Network Security Friends and enemies: Alice, Bob, Trudy

Network Security Friends and enemies: Alice, Bob, Trudy

Network Security

CS446 / CS646 - Networking

Instructor: Bo Sheng

1

Chapter 8: Network Security

Chapter goals:

• understand principles of network security:

– cryptography and its many uses beyond

“ confidentiality”

– authentication

– message integrity

• security in practice:

– firewalls and intrusion detection systems

– security in application, transport, network, link layers

Security

8-2

Chapter 8 roadmap

8.1 What is network security?

8.2

Principles of cryptography

8.3

Message integrity, authentication

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-3

What is network security?

confidentiality: only sender, intended receiver should

“understand” message contents

– sender encrypts message

– receiver decrypts message

authentication:

sender, receiver want to confirm identity of each other

message integrity:

sender, receiver want to ensure message not altered (in transit, or afterwards) without detection

access and availability

: services must be accessible and available to users

Security

8-4

Friends and enemies: Alice, Bob, Trudy

• well-known in network security world

• Bob, Alice (lovers!) want to communicate “securely”

• Trudy (intruder) may intercept, delete, add messages

Alice Bob data secure sender channel data, control messages s secure receiver data

Trudy

8-5

Who might Bob, Alice be?

• … well, real-life Bobs and Alices!

• Web browser/server for electronic transactions (e.g., on-line purchases)

• on-line banking client/server

• DNS servers

• routers exchanging routing table updates

• other examples?

Security

8-6

1

There are bad guys (and girls) out there!

Q:

What can a “bad guy” do?

A:

A lot! See section 1.6

eavesdrop: intercept messages

– actively

insert

messages into connection

impersonation: can fake (spoof) source address in packet

(or any field in packet)

hijacking:

“take over” ongoing connection by removing sender or receiver, inserting himself in place

denial of service: prevent service from being used by others (e.g., by overloading resources)

Security

8-7

Chapter 8 roadmap

8.1

What is network security?

8.2 Principles of cryptography

8.3

Message integrity, authentication

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-8 plaintext

Encryption/Decryption

encryption ciphertext decryption plaintext key key

• Plaintext: a message in its original form

• Ciphertext: a message in the transformed, unrecognized form

• Encryption: the process that transforms a plaintext into a ciphertext

• Decryption: the process that transforms a ciphertext to the corresponding plaintext

• Key: the value used to control encryption/decryption.

9

Cryptanalysis

• “code breaking”, “attacking the cipher”

• Difficulty depends on

– sophistication of the cipher

– amount of information available to the code breaker

• Any cipher can be broken by exhaustive trials, but rarely practical

10

Caesar Cipher

• Replace each letter with the one 3 letters later in the alphabet

– ex.: plaintext CAT  ciphertext FDW plaintext alphabet

A B C D E F G H I J K … ciphertext alphabet

A B C D E F G H I J K …

Trivial to break

11

Mono-Alphabetic Ciphers

• Generalized substitution cipher: an arbitrary (but fixed) mapping of one letter to another

– 26! ( ≈ 4.0*10 26 ≈ 2 88 ) possibilities plaintext alphabet

A B C D E F G H I J K … ciphertext alphabet

A B C D E F G H I J K …

12

2

Attacking Mono-Alphabetic Ciphers

• Broken by statistical analysis of letter, word, and phrase frequencies of the language

• Frequency of single letters in English language, taken from a large corpus of text:

The “Weakest Link” in Security

• Cryptography is rarely the weakest link

• Weaker links

– Implementation of cipher

– Distribution or protection of keys

– … …

13 14

Secret Keys vs Secret Algorithms

• Security by obscurity

– We can achieve better security if we keep the algorithms secret

– Hard to keep secret if used widely

– Reverse engineering, social engineering

• Publish the algorithms

– Security of the algorithms depends on the secrecy of the keys

– Less unknown vulnerability if all the smart (good) people in the world are examine the algorithms

15

Basic Components

• Secret key cryptography

• Public key cryptography

• Hash functions

16 plaintext

Secret Key Cryptography

encryption ciphertext decryption plaintext key Same key key

• Same key is used for encryption and decryption

• Also known as

– Symmetric cryptography

– Conventional cryptography

17

Secret Key Cryptography

• Stream cipher

• Block cipher

– Converts one input plaintext block of fixed size k bits to an output ciphertext block of k bits

– DES, IDEA, AES, …

– AES

• Selected from an open competition, organized by NSA

• Joan Daemen and Vincent Rijmen (Belgium)

• Block size=128 bits, Key Size= 128/192/256 bits

18

3

Initialization

Vector

Key

Cipher Block Chaining (CBC)

M

1

M

2

M

3

M

4

128 128

128

46 + padding

E E

E E

C

1

128

C

2

128

C

3

128

C

4

128

19 plaintext

Public Key Cryptography

ciphertext plaintext encryption decryption

Public key Private key

• A public/private key pair is used

– Public key can be publicly known

– Private key is kept secret by the owner of the key

• Much slower than secret key cryptography

• Also known as asymmetric cryptography

• Another mode: digital signature

20 plaintext

Public Key Cryptography

ciphertext

Sign Verify plaintext

Private key Public key

• Digital signature

– Only the party with the private key can create a digital signature.

