that cookie is sent by that domain

16. Security violations? • Cookies potentially violate privacy • E.g., connecting to one server results in a cookie that will be transmitted to another • Storing authentication information in a cookie is also potentially dangerous (unless cookie is kept confidential, or other methods are used)