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Authentication

Topics today

Background: Hash Functions Password Graphical passwords Challenge-Response Biometrics

HASH FUNCTIONS

slide 3

Hash Functions: Main Idea

bit strings of any length n-bit bit strings

. .

. . .

x’ x’’

x

y’ y

hash function H

Hash function H is a lossy compression function H(x) should look “random”

• Every bit (almost) equally likely to be 0 or 1

“message digest”

message

Collisions

Collision: H(x)=H(x’) for some inputs x≠x’

Collisions must happen • Length of x unlimited, length of H(x) is n-bits ! • Even if length of x is bounded |x| = b, and b > n,

there will be collisions • “Pigeonhole principle” "עקרון שובך היונים "

A hash function cannot be 1-to-1 Cryptographic hash function must have certain

properties slide 5

Property 1: One-Way

Intuition: hash should be hard to invert • “Preimage resistance” • Given random y, it should be “hard” to find any x

such that h(x)=y

How hard? • Brute-force: try every possible x, see if h(x)=y • SHA-1 (common hash function) has 160-bit output

– Suppose have hardware that’ll do 230 trials a pop – Assuming 234 trials per second, can do 289 trials per year – Will take 271 years to invert SHA-1 on a random image

Probability background: “Birthday Paradox”

How likely is it that some people have the same birthday in a class of T students?

If there are T=366 people in class then • Prob(not all different birthdays | T people) = 0

How big should T be so • Prob(not all different birthdays | T people) >= ½ ??

Let’s experiment !

“Birthday Paradox” analysis

How many possibilities that are all different? • (K)T = K(K-1)…(K-T+1) - samples without replacement

Probability of no repetition? 𝑘 ∗ 𝑘 − 1 ∗ 𝑘 − 2 ∗ ⋯ (𝑘 − 𝑡 + 1)

𝑘 ∗ 𝑘 ∗ 𝑘… ∗ 𝑘=

𝑘𝑘∗𝑘 − 1𝑘

∗ ⋯∗ 𝑘 − 𝑡 + 1

𝑘= 1 ∗ 1 −

1𝑘

∗ 1 −2𝑘

∗ ⋯∗ 1 −𝑡 − 1𝑘

≤

𝑒−1𝑘 ∗ 𝑒−

2𝑘 ∗ 𝑒−

3𝑘 ∗ ⋯∗ 𝑒−

𝑡−1𝑘 ≅ 𝑒−

𝑡22𝑘

Birthday Paradox continued

𝑒−𝑡22𝑘 ≤

12

⇔ 𝑡2

2𝑘≥ ln 2 ⇔

𝑡2 ≥ 2 (ln 2)𝑘 = 1.38𝑘⇔

𝑡 ≥ 1.38𝑘 Bottom line: For k=365, t=23 suffices

In general 𝑡 = 𝛀( 𝑘) suffices slide 9

Property 2: Collision Resistance

Hard to find some x≠x’ such that h(x)=h(x’) Brute-force collision search is O(2n/2), not O(2n)

• n = number of bits in the output of hash function • For SHA-1, this means O(280) vs. O(2160)

Reason: birthday paradox • Let T be the number of values x1, x2, x3, … we need to

look at before finding the first pair xi≠xj s.t. h(xi)=h(xj) • Assuming h is random, how big should T be so the

probability of finding a repetition >= ½ ? • Total number of possible values? • Conclusion:

K=2n T ≈ 2n/2

Property 3: Weak Collision Resistance

Given a randomly chosen x, hard to find x’ such that h(x)=h(x’) • Attacker must find collision for a specific x… • by contrast, to break collision resistance, enough

to find any collision • Brute-force attack requires O(2n) time

Weak collision resistance does not imply collision resistance (why?)

Hashing vs. Encryption

Hashing is one-way. There is no “uh-hashing”! • In contrast, encryption is two-way: can decrypt a

ciphertext with the decryption key • Hashes have no equivalent of “decryption”

Hash(x) looks “random”… but can be compared for equality with Hash(x’)

Hashes are also known as “cryptographic checksums”

Application: Password Hashing

Instead of user password, store H(password) When user enters password, compute its hash

and compare with entry in password file • System does not store actual passwords! • Difficult to go from hash to password!

