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ISA 662 Information System Security

Authentication

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Chapter 12: Authentication Basic Concept Password Challenge-Response Biometrics

and gummy bears

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Basic Concept Authentication: to prove an identity

Strong authentication: without transmitting password Based on

What the user knows – e.g., password, PIN What the user possesses – e.g., ID card, ticket What the user is – e.g., fingerprint, eye scanning,

DNA Attack

Active: e.g., password guessing, stealing password file from server, authentication replay

Passive: e.g., password sniffer

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Chapter 12: Authentication Basic Concept Password Challenge-Response Biometrics

and gummy bears

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Basic Model

Alice server

Alice acbd18db4cc2f85cedef654fccc4a4d8

Bob 0b180078d994cb2b5ed89d7ce8e7eea2

… ……

password=foo, user ID=Alice

md5(‘foo’)=acbd18db4cc2f85cedef654fccc4a4d8

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Dictionary Attacks

Eve server

Alice acbd18db4cc2f85cedef654fccc4a4d8

… ……

password=a, user ID=Alicepassword=aa, user ID=Alicepassword=aaa, user ID=Alice …password=foo, user ID=Alice

Online dictionary attack

aabcapple…foo

EveAlice acbd18db4cc2f8

5cedef654fccc4a4d8

… ……

md5(a)=0cc175b9c0f1b6a831c399e269772661… …

md5(foo)=acbd18db4cc2f85cedef654fccc4a4d8

Offline dictionary attack

aabcapple…foo

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Countermeasure Overview

Eve serverAlice acbd18db4cc2f8

5cedef654fccc4a4d8

… ……

password=abc, user ID=Alicepassword=foo, user ID=Alice

md5(‘foo’)=acbd18db4cc2f85cedef654fccc4a4d8

• Keep the server-side table secret (e.g., shadowed pass file)

• Use strong password• Salting: md5(‘foo’+r)

• Restrict repetitive attempts• Password aging

EveAlice acbd18db4cc2f8

5cedef654fccc4a4d8

… ……

md5(a)=0cc175b9c0f1b6a831c399e269772661 …md5(‘foo’)=acbd18db4cc2f85cedef654fccc4a4d8

aabcapple…foo

abc…foo

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Choice of Passwords Dilemma

Password should be random Uniform distribution maximizes entropy (hence

dictionary) Password should be easy to remember

Attacker sorts dictionary in descending order of likelihood An easy-to-remember password requires less

time to guess

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Choice of Passwords (Cont’d) Users tend to choose weak passwords

Based on user ID, user or relative’s name, computer name, place name, DOB

English words Too short, digits only, letters only License plates, acronyms, SSN, pet name, job,

etc.

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          Choice of Passwords (Still Cont’d)

Force user to choose good passwords Through education and policy Through proactive mechanism

Cannot be bypassed Knows what is bad password Discriminate on per-user, per-site basis

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Salting Goal: making offline dictionary attacks (on

a large number of passwords) harderAlice

md5(‘foo’)

... ...Bob md5(‘abc’)

aabcapple…foo

md5(‘a’)md5(‘abc’) …md5(‘foo’)...

Alice

md5(‘foo’||r1)

r1

... ... ...Bob md5(‘abc’||

r2)r2

aabcapple…foo

md5(‘a’||r1)

md5(‘abc’|| r1)

md5(‘foo’|| r1)...md5(‘a’||r2)

md5(‘abc’|| r2)

md5(‘foo’|| r2)...

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Example: UNIX Password UNIX system password authentication

First 8 characters of input password Concatenated by a 12 bit salt Used as a key To encrypt a constant For 25 times The result and the salt stored in password file

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Prevent Repetitive Attempts Make online attacks harder

Backoff: wait 2n seconds before nth attempt Disconnection: modem user Disabling: only administrators can reactivate

OSF1 uses both backoff and disconnection

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Password Aging Force users to change passwords after

some time has expired How do you force users not to re-use

passwords? Record previous passwords Block changes for a period of time

Give users time to think of good passwords Warn them of expiration days in advance

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Chapter 12: Authentication Basic Concept Password Challenge-Response Biometrics

and gummy bears

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One-Time Password Password replay

sniff password then replay Use password exactly once!

