Security, Privacy and Trust - Lecture 11 - Web Information Systems (4011474FNR)

2 December 2005

Web Information Systems Security, Privacy and Trust

Prof. Beat Signer

Department of Computer Science

Vrije Universiteit Brussel

http://www.beatsigner.com

http://www.beatsigner.com/

Beat Signer - Department of Computer Science - [email protected]

2 December 12, 2013

Security Aspects

Authenticity knowing the sender or receiver of data

- who is trying to access data on a web server

- who is offering a service

- who sent an email

- …

Privacy keeping information private

- protect credit card information that is sent to a server

- protect information sent in emails

- …

Integrity ensuring that information is not changed when transferred


3 December 12, 2013

HTTP Authentication

Native authentication functionality offered by HTTP instead of directly sending a response for a given request, the

server can always respond with an authentication challenge (401 status code)

HTTP is extensible to support different authentication

protocols and offers the following two standard protocols basic access authentication

- simple Base64 encoding of the string <username>:<password>

digest access authentication

Protected resources can be grouped in security realms

with different sets of authorised users or groups of users


4 December 12, 2013

Basic Access Authentication

Client Server

GET /wise/exam.pdf HTTP/1.0

Client Server

Client Server

Client Server

ask

password

try to access

a protected

resource

HTTP/1.0 401 Authorization Required WWW-Authenticate: Basic realm="WISE"

GET /wise/exam.pdf HTTP/1.0 Authorization: Basic YmVhdDpydWxleg==

HTTP/1.0 200 OK Content-type: application/pdf

Internet


5 December 12, 2013

Base64 Encoding

Base64 encoding can be used to represent binary data

in a portable format (alphabet) used by MIME for content transfer encoding

used to embed binary data in XML files (e.g. in XML-RPC)

note that Base64 encoded data needs more space

Takes a sequence of bytes (8-bit) and breaks it into 6-bit

chunks padding with 0s to make it a multiple of 24 (LCM of 6 and 8)

complete 6-bit padding chunks are represented by the special character '='

Each 6-bit chunk is then represented by a character from

a 64-character alphabet


6 December 12, 2013

Base64 Encoding Example

Let us encode the string

'Ja' to Base64

padding to 24 bit

lookup of 6-bit chunks in

index table

use '=' for completely padded

6-bit chunks

val 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

char

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

val

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

char

Q

R

S

T

U

V

W

X

Y

Z

a

b

c

d

e

f

val

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

char

g

h

i

j

k

l

m

n

o

p

q

r

s

t

u

v

val

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

char

w

x

y

z

0

1

2

3

4

5

6

7

8

9

+

/

01001110

N o

01101111 00000000

19 38 60

T m 8 =

Base64 index table

Text

Bit Pattern

Index

Base64

padding


7 December 12, 2013

Proxy Authentication

We can use the same authentication approach for

controlling access to proxy servers

The proxy will return slightly different HTTP headers

HTTP/1.0 407 Proxy Authentication Required Proxy-Authenticate: Basic realm="WISE"


8 December 12, 2013

Web Server Configuration

Example configuration for an Apache HTTP Server

Create a new password file (using the –c parameter)

Put an .htaccess file with the configuration into the

directory that has to be protected alternatively add information to httpd.conf

#htpasswd -c /usr/local/apache/admin/passwords nelson New password: nelson123 Re-type new password: nelson123 Adding password for user nelson

AuthType Basic AuthName "WISE" AuthUserFile /usr/local/apache/admin/passwords Require user nelson


9 December 12, 2013

Basic Access Authentication ...

Basic access authentication is not secure username and password are sent almost in "cleartext"

- Base64 value can be very easily decoded

easy to do replay attacks

- simply reuse the username and the password

Potential solutions combine the basic access authentication with an encrypted data

transfer (e.g. via TLS/SSL)

- does not prevent replay attacks

use of alternative digest access authentication


10 December 12, 2013

Digest Access Authentication

Password is no longer sent in cleartext only a one-way digest that is computed out of the password

(one-way hash function) is sent to the server

Message Digest #5 (MD5) is a popular digest function

What about digest replay attacks? server sends a special token (nonce) that changes frequently

client adds the nonce to the password before computing the MD5

- any changes of the nonce result in changes of the digest which helps to

prevent replay attacks

h1 = MD5(username:realm:password) h2 = MD5(httpMethod:requestedURI) response = MD5(h1:nonce:h2)

Computed response based on MD5



Digest Access Authentication ...