– The digital signature is verifiable by anyone who knows the public key.

– The signer cannot deny that he/she has done so.

21

Public Key Cryptography

• It must be computationally

easy to generate a public / private key pair

hard to determine the private key, given the public key

• It must be computationally

easy to encrypt using the public key

easy to decrypt using the private key

hard to recover the plaintext message from just the ciphertext and the public key

22

Symmetric vs Asymmetric

• Symmetric algorithms are much faster

– In the order of a 1000 times faster

• Symmetric algorithms require a shared secret

– Impractical if the communicating entities don’t have another secure channel

• Both algorithms are combined to provide practical and efficient secure communication

– E.g., establish a secret session key using asymmetric crypto and use symmetric crypto for encrypting the traffic

23

Public key encryption algorithms

requirements:

1

B

.

-

B

.

K (K (m)) = m

B

+

2 impossible to compute private

key K

B

RSA:

Rivest, Shamir, Adelson algorithm

8-24

4

Prerequisite: modular arithmetic

• x mod n = remainder of x when divide by n

• facts:

[(a mod n) + (b mod n)] mod n = (a+b) mod n

[(a mod n) - (b mod n)] mod n = (a-b) mod n

[(a mod n) * (b mod n)] mod n = (a*b) mod n

• thus

(a mod n) d mod n = a d mod n

• example: x=14, n=10, d=2:

(x mod n) d mod n = 4 2 mod 10 = 6 x d = 14 2 = 196 x d mod 10 = 6

Security

8-25

RSA: getting ready

• message: just a bit pattern

• bit pattern can be uniquely represented by an integer number

• thus, encrypting a message is equivalent to encrypting a number

example:

• m= 10010001 . This message is uniquely represented by the decimal number 145.

• to encrypt m, we encrypt the corresponding number, which gives a new number (the ciphertext).

Security

8-26

RSA: Creating public/private key pair

1.

choose two large prime numbers p, q.

(e.g., 1024 bits each)

2.

compute

n

= pq, z = (p-1)(q-1)

3.

choose

e

(with e<n) that has no common factors with z (e, z are “relatively prime”).

4.

choose

d

such that ed-1 is exactly divisible by z.

(in other words: ed mod z = 1 ).

5.

public key is (

n,e

). private key is (

n,d ).

K

+

B

K

-

B

Security

8-27

RSA: encryption, decryption

0.

given (

n,e

) and (

n,d

) as computed above

1.

to encrypt message m (<n), compute

c = m mod n

2.

to decrypt received bit pattern, c, compute

m = c mod n

magic happens!

m = (m

mod n)

d

mod n c

Security

8-28

RSA example:

Bob chooses p=5, q=7. Then n=35, z=24.

e=5

(so e, z relatively prime).

d=29

(so ed-1 exactly divisible by z).

encrypting 8-bit messages.

encrypt: bit pattern m

0000l000 12 me

24832 17 decrypt: c cd

17

481968572106750915091411825223071697

Security

12

8-29

Why does RSA work?

• must show that c d mod n = m where c = m e mod n

• fact: for any x and y: x y mod n = x (y mod z) mod n

– where n= pq and z = (p-1)(q-1)

• thus, c d mod n = (m e mod n) d mod n

= m ed mod n

= m (ed mod z) mod n

= m 1 mod n

= m

Security

8-30

5

RSA: another important property

The following property will be

very

useful later:

-

K

(

K (m)

)

= m

=

K

(

K (m)

) use public key first, followed by private key use private key first, followed by public key

result is the same!

Security

8-31

Why

-

K

(

K (m)

B

)

= m

=

K

(

K (m)

) ?

follows directly from modular arithmetic:

(m e mod n) d mod n = m ed mod n

= m de mod n

= (m d mod n) e mod n

8-32

Why is RSA secure?

• suppose you know Bob’s public key (n,e).

How hard is it to determine d?