How do user and server synchronize? Lamport’s scheme

Challenge-response mechanism View response as the password

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Lamport’s Scheme How do user and server synchronize on which password to

use? h: one-way hash function (e.g., MD5 or SHA-1) hn(k): h(h(h(...h(k)...)))

Registration

User Server

k ksecure channel

n

hn(k)

1st Authenticationk hn(k)hn-1(k) hn-1(k)

hn-1(k)

2nd Authenticationk hn-1(k)hn-2(k) hn-2(k)

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Challenge-Response

DES, RSA, HMAC, etc.

k

User Server

challenge

response

k

response

time

time

challenge

DES, RSA, HMAC, etc.

dictionary attack!

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Pre-Encrypted Key Exchange

Alice BobAlice || {ks}p

Alice Bob

Alice Bob

{ChallengeB}ks

Alice Bob

p, ksp

p, ks

{ChallengeA ChallengeB}ks

{ChallengeA}ks

Quiz: how is offline dictionary attack possible?

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Encrypted Key Exchange (RSA) Goal: to defeat off-line dictionary attacks

Alice BobAlice || {eAlice}p

Alice Bob{{(ks}eAlice}p

Alice Bob{ChallengeA}ks

Alice Bob

Alice Bob

p, eAlicep

p, eAlice p, eAlice, ks

p, eAlice, ks

{ChallengeA ChallengeB}ks

{ChallengeB}ks

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Chapter 12: Authentication Basic Concept Password Challenge-Response Biometrics

and gummy bears

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Biometrics Automated measurement of biological,

behavioral features that identify a person Fingerprints: graph isomorphism Voices: statistical techniques Eyes: patterns in irises Faces: image, or specific characteristics like

distance from nose to chin Keystroke dynamics: keystroke intervals,

pressure, duration of stroke, where key is struck

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Can Biometrics Save the World? Gummy bears say: No! Fingerprint scanners are believed to be very

secure, until It was shown they can be reliably fooled

with $10 worth of household supplies From both real fingers or residue fingerprints Can wear a transparent fake finger on real finger Using

Gelatin, what Gummy Bears are made out of Digital camera, inkjet printer, photo-sensitive PCB, etc. Fool all 11 commercial fingerprint scanners, 80% of

time

For more details, refer to Tsutomu Matsumoto’s talk ‘Importance of Open Discussion on Adversarial Analyses for Mobile Security Technology’ whose link is on class web page

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Key Points Authentication is based on

what users know what they possess what they are

Passwords are here to stay How to use passwords is important Remember the Gummy bears

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ISA 662 Information System Security

Identity

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Overview Certificate and distinguished name Web identity and anonymity

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Certificates and Identity Certificates issued to a user

User must be uniquely identified for this purpose What to be the identity?

Name: usually ambiguous “Your name please; your address please; your

telephone no. please; your last 4-digit SSN please”

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Distinguished Names Include ancillary information in identity

Enough to identify principal uniquely Example: X.509v3 Distinguished Names

/O=University of California/OU=Davis campus/OU=Department of Computer Science/CN=Matt Bishop/refers to the Matt Bishop (CN is common name) in the Department of Computer Science (OU is organizational unit) on the Davis Campus of the University of California (O is organization)

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CAs and Policies Matt Bishop wants a certificate from Certs-

from-Us How does Certs-from-Us know this is “Matt

Bishop”? CA’s authentication policy says what type and

strength of authentication is needed to identify Matt Bishop to satisfy the CA that this is, in fact, Matt Bishop