Client Server

GET /wise/exam.pdf HTTP/1.0

Client Server

Client Server

Client Server

ask

password

HTTP/1.0 401 Unauthorized WWW-Authenticate: Digest realm="WISE", qop="auth,auth-int" nonce="6G543RED"

GET /wise/exam.pdf HTTP/1.0 Authorization: Digest username="nelson", realm="WISE", nonce="6G543RED", qop="auth", response="HF779RW47R7HF", ...

HTTP/1.0 200 OK Authorization-Info: nextnonce="7HZT7F6" ...

Internet

try to access

a protected

resource



Digest Access Authentication ...

The Authorization-Info: nextnonce="..." is used

to send the next nonce in advance client can send the computed hash value already with the original

request (preemptive authorization)

The quality of protection (qop) field is used to

negotiate different protection mechanisms auth

- authentification

auth-int

- authentification and message integrity protection

- add an MD5 of the body



Transport Layer Security (TLS)

Cryptographic protocol to

ensure secure network

communication

successor of the Secure

Socket Layer (SSL) protocol

situated at the TCP/IP

Application Layer or OSI

Presentation Layer

Types of authentification

unilateral authentification

- only server authentification

mutual authentification

- client and server authentification

Application

TLS Presentation SSL

Session

Transport

Network

Data Link

Physical 1

2

3

4

5

6

7

OSI Reference Model



Transport Layer Security (TLS)

Features server authentication

client authentication

confidentiality through data encryption

data integrity

Protection against man-in-the-middle attacks

replay attacks



Cryptography

In cryptography a cipher (coding scheme)

is used in combination with a key to create

a ciphertext out of a plaintext

Cryptanalysis tries to get information out of the ciphertext

without having access to the secret information (key)

MEET ME AT NOON

PHHW PH DW QLLQ

MEET ME AT NOON cipher

(encoder)

cipher

(decoder) ciphertext

key key

plaintext plaintext



Symmetric Key Cryptography

A symmetric key cipher uses the same key for the

encoding and decoding of a plaintext message

Many existing symmetric key ciphers DES, Triple DES, Blowfish, Rijndael/AES, ...

The algorithms are often common knowledge and the

key is the only secret thing key has to be kept secret

Brute force attack (enumeration attack) tries all keys

The key length defines the number of potential keys e.g. 128 bit key considered safe today

- can change with more powerful machines



Symmetric Key Cryptography ...

One problem of symmetric key cryptography is that we

have to secretly share the common key before we can

exchange any messages this has to be repeated with different keys for any two partners

willing to establish a secret communication

how should we establish the exchange over the Internet? - insecure channel

where should we secretly store all those keys?



Public Key Cryptography

Instead of a single key, public key cryptography uses an

asymmetric pair of keys publicly available key for the encoding

secret key for the decoding

Each party has only a single public key which is used by

everybody to encode messages to this party only the receiver can decode message with their private key

MEET ME AT NOON

hJ7FHDuKJF Z8e fsdlgi MEET ME

AT NOON cipher

(encoder)

cipher

(decoder) ciphertext

public key B private key B

plaintext plaintext

A B



Public Key Cryptography ...

Public key cryptography can be used to establish secure

Internet connections to any computer around the world

without having to secretly share a key beforehand

An asymmetric public key cipher has to ensure that an

attacker cannot compute the private key based on any

information they can intercept public key

ciphertext (with corresponding plaintext)

- can easily be created by any party by using the public key

A well known public key algorithm is the RSA cipher



RSA Cipher (Rivest, Shamir and Adleman)

Public-key cipher that can

be used for encryption as

well as signing published in 1978 by Rivest,

Shamir and Adleman while they were at MIT

The public and private keys are

generated based on two large distinct prime numbers the potential attacker will know about the product of the two prime

numbers but nothing about the numbers themselves

use modular arithmetic for the encoding/decoding

as long as the attacker is not able to do a factorisation into the two prime numbers, RSA is assumed to be secure

Adi Shamir, Ron Rivest and Len Adleman



Public Key Cryptography ...