System cannot “restore” your password!

Application: Software Integrity

goodFile

Software manufacturer wants to ensure that the executable file is received by users without modification… Sends out the file to users and publishes its hash in the NY Times The goal is integrity, not secrecy

Idea: given goodFile and hash(goodFile), very hard to find badFile such that hash(goodFile)=hash(badFile)

BigFirm™ User

VIRUS

badFile

The Times

hash(goodFile)

http://msn.cwusa.tv/images/Bill-Gates-08-Formal.jpg

Which Property Do We Need?

UNIX passwords stored as hash(password) • One-wayness: hard to recover entire password • Are passwords random?

Integrity of software distribution • Weak collision resistance • But software images are not really random… maybe

need full collision resistance

Common Hash Functions

MD5 • 128-bit output • Still used very widely • Completely broken by now

RIPEMD-160 • 160-bit variant of MD-5

SHA-1 (Secure Hash Algorithm) • 160-bit output • US government (NIST) standard as of 1993-95

– Also the hash algorithm for Digital Signature Standard (DSS)

Overview of MD5

Designed in 1991 by Ron Rivest Iterative design using compression function

M1 M2 M3 M4 IHV0

Com- press

Com- press

Com- press

Com- press

IHV4

slide 17

History of MD5 Collisions

2004: first collision attack • Only difference between colliding messages is 128

random-looking bytes

2007: chosen-prefix collisions • For any prefix, can find colliding messages have this

prefix and differ up to 716 random-looking bytes

2008: rogue certificates • End of MD5 • Talk about this in more detail when discussing PKI

Basic Structure of SHA-1 Against padding attacks

Split message into 512-bit blocks

Compression function • Applied to each 512-bit block and current 160-bit buffer • This is the heart of SHA-1

160-bit buffer (5 registers) initialized with magic values

How Strong Is SHA-1?

Every bit of output depends on every bit of input • Very important property for collision-resistance

Brute-force inversion requires 2160 ops, birthday attack on collision resistance requires 280 ops

Some recent weaknesses (2005) • Collisions can be found in 263 ops

•⇔ •⇔

AUTHENTICATION: PASSWORDS AND SECURITY QUESTIONS

slide 21

Basic Problem

?

How do you prove to someone that you are who you claim to be?

Any system with access control must solve this problem

Many Ways to Prove Who You Are

What you know • Passwords • Answers to questions that only you know

Where you are • IP address

What you are • Biometrics

What you have • Secure tokens

Password-Based Authentication

User has a secret password. System checks it to authenticate the user. How is the password communicated?

• Eavesdropping risk

How is the password stored? • In the clear? Encrypted? Hashed?

How does the system check the password? How easy is it to guess the password?

• Easy-to-remember passwords tend to be easy to guess • Password file is difficult to keep secret

Passwords and Computer Security

Physical intrusion: install sniffer or keylogger to steal passwords

Run cracking tools on password files • Cracking needed because modern systems usually do

not store passwords in the clear (how are they stored?)

In Mitnick’s “Art of Intrusion”, 8 out of 9 exploits involve password stealing and/or cracking

Default Passwords

Pennsylvania ice cream shop phone scam • Voicemail PIN defaults to last 4 digits of phone number;

criminals change message to “I accept collect call”, make $8600 on a 35-hour call to Saudi Arabia

Examples from Mitnick’s “Art of Intrusion” • U.S. District Courthouse server: “public” / “public” • NY Times employee database: pwd = last 4 SSN digits • “Dixie bank”: break into router (pwd=“administrator”),

then into IBM AS/400 server (pwd=“administrator”), install keylogger to snarf other passwords

– “99% of people there used ‘password123’ as their password”

Storing Passwords

t4h97t4m43 fa6326b1c2 N53uhjr438 Hgg658n53 …

user system password file “cypherpunk”

hash function

Password Hashing

Instead of user password, store Hash(password) When user enters password, compute its hash

and compare with entry in password file • System does not store actual passwords! • Difficult to go from hash from password!

Hash function H must have some properties • Given H(password), hard to find string X such that

H(X)=H(password) - why?

