Biometrics, 2-factor Authentication, Single Sign-on, Cookies

CS166

Chris Pollett

Oct 17, 2012

Outline

1936 -- suggested by Frank Burch
1980s -- James Bond films
1986 -- first patent appeared
1994 -- John Daugman patented best current approach. Patent owned by Iridian Technologies

Based on Hamming distance
Define `d(x,y)` to be:
- # of non match bits / # of bits compared
- d(0010,0101) = 3/4 and d(101111,101001) = 1/3
Compute `d(x,y)` on 2048-bit iris code
- Perfect match is d(x,y) = 0
- For same iris, expected distance is 0.08
- At random, expect distance of 0.50
- Accept iris scan as match if distance < 0.32

Good photo of eye can be scanned -- Attacker could use photo of eye
Afghan woman was authenticated by iris scan of old photo from National Geographic. http://news.bbc.co.uk/2/hi/south_asia/1870382.stm
To prevent attack, scanner could use light to be sure it is a "live" iris

Equal error rate (EER): fraud == insult rate
Fingerprint biometric has EER of about 5%
Hand geometry has EER of about `10^(-3)`
In theory, iris scan has EER of about `10^(-6)`.
- But in practice, may be hard to achieve
- Enrollment phase must be extremely accurate
Most biometrics much worse than fingerprint!
Biometrics useful for authentication...
- ...but identification biometrics almost useless today

Biometrics are hard to forge
But attacker could
- Steal Alice's thumb
- Photocopy Bob's fingerprint, eye, etc.
- Subvert software, database, "trusted path"...
And how to revoke a "broken" biometric?
Biometrics are not foolproof
Biometric use is limited today
That should change in the (near?) future

Problem 7.6 This problem deals with storing passwords in a file.

Why is it a good idea to hash passwords that are stored in a file? Answer. This prevents an unauthorized person who manages to acquire a password file from immediately getting everyone's password.
Why is it a much better idea to hash passwords stored in a file than to encrypt the password file? Answer. Encrypting the password file would require the file be decrypted for password checking. This might be both computationally expensive and result in a plaintext file appearing as an intermediate file which could be copied.
What is a salt and why should a salt be used whenever passwords are hash? Answer. A salt is a string concatenated to the password before hashing. In the password file we store the hashed result together which the plaintext salt. This prevents the attacker from using hashes of a password dictionary for more than one forward attack.

Something in your possession
Examples include following...
- Car key
- Laptop computer (or MAC address)
- Password generator (next)
- ATM card, smartcard, etc.

A password generator is a small device that has a key `K` known to Bob (but not Alice) coded into its firmware. It computes a hash based on this key and a string fed to it.
Alice receives random "challenge" `R` from Bob
Alice enters PIN and `R` in password generator
Password generator hashes symmetric key `K` with `R`
Alice sends "response" `h(K,R)` back to Bob
Bob verifies response
Note: Alice has password generator and knows PIN

Requires any 2 out of 3 of
- Something you know
- Something you have
- Something you are
Examples
- ATM: Card and PIN
- Credit card: Card and signature
- Password generator: Device and PIN
- Smartcard with password/PIN

A hassle to enter password(s) repeatedly
- Alice wants to authenticate only once
- "Credentials" stay with Alice wherever she goes
- Subsequent authentications transparent to Alice
Kerberos --- example single sign-on protocol
Single sign-on for the Internet?
- Microsoft: Passport
- Everybody else: Liberty Alliance
- Security Assertion Markup Language (SAML)

When you browse the web using a browser, websites often send "cookies" to be stored by your browser.
The protocol which governs how browsers and web servers communicate, HTTP -- hypertext transfer protocol, is a a stateless protocol. Cookies, however, allow servers to maintain the state of an interaction.
More specifically, in response to a browser request, a website sends an HTTP header (Set-Cookie:) with a session string and expiration. These are then stored by your browser.
When a browser makes a second request of the website, as part of its HTTP request, it sends any relevant cookies for the site (Cookie: header)
This can be then used to look up any information about that session and its current state.
Sorta like a single sign-on for a website
There are privacy concerns... (Tracking cookies, Flash cookies, E-tags for tracking, etc.)
Also, Cookies are a very, very weak form of authentication...

CSRF stands for Cross-site Request Forgery. Here is the idea of this attack which affects cookies.
Suppose a user logs into his online bank.
The bank stores a cookie on his machine and only uses that to check if he is logged in.
The user browses to some other site (not the bank) with malware in it and clicks on an image link that actually contains a URL for the bank together with an action like do a money transfer to a bad person.
Since the user still has the banks cookie, the transaction goes ahead without the user even knowing.

Include user specific tokens in forms, so form data won't be used to take actions on databases unless that token is present.
Check the referrer of any data sent to your server.
As a user make sure to log-off sites before browsing elsewhere.