A drawback of asymmetric public key cryptography is the

fact that the algorithms are much slower than symmetric

ciphers

Hybrid solutions combine public key with symmetric key

cryptography the public key encryption is only used in the setup phase to

securely exchange a pair of symmetric keys

afterwards a secure channel is established based on the symmetric keys

Security of public key cryptography? new developments (e.g. quantum computing) might break public

key cryptography



Digital Signatures

A digital signature can be used for two purposes to prove the authenticity of a message

to guarantee that a message has not been changed during the transfer (integrity)

Sender creates a plaintext digest, encodes it with the

private key and adds it as a signature to the message the receiver creates the same digest and compares it with the

decoded signature

cipher cipher

private key A public key A

plaintext plaintext plaintext

signature

digest digest digest

same?

A B



Digital Certificates

Information about a

person/company that is

digitally signed by a

certificate authority (CA)

owner's name

validity time

signature of the CA

owner's public key



Digital Certificates ...

No single standard but most certificates store their

information in the X.509 v3 certificate standard form

Basically digital certificates can be used on the server

side as well as on the client side in practice client-side certificates are not often used



HTTP Secure (HTTPS)

Secure version of HTTP combines HTTP with asymmetric, symmetric and certificate-

based cryptography

HTTP sent over TLS/SSL

HTTPS protocol is selected by the https:// URL prefix

Browser connects to the HTTPS default port (port 443) Initial SSL handshake

- negotiate protocol versions

- negotiate common cipher

- authentication

- generate temporary symmetric session keys



Email Security

Emails are generally sent as unencrypted plain text

An email is stored on multiple intermediary servers

before reaching its target relatively easy to intercept

would you also put anything you write in an email on a postcard?

Note that the sender of an email can easily be faked

If we want to fix these problems we have to use third-

party tools such as Pretty Good Privacy (PGP) privacy

- strong encryption

authentication

- digital signatures



Email SPAM

Abuse of an electronic messaging

system (email) to deliver unwanted messages

A major part of all SPAM is sent by only a few hundred

spammers

It is estimated that SPAM costs businesses more than

100 billion dollars per year

SPAM is illegal in many countries and some spammers

have already been sentenced to jail

"Solutions" SPAM filters

micropayments for emails



Email SPAM ...

Phishing attacks send emails that look like coming from an official authority

and contain a request for sensitive data (e.g. password)

send emails with links to websites that look like official companies (e.g. your homebank)

Spammers often use botnets to send their SPAM



Botnets

Computers infected by malicious software become part

of a large botnet that can be remotely controlled the largest botnets contain more than 1 million machines

An attacker can buy part of such a botnet to perform

various harmful tasks including the distribution of SPAM

distributed denial of service attacks (DDOS)

Distributed denial of service attacks are a very powerful

weapon as it has for example been shown when Estonia

was attacked in May 2007 cannot easily be detected and filtered by firewalls since the traffic

is created by many different machines



Firewalls

Software and hardware firewalls introduce artifical

"bottlenecks" that have to be passed by all the traffic block specific ports

filter and block content

protect private intranets from incoming Internet traffic

- often only a subnetwork (demilitarised zone) is connected to the Internet

Internet

Client Server

Firewall



Privacy

While users access information over the Internet,

there is a continuous logging of their requests

Each server stores information about clients who

accessed specific resources

Data mining techniques can be used to combine this

logging information and create user profiles can for example be used for user-targeted advertising

Users also "deliberately" publish personal information e.g. on Facebook

Published information often cannot be easily deleted e.g. still accessible via Internet Archive (http://www.archive.org)



Web Log

Log entry created every time a web server is accessed

A log entry typically contains information about IP address of the requesting machine

accessed URL

request time

refer link (previous page accessed by the client)

- sent as part of the HTTP Request

browser type

errors that occured

...



