1. Maintaining a Cache of Previously Queried Prefixes “Telepathwords: Preventing weak passwords by reading users’ minds.” Saranga Komanduri, Richard Shay, Lorrie Faith Cranor, Cormac Herley, and Stuart Schechter. In 23rd USENIX Security Symposium (USENIX Security 14). San Diego, CA: USENIX Association, pp. 591-606. 2014. Presented by: Nazish Khan

2. Summary  Requires efficient algorithms to model users’ behaviour and employ already-typed characters to predict subsequent ones  Real time predictions based on numerous predictors  Common character sequences  Keyboard movements  Repeated strings  Interleaved strings  Compared Telepathwords with composition rules 2 Feedback bar Prediction display

3. Storing Previous Queries  In an ideal situation, we would want no evidence of past requests  Authors take a security risk by maintaining a cache of previously queried prefixes on the server 3 Cache of past requests ---  Removal of past requests

4. Issues Why is it a security risk?  Cache becomes central point of storage (of previous queries- prone to attacks)  Is confidentiality guaranteed? No  Is integrity guaranteed? No  Protect the log but what about the cache? 4

5. Securing the Log 5 Requests one-time session key Generates session key, encrypt it with a public key and writes encrypted session key to the log Log Sends the session key XORs traffic with stream cipher and using symmetric encryption (AES)

6. Justification Authors hardly justify their decision to go ahead with this risk. Why did they take this risk?  Reuse queries  Faster processing  Route all client-server communications over HTTPS  Server is unable to read the contents of the online log 6

7. Conclusion 7  Cache is not protected  An attacker could gain access to the data  No confidentiality or integrity  When a user is typing text, no protection mechanism in memory.  Log has only been encrypted to cater for confidentiality  An attacker could modify its contents- threat to integrity

8. Thank you Questions?

9. My opinion  I have my doubts regarding the realistic use of this system 9  Need to give some crucial thought to the following questions:  Is security really important in such a system?  How to cater for the trade-off between security and usability? What’s more important? To whom?