• essentially need to find factors of n without knowing the two factors p and q

– fact: factoring a big number is hard

Security

8-33

RSA in practice: session keys

• exponentiation in RSA is computationally intensive

DES is at least 100 times faster than RSA

• use public key crypto to establish secure connection, then establish second key – symmetric session key – for encrypting data

session key, K

S

Bob and Alice use RSA to exchange a symmetric key K once both have K

S

S

, they use symmetric key cryptography

8-34

Hash Function

Message of arbitrary length

Hash

A fixed-length short message

• Also known as

– Message digest

– One-way transformation

– One-way function

– Hash

• Length of H(m) much shorter than length of m

• Usually fixed lengths: 128 or 160 bits

35

Properties of Hash

• Consider a hash function H

– Performance: Easy to compute H(m)

– One-way property: Given H(m) but not m, it’s computationally infeasible to find m

– Weak collision resistance (free): Given H(m), it’s computationally infeasible to find m’ such that H(m’) = H(m).

– Strong collision resistance (free): Computationally infeasible to find

m

1

, m

2 such that H(m

1

) = H(m

2

)

36

6

Hash Applications

• File / Message integrity

– Check if a downloaded file is corrupted

– Detect if a file has been changed by someone after it was stored

– Compute a hash H(F) of file F

openssl dgst -md5 filename

37

Hash Applications

• Password verification

– Password cannot be stored in plaintext

– In a hashed format

– Linux: /etc/passwd, /etc/shadow

cat /etc/shadow

38

Hash Applications

• User authentication

– Alice wants to authenticate herself to Bob

– Assuming they already share a secret key K

Alice Bob

computes

Y=H(R|K) verifies that

Y=H(R|K)

39

Modern Hash Functions

• MD5 (128 bits)

– Previous versions (i.e., MD2, MD4) have weaknesses.

– Broken; collisions published in August 2004

– Too weak to be used for serious applications

• SHA (Secure Hash Algorithm)

– Weaknesses were found

• SHA-1 (160 bits)

– Broken, but not yet cracked

– Collisions in 2 of 2 80

69 operations hash operations, much less than the brute-force attack

– Results were circulated in February 2005, and published in CRYPTO

’05 in August 2005

• SHA-256, SHA-384, …

40

Chapter 8 roadmap

8.1

What is network security?

8.2

Principles of cryptography

8.3

Message integrity

, authentication

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-41

Authentication

Goal:

Bob wants Alice to “prove” her identity to him

Protocol ap1.0:

Alice says “I am Alice”

“ I am Alice”

Failure scenario??

Security

8-42

7

Authentication

Goal:

Bob wants Alice to “prove” her identity to him

Protocol ap1.0:

Alice says “I am Alice”

“ I am Alice” in a network,

Bob can not “see” Alice, so Trudy simply declares herself to be Alice

Security

8-43

Authentication: another try

Protocol ap2.0:

Alice says “I am Alice” in an IP packet containing her source IP address

Alice’s

IP address

“ I am Alice”

Failure scenario??

Security

8-44

Authentication: another try

Protocol ap2.0:

Alice says “I am Alice” in an IP packet containing her source IP address

Alice’s

IP address

I am Alice”

Trudy can create a packet

“ spoofing”

Alice’s address

Security

8-45

Authentication: another try

Protocol ap3.0:

Alice says “I am Alice” and sends her secret password to “prove” it.

Alice’s

IP addr

Alice’s password

I’m Alice”

Alice’s

IP addr

OK

Failure scenario??

Security

8-46

Authentication: another try

Protocol ap3.0:

Alice says “I am Alice” and sends her secret password to “prove” it.

Alice’s

IP addr

Alice’s password

“ I’m Alice”

Alice’s

IP addr

OK

playback attack:

Trudy records Alice’s packet and later plays it back to Bob

Alice’s

IP addr

Alice’s password

“ I’m Alice”

Security

8-47

Authentication: yet another try

Protocol ap3.1:

Alice says “I am Alice” and sends her

encrypted

secret password to “prove” it.

Alice’s

IP addr encrypted password

“ I’m Alice”

Alice’s

IP addr

OK

Failure scenario??

Security

8-48

8

Authentication: yet another try

Protocol ap3.1:

Alice says “I am Alice” and sends her

encrypted

secret password to “prove” it.

Alice’s

IP addr encrypted password

“ I’m Alice”

Alice’s

IP addr

OK

Alice’s

IP addr encrypted password

“ I’m Alice”

Security record and playback

still

works!

8-49

Authentication: yet another try

Goal:

avoid playback attack

nonce:

number (R) used only

once-in-a-lifetime ap4.0:

to prove Alice “live”, Bob sends Alice

nonce

, R. Alice must return R, encrypted with shared secret key

I am Alice”

R

Failures, drawbacks?

