1. Securing Passwords against Dictionary Attacks
Benny Pinkas, Tomas Sander
HP Labs
(most work done at STAR Lab, Intertrust)

2. In this talk Online dictionary attacks against passwords
Current countermeasures are insufficient and introduce risks
A solution using Reverse Turing Tests
Prevent online dictionary attacks, while preserving the advantages of using passwords (low costs, portability, user friendliness…)

3. Motivation Passwords are the most common authentication method
They are inherently insecure
How can a password based authentication system be secured against online dictionary attacks?

4. 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

5. Previous suggestions: securing passwords against online attacks
Enterprise:
hardware tokens. (Cost? Usability?)
Server defined passwords. (Usability?)
Consumer:
Key stroke timing [Bell Labs] (Reliability?)
Graphical passwords [Microsoft, Berkeley]
(Usability?)
None of these methods is as popular as plain passwords

6. Possible attacks on passwords
Eavesdropping. (Solution: encrypt the channel, e.g. using SSL or SSH.)
Offline dictionary attacks. (Solution: limit access to password file, use salt.)
Online dictionary attacks: Attacker guesses a username/password pair and tries to login.

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

8. Global Password Attack: Countering the countermeasurs
Username-1 / pwd-1
Answer 1
Pipelining guesses:
High throughput
Username-2 / pwd-2
Answer 2
Username-100 / pwd-100
Use different
usernames -
no locking
Answer 100

9. 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 – cost is $20-50 per call.

10. Using Pricing via Processing [DN]
Idea: each login attempt must be accompanied by H(username,pwd,t,r) s.t. 20 least significant bits are 0.
Negligible overhead for a single request.
A dictionary attack is slowed by a factor of 220 (must find r for every pwd guess).
Implementation problems:
Clients must use a special software.
Legitimate user with a slow machine.
Describe what the server is doing.
Verification is easy.

11. Our Approach
Legitimate logins – done by humans. Dictionary attacks – run by programs.
Login attempts must be accompanied by a computation that is easy for humans and hard for programs.
Other requirements: Little impact on usability, portability, no additional hardware, easy implementation and integration.

12. Reverse Turing Test (RTT)
Verifies “human in the loop”. A challenge from a domain in which humans excel and computers fail.
Please type the following word:

13. Properties of Reverse Turing Tests (RTT, Captcha, ATT)
Automated generation and verification.
Easy for humans.
Hard for computer programs.
Small probability of guessing the answer (I.e. not a yes/no answer).

14. Reverse Turing Tests (RTT)
Suggested by Moni Naor in 1996.
Captcha project, CMU.
Used to prevent automated programs from accessing different features of web sites (Yahoo!, Paypal, AltaVista).
Possible accessibility problems?

15. Security of RTTs
Alta Vista: # of url submissions down by 90% after RTT were required.
Pessimal print – “…RTTs are, and will be, hard for OCR programs” [CBF].
Unfortunately, simple RTTs (Yahoo!’s), displaying English text, can be broken with high probability [MM2002].
There will be an arms race. We only need that breaking RTTs isn’t too easy.
CBF – Coates, baird, fateman.
MM- Mori, Malik
Explain why we only need that breaking is not too easy

16. Simple method I want to login RTT id, pwd, RTT answer
(id,pwd) valid, and
RTT answer is correct
Welcome!
Go away!
Otherwise

17. Properties Securitya: Usability: User’s experience is more annoying
Each password guess requires an RTT.
Hard to guess RTT answer.
Password space of size N requires adversary to answer N RTTs
Usability: User’s experience is more annoying
Scalability: server must generate many RTTs (one per login attempt).

18. Improved Authentication Method
Each user typically uses a limited set of computers.
Dictionary attacks originate from other computers.
Servers can identify machines (e.g. using cookies or ip addresses).

19. Improved Authentication Method
cookie, id, pwd
If password is correct:
Cookie indicates previous successful login to same account?
Yes No
Grant access
RTT?
Solution?
Yes: Grant
No: Deny!
If password is incorrect:
With prob 90% deny access
With prob p=10% ask for an RTT
and then deny access

20. Properties Usabilitya- user has to answer RTT
In the first login from a new computer
If entered wrong password
Scalabilitya: Server generates RTTs only for 10% of incorrect login attempts.

21. Security User must receive identical feedback if,
(id,pwd) pair is correct but RTT is required
(id,pwd) pair is incorrect and RTT is required
Attacker can easily identify a set of pN candidate passwords.
To check these passwords, has to “pay” with an RTT answer per password.
(We can also protect against cookie theft)
Implication for timing

22. Security - example
Parameters: N=106 passwords, 1000 possible answers for RTT, p=10%.
Attacks:
Attacker guesses RTT answer: succeeds with prob 10-8.
Attacker breaks RTT in 3 seconds (automatically or using humans): expected to invest 42 hours per account.

23. And if RTT is broken… Identify a successful attack: Countermeasures:
Monitor fraction of login attempts that solve the RTT but fail in entering password.
Set alarm when this fraction increases.
Countermeasures:
Increase p (fraction of logins requiring RTT).
Switch to an RTT from a different domain.
Notify administrator

24. Implications wrt Account locking
Common practice today: lock account after L unsuccessful login attempts.
Risks: Denial of service, service calls.
Assume: A secure RTT with 1000 possible answers, RTT needed for 10% of pwd guesses.
Pwd space increases by a factor of 100.
Therefore, can lock accounts after L*100 unsuccessful login attempts…

25. Benefits to server Better security against break-ins.
Visible security measures, but with few usability effects.
Easy implementation and integration.
Less account locking
Less denial of service attacks – important for corporates, auctions,…
Save money - less customer support calls

26. Scores wrt Different Criteria
Availability and portability: account can be accessed from everywhere.
User friendliness: easy learning curve
Robustness: less account locking
Low implementation and operation costs
Passwords score well.
Our solution scores well, and provides better security.