CA authenticates user and embed its trust in certificate, later the certificate is used by others for authentication and that trust propagates

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Example: Verisign CAs Class 1: CA issued certificates to

individuals authenticated by email address Certificate used for sending, receiving email

with various security services at that address Class 2: CA issued certificates to

individuals Authenticated by verifying user-supplied real name and address through an online database Certificate used for online purchasing

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Example: Verisign CAs (Cont’d) Class 3: CA issued certificates to

individuals Authenticated by background check from investigative service Higher level of assurance of identity than Class

1 and Class 2 CAs Class 4: CA issued certificates to web

servers Same authentication policy as Class 3 CA Consumers using these sites had high degree

of assurance the web site was not spoofed

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Internet Certification Hierarchy Tree structured arrangement of CAs

Root is Internet Policy Registration Authority, or IPRA

Sets policies all subordinate CAs must follow Certifies subordinate CAs (called policy certification

authorities, or PCAs), each of which has own authentication, issuance policies

Does not issue certificates to individuals or organizations other than subordinate CAs

PCAs issue certificates to ordinary CAs Does not issue certificates to individuals or

organizations other than subordinate CAs CAs issue certificates to organizations or

individuals

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Example: UValmont

IPRA

PCA-1

UValmontStudent CA

student student

PCA-2

UValmontStaff CA

staff staff

high assurancePCA

low assurancePCA

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Certificate Differences Student, staff certificates signed using

different private keys (for different CAs) Student’s signed by key corresponding to low

assurance certificate signed by first PCA Staff’s signed by key corresponding to high

assurance certificate signed by second PCA To see how much trust should be placed in

the certificate (what policy used to authenticate): Determine CA signing certificate, check its policy Also go to PCA that signed CA’s certificate

CAs are restricted by PCA’s policy, but CA can (only) restrict itself further

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Types of Certificates Organizational certificate

Issued based on principal’s affiliation with organization

Example Distinguished Name/O=University of Valmont/OU=Computer Science Department/CN=Marsha Merteuille/

Residential certificate Issued based on where principal lives No affiliation with organization implied Example Distinguished Name

/C=US/SP=Louisiana/L=Valmont/PA=1 Express Way/CN=Marsha Merteuille/

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Types of Certificates (Cont’d) Certificate tied to a role

UValmont wants comptroller to have a certificate

This way, she can sign contracts and documents digitally

Distinguished Name/O=University of Valmont/OU=Office of the Big Bucks/RN=Comptrollerwhere “RN” is role name; note the individual using the certificate is not named, so no CN

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Naming Conflicts John Smith, John Smith Jr. live at same

address John Smith Jr. applies for residential certificate

from Certs-from-Us, getting the DN of:/C=US/SP=Maine/L=Portland/PA=1 First Ave./CN=John Smith/

Now his father applies for residential certificate from Quick-Certs, getting DN of:/C=US/SP=Maine/L=Portland/PA=1 First Ave./CN=John Smith/because Quick-Certs has no way of knowing that DN has been taken

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Solutions Organizational certificates

All CA DNs must be superior to that of the principal

Example: for Marsha Merteuille’s DN:/O=University of Valmont/OU=Computer Science Department/CN=Marsha Merteuille/DN of the CA must be either:/O=University of Valmont/(the issuer being the University) or/O=University of Valmont/OU=Computer Science Department/(the issuer being the Department)

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Related Problem Single CA issues two types of certificates

under two different PCAs Example

UValmont issues both low assurance, high assurance certificates under two different PCAs

How does validator know under which PCA the certificate was issued?