Web Log

Web logs can be combined with other information e.g. login information can be used to reveal a user's identity

Refer link enables access to potentially private information

e.g. if previous request was an HTML form request using the GET method then all the data will be available as part of the URL

XXX.XXX.XXX.193 - - [02/Dec/2009:05:50:40 +0100] "GET /knives-shun-c-81_114-l-en.html?gclid=CLOFucf5tp4CFc5L5Qod8jQzpA HTTP/1.1" 200 65478 "http://guelph.kijiji.ca/f-Shun-Classifieds-W0QQKeywordZShunQQisSearchFormZtrue" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.0.15) Gecko/2009101601 Firefox/3.0.15" XXX.XXX.XXX.116 - - [02/Dec/2009:05:50:42 +0100] "GET /images/Jamie%20Oliver/flavourShakerSchwarz.jpg HTTP/1.1" 200 3594 "http://www.tenera.ch/kenwood-pasta-roller-at970a-for-lasagne-base-unit-p-1314-l-en.html" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; GTB5; .NET CLR 1.1.4322; MS-RTC LM 8; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)" XXX.XXX.XXX.139 - - [02/Dec/2009:05:52:19 +0100] "GET /stylesheet.css HTTP/1.1" 200 10185 "http://www.tenera.ch/kai-seki-magoroku-redwood-nakirimesser-165-cm-p-1433-l-de.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) )" XXX.XXX.XXX.139 - - [02/Dec/2009:05:52:19 +0100] "GET /kai-seki-magoroku-redwood-nakirimesser-165-cm-p-1433-l-de.html HTTP/1.1" 200 60636 "http://www.google.ch/search?hl=de&source=hp&q=seki+magoroku&meta=&aq=0&oq=seki+ma" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) )" XXX.XXX.XXX.139 - - [02/Dec/2009:05:52:21 +0100] "GET /images/pixel_trans.gif HTTP/1.1" 200 43 "http://www.tenera.ch/kai-seki-magoroku-redwood-nakirimesser-165-cm-p-1433-l-de.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) )" ...

web log with refer links



Web Log File Analysis

Site owner can use

various tools to analyse

the log files e.g. Webalizer

How much information do

we give away when

accessing a website?

What is happening with the logged data? combined with other information to reveal IP addresses?

combined with log files from other sites?

- user profiling

intended use of data should be mentioned in the privacy policy



Cookies Revisited

Persistent cookies can be used to track a

user over time similar to IP address but more precise

Third-party cookies can be used to build an anonymous

user profile if a website contains elements that have to be accessed from

another server (e.g. banner ads), then the server can set a cookie

- the third-party server creates a unique resource URL for every page on which

the resource has been embedded

- the user can be tracked on any site that uses the same service (e.g. banner

ads) and an anonymous user profile can be created

Cookies should not be used for authentication can be modified by a user to forge identity (cookie poisoning)



Web Bugs

User tracking based on the same idea as

with third-party cookies

Embed a small object (e.g. 1 pixel image) in a webpage

and get informed every time the webpage is accessed request containing the IP address is sent to the server

The web bugs approach cannot only be used for

webpages but also for other resources such as email,

Word documents etc. if the user reads an email containing an embedded HTML web

bug, the server knows when the email has been read but also gets information about the IP address of the mail client



Other Services with Privacy Issues

Google Earth shows a lot of sensitive information e.g. military bases etc.

Google Street View shows not only streets and buildings

but also citizens privacy of individuals might be violated since they are shown at

strange places or in weird situations

since the blurring of faces and number plates does not always work, some countries would like to stop the service

Many other free services from Google as well as other

companies harvest personal information and use it, for

example, for customer-targeted advertising



Video: Google Analytics



Google Analytics

Very nice tool for web administrators to analyse their

web traffic easy to "install" over the Web

website administrators have to add a piece of JavaScript code to their website

- similar to web bug approach shown earlier

Google gets information about site visitors

While a user can normally choose to use a free service

(e.g. Gmail) or not, the user has no choice when it

comes to the tracking via Google Analytics

How save is the captured data? what if somebody manages to steal the data?



Exercise 11

Security



References

David Gourley et al., HTTP: The Definitive

Guide, O'Reilly Media, September 2002

Google Analytics Video http://www.youtube.com/watch?v=rHeKRvo6OhI

R.L. Rivest, A. Shamir and L. Adleman, A Method for

Obtaining Digital Signatures and Public-Key

Cryptosystems Authentication, Communications of the

ACM, February 1978

2 December 2005

Next Lecture Future Trends and Summary

Security, Privacy and Trust - Lecture 11 - Web Information Systems (4011474FNR)

Education

transport layer security

anonymous user profile

symmetric key cryptography

public key cryptography

basic access authentication

digest access authentication

web server

basic realm