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

Securing Passwords against Dictionary Attacks

Benny Pinkas, Tomas SanderHP Labs

(most work done at STAR Lab, Intertrust)

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…)

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?

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

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

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.

Countermeasures against offline dictionary attacks

Username / pwd-1

Username / pwd-2

Username / pwd-5

Answer 2 (No)

Answer 1 (No)

Answer 5 (No)

Delayed answer

Account locked

Global Password Attack: Countering the countermeasurs

Username-1 / pwd-1

Username-2 / pwd-2

Username-100 / pwd-100

Answer 2

Answer 1

Answer 100

Pipelining guesses:High throughput

Use differentusernames -no locking

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.

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.

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.

Reverse Turing Test (RTT)

Please type the following word:

Verifies “human in the loop”. A challenge from a domain in which humans excel and computers fail.

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).

Reverse Turing Tests (RTT)

• Suggested by Moni Naor in 1996.• Captcha project, CMU.

http://www.captcha.net• Used to prevent automated programs

from accessing different features of web sites (Yahoo!, Paypal, AltaVista).

• Possible accessibility problems?

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.

Simple methodI want to login

RTT

Welcome!

id, pwd, RTT answer

Go away!

(id,pwd) valid, and RTT answer is correct

Otherwise

Properties• Security:

– 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).

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).

Improved Authentication Method

• If password is correct:– Cookie indicates previous

successful login to same account?• Yes• No

cookie, id, pwd

Grant access

With prob 90% deny accessWith prob p=10% ask for an RTT and then deny access

RTT?

• If password is incorrect:Solution?• Yes: Grant• No: Deny!

Properties

• Usability- user has to answer RTT– In the first login from a new

computer– If entered wrong password

• Scalability: Server generates RTTs only for 10% of incorrect login attempts.

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)

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.

And if RTT is broken…

• Identify a successful attack:– 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

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…

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

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.