Reflects on assurance of the identity of the principal to whom certificate was issued

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Solution CA Distinguished Names need not be

unique CA (Distinguished Name, public key) pair

must be unique Example

In earlier UValmont example, student validation required using first PCA’s public key; validation using second PCA’s public key would fail

Keys used to sign certificate indicate the PCA, and the policy, under which certificate is issued

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Persona Certificate Anonymous certificate

Certificate with meaningless Distinguished Name Example:

Government requires all citizens with gene X to register because people with this gene more likely commit crimes

No scientific evidence, and government wants no civil rights fuss

Government employee wants to alert media Government will deny plan, change approach Government employee will be fired, prosecuted

Must notify media anonymously

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Persona Certificate (Cont’d) Employee gets persona certificate, sends copy

of plan to media Media knows message unchanged during transit

(data integrity) But they do not know who sent it (no authentication) Government denies plan, changes it

Employee sends copy of new plan signed using same certificate Media can tell it’s from original whistleblower

(linkability) Media cannot track back whom that whistleblower is

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Overview Certificate and distinguished name Web identity and anonymity

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Host Identity Host identity at different layers

Ethernet address: 00:05:02:6B:A8:21 IP address: 192.168.35.89 Host name: cherry.orchard.chekhov.ru

These can be spoofed Write your own Ethernet frames Fake source IP address Fake host name

Above protocols rely on spoofed identity

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Domain Name Server Maps transport identifiers (host names) to

network identifiers (host addresses) Forward records: host names IP addresses

PING Reverse records: IP addresses host names

NSLOOKUP

Weak authentication Not cryptographically based E.g., reverse domain name lookup IP -> host name via DNS -> IP via DNS ->

matching the two IPs

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DHCP and NAT DHCP server dynamically assigns clients

unused IP addresses Gateway assigns clients internal address

NAT translates all outgoing and incoming packets

Invisible to both inside clients and outsiders Your desktop/laptop ->

router (DHCP and NAT) -> Internet

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Attacks on DNS Change records on server Cache poisoning

Attacker sends victim request that must be resolved by asking attacker

Legitimately piggyback extra record to response, but have fake name/IP association in the record

The fake record will be cached

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Anonymity Servers and observers know who is visiting

Sometimes not desirable Anonymizer: a site that hides origins of

connections What about a simple proxy server

Server sees the proxy, not client

client1

client2

proxy server1

server2

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Problem With Simple Proxy The proxy knows who are the client and

server And we all know this fact

Example: anon.penet.fi Anonymous re-mailer Material claimed to be copyrighted sent through

site Finnish court directed owner to reveal mapping

so plaintiffs could determine sender Owner appealed, subsequently shut down site

What about a chain of proxies?

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Chain of Proxies Proxies operated by multiple organizations

No single proxy can know client/server association

Traffic encrypted Cannot tell from address in header

client1

client2

proxy server1

server2

proxy proxy

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Weaknesses of Chain of Proxies First and last proxy collude

One knows client one knows server Traffic analysis

Observes in, out flows of traffic timing message size

What if we mix multiple messages

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MIX Each mix server

Padding messages to fixed size blocks - size Reorder the blocks - first/last collude Wait before transmitting - timing

client1

client2

mix1

server1

server2

mix2 mix3

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MIX (Cont’d) How can mix server know where to send

message? Nested public-key encryption (later in Onion

Routing)

client1

client2

mix1

server1

server2

mix2

Mix1, {Mix2, {m,r2}e2, r1}e1

Mix2, {m, r2}e2

m

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Crowds Intuition

You can never trace back to the origin of a rumor, because “I too heard about it from someone else”

Each node (Jondo) Sends request to server with probability p;

forward it to another node with probability (1-p)

server

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Privacy Anonymity protects privacy by obstructing

amalgamation of individual records Important, because amalgamation poses 3 risks:

Incorrect conclusions from misinterpreted data Harm from erroneous information Not being let alone

Also hinders monitoring to deter or prevent crime Conclusion: anonymity can be used for good or ill

Right to remain anonymous entails responsibility to use that right wisely

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Key Points Certificates use Distinguished name as

identity Web identity based on addresses is weak Anonymity can be achieved with

sophisticated proxies