UNIX Password System

Uses DES encryption as if it were a hash function • Encrypt NULL string using password as the key

– Truncates passwords to 8 characters!

• Artificial slowdown: run DES 25 times (why?) • Can instruct modern UNIXes to use MD5 hash function

Problem: passwords are not truly random • With 52 upper- and lower-case letters, 10 digits and

32 punctuation symbols, there are 948 ≈ 6 quadrillion possible 8-character passwords

• Humans like to use dictionary words, human and pet names ≈ 1 million common passwords

Dictionary Attack

Password file /etc/passwd is world-readable • Contains user IDs and group IDs which are used by

many system programs

Dictionary attack is possible because many passwords come from a small dictionary • Attacker can pre-compute H(word) for every word in

the dictionary – this only needs to be done once!! – This is an offline attack – Once password file is obtained, cracking is instantaneous

• With 1,000,000-word dictionary and assuming 10 guesses per second, brute-force online attack takes 50,000 seconds (14 hours) on average

Old Password Surveys

Klein (1990) and Spafford (1992) • 2.7% guessed in 15 minutes, 21% in a week • Much more computing power is available now!

U. of Michigan: 5% of passwords were “goblue” • (cheer for their football team)

Zviran and Haga (1999) • Password usage at a DoD facility in California • 80% of passwords were 4-7 characters in length, 80%

used alphabetic characters only, 80% of the users had never changed their password

Password Guessing Techniques

Dictionary with words spelled backwards First and last names, streets, cities Same with upper-case initials All valid license plate numbers in your state Room numbers, telephone numbers, etc. Letter substitutions and other tricks

• If you can think of it, attacker will, too

Storing Passwords : Salt

shmat:fURxfg,4hLBX:14510:30:Vitaly:/u/shmat:/bin/csh

/etc/passwd entry salt (chosen randomly when password is first set)

hash(salt,pwd) Password

• Users with the same password have different entries in the password file

• Offline dictionary attack becomes much harder

Advantages of Salting

Without salt, attacker can pre-compute hashes of all dictionary words once for all password entries • Same hash function on all UNIX machines; identical

passwords hash to identical values • One table of hash values works for all password files

With salt, attacker must compute hashes of all dictionary words once for each combination of salt value and password • With 12-bit random salt, same password can hash to

4096 different hash values

Shadow Passwords

shmat:x:14510:30:Vitaly:/u/shmat:/bin/csh

• Store hashed passwords in /etc/shadow file which is only readable by system administrator (root)

• Add expiration dates for passwords • Early Shadow implementations on Linux called the

login program which had a buffer overflow!

Hashed password is not stored in a world-readable file

/etc/passwd entry

How People Use Passwords

Write them down Use a single password at multiple sites

• Do you use the same password for Amazon and your bank account? Do you remember them all?

Forget them… many services use “security questions” to reset passwords • “What is your favorite pet’s name?” • Paris Hilton’s T-Mobile cellphone hack

Problems with Security Questions

Inapplicable • What high school did your spouse attend?

Not memorable • Name of kindergarten teacher? Price of your first car?

Ambiguous • Name of college you applied to but did not attend?

Easily guessable • Age when you married? Year you met your spouse?

Favorite president? Favorite color?

Automatically attackable (using public records!)

[Rabkin, “Security questions in the era of Facebook”]

Answers Are Easy to Find Out…

Make of your first car? • Until 1998, Ford had >25% of market

First name of your best friend? • 10% of males: James/Jim, John, Robert/Bob/Rob

Name of your first / favorite pet? • Max, Jake, Buddy, Bear… • Top 500 (covers 65% of names) available online

Information available from Facebook, etc. • Where you went to school, college athletic rivals,

favorite book/movie/pastime, high school mascot

GRAPHICAL PASSWORDS

slide 39

Graphical Passwords

Images are easy for humans to remember • Especially if you invent a memorable story to go

along with the images

Dictionary attacks on graphical passwords are believed to be difficult • Images are very “random” (is this true?)

Still not a perfect solution • Need infrastructure for displaying and storing images • Shoulder surfing

How Passfaces Works

Users Are Assigned a Set of 5* Passfaces

User Interface Library of Faces

* Typical implementation – 3 to 7 possible as standard

How Passfaces Works

5 Passfaces are Associated with 40 associated decoys Passfaces are presented in five 3 by 3 matrices each having 1

Passface and 8 decoys

Empirical Results

Experimental study of 154 computer science students at Johns Hopkins and Carnegie Mellon

Conclusions: • “… faces chosen by users are highly affected by the

race of the user… the gender and attractiveness of the faces bias password choice… In the case of male users, we found this bias so severe that we do not believe it possible to make this scheme secure against an online attack…”

2 guesses enough for 10% of male users 8 guesses enough for 25% of male users

User Quotes

“I chose the images of the ladies which appealed the most”

“I simply picked the best looking girl on each page”

“I picked her because she was female and Asian and being female and Asian, I thought I could remember that”

“I started by deciding to choose faces of people in my own race…”

“… Plus he is African-American like me”

CHALLENGE - RESPONSE

slide 45

Security Against Eavesdropping

Idea: use a shared secret to derive a one-time password

If the attacker eavesdrops on the network, he’ll learn this password but it will be useless for future logins

Challenge-Response

user system secret

challenge value

f(secret,challenge)

Why is this better than the password over a network?

secret

Challenge-Response Authentication

User and system share a secret (key or password) Challenge: system presents user with some string Response: user computes response based on the

secret and the challenge • Secrecy: difficult to recover key from response

– One-way hashing or symmetric encryption work well

• Freshness: if challenge is fresh, attacker on the network cannot replay an old response

– For example, use a fresh random number for each challenge

Good for systems with pre-installed secret keys • Car keys; military friend-or-foe identification

SecurID

Alice Bob

KEY

v= F(KEY, 0)

KEY

Verifies v=F(KEY,0) ?

Advancing the counter • Time-based (60 seconds) or every button press

Allow for skew in the counter value • RSA SecurID: 5-minute clock skew by default

Setup: generate random key

Counter: Counter:

v= F(KEY, 1)

Verifies v=F(KEY,1) ? …

0 1 … 0 1 …

RSA uses a custom function Input: 64-bit key, 24-bit ctr Output: 6-digit value

BIOMETRIC AUTHENTICATION

slide 50

Biometric Authentication

Nothing to remember Passive

• Nothing to type, no devices to carry around

Can’t share (usually) Can be fairly unique

• … if measurements are sufficiently accurate

Problems with Biometrics

How hard are biometric readings to forge?

• Difficulty of forgery is routinely overestimated • Analysis often doesn’t take into account the possibility

of computer-generated forgery

Revocation is difficult or impossible

Biometric Error Rates (Benign)

“Fraud rate” vs. “insult rate” • Fraud = system accepts a forgery (false accept) • Insult = system rejects valid user (false reject)

Increasing acceptance threshold increases fraud rate, decreases insult rate

For biometrics, U.K. banks set target fraud rate of 1%, insult rate of 0.01% [Ross Anderson]

• Common signature recognition systems achieve equal error rates around 1% - not good enough!

Biometrics (1)

Face recognition (by a computer algorithm) • Error rates up to 20%, given reasonable variations in

lighting, viewpoint and expression

Fingerprints • Traditional method for identification • 1911: first US conviction on fingerprint evidence • U.K. traditionally requires 16-point match

– Probability of false match is 1 in 10 billion – No successful challenges until 2000

• Fingerprint damage impairs recognition – Ross Anderson’s scar crashes FBI scanner

Biometrics (2)

Iris scanning • Irises are very random, but stable through life

– Different between the two eyes of the same individual

• 256-byte iris code based on concentric rings between the pupil and the outside of the iris

• Equal error rate better than 1 in a million • Best biometric mechanism currently known

Hand geometry • Used in nuclear premises entry control, INSPASS

(discontinued in 2002)

Voice, ear shape, vein pattern, face temperature

Surgical Change

Stealing Biometrics

Involuntary Cloning

Clone a biometric without victim’s knowledge or assistance

“my voice is my password” cloned retina Fingerprints from

beer bottles Eye laser scan Bad news: it works!

Cloning a Finger [Matsumoto]

Molding [Matsumoto]

The Mold and the Gummy Finger [Matsumoto]

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