Security

Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!

8-50

Authentication: ap5.0

ap4.0 requires shared symmetric key

• can we authenticate using public key techniques?

ap5.0:

use nonce, public key cryptography

R

I am Alice”

“ send me your public key”

Bob computes

K

(K (R)) = R and knows only Alice could have the private key, that encrypted R such that

-

(K (R)) = R

Security

8-51

ap5.0: security hole

man (or woman) in the middle attack:

Trudy poses as

Alice (to Bob) and as Bob (to Alice)

I am Alice

R

I am Alice

R

Send me your public key

K

-

A A

Trudy gets m = K (K (m))

T

+ encrypted with

Alice’s public key

Security

+

K (m)

8-52

ap5.0: security hole

man (or woman) in the middle attack:

Trudy poses as

Alice (to Bob) and as Bob (to Alice) difficult to detect:

 Bob receives everything that Alice sends, and vice versa.

(e.g., so Bob, Alice can meet one week later and recall conversation!)

 problem is that Trudy receives all messages as well!

Security

8-53

Chapter 8 roadmap

8.1

What is network security?

8.2

Principles of cryptography

8.3 Message integrity,

authentication

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-54

9

Digital signatures

cryptographic technique analogous to handwritten signatures:

• sender (Bob) digitally signs document, establishing he is document owner/creator.

verifiable, nonforgeable:

recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document

Security

8-55

Digital signatures

simple digital signature for message m:

• Bob signs m by encrypting with his private key K

-

B

, creating

“ signed” message, K

-

B

(m)

Bob’s message, m

Dear Alice

Oh, how I have missed you. I think of you all the time! …(blah blah blah)

Bob

K

-

B

Bob’s private key

Public key encryption algorithm m,K

-

B

(m)

Bob’s message, m, signed

(encrypted) with his private key

Security

8-56

Digital signatures

 suppose Alice receives msg m, with signature: m, K

-

B

(m)

 Alice verifies m signed by Bob by applying Bob’s public key K to K

-

B

(m) then checks K

+

B

(K

-

B

(m) ) = m.

+

B

 If K (K

-

B

(m) ) = m, whoever signed m must have used Bob’s private key.

Alice thus verifies that:

 Bob signed m

 no one else signed m

 Bob signed m and not m’ non-repudiation:

 Alice can take m, and signature K signed m

-

B

(m) to court and prove that Bob

Security

8-57

Message digests

large message m

H: Hash

Function computationally expensive to public-key-encrypt long messages

goal:

fixed-length, easy- tocompute digital “fingerprint”

• apply hash function H to m, get fixed size message digest, H(m).

H(m)

Hash function properties:

• many-to-1

• produces fixed-size msg digest

(fingerprint)

• given message digest x, computationally infeasible to find m such that x = H(m)

Security

8-58

Internet checksum: poor crypto hash function

Internet checksum has some properties of hash function:

• produces fixed length digest (16-bit sum) of message

• is many-to-one

But given message with given hash value, it is easy to find another message with same hash value: message

I O U 1

0 0 . 9

9 B O B

ASCII format

49 4F 55 31

30 30 2E 39

39 42 D2 42

message

I O U 9

0 0 . 1

9 B O B

ASCII format

49 4F 55 39

30 30 2E 31

39 42 D2 42

B2 C1 D2 AC

different messages but identical checksums !

B2 C1 D2 AC

Security

8-59

Digital signature = signed message digest

Bob sends digitally signed message:

Alice verifies signature, integrity of digitally signed message: encrypted large message m

H: Hash function

H(m)

K

B

(H(m))

+

Bob’s private key

K

digital signature

(encrypt) encrypted msg digest

K

-

B

(H(m)) large message m

H: Hash function

Bob’s public key

K

+

H(m) equal

?

digital signature

(decrypt)

H(m)

Security

8-60

10

Hash function algorithms

• MD5 hash function widely used (RFC 1321)

– computes 128-bit message digest in 4-step process.

– arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x

SHA-1 is also used

– US standard [

NIST, FIPS PUB 180-1]

– 160-bit message digest

Security

8-61

Recall: ap5.0 security hole

man (or woman) in the middle attack:

Trudy poses as

Alice (to Bob) and as Bob (to Alice)

I am Alice

R

I am Alice

R

Send me your public key

K

-

A

Trudy gets

m = K (K (m)) sends m to Alice encrypted with

Alice’s public key

Security

8-62

Certification authorities

certification authority (CA):

binds public key to particular entity, E.

• E (person, router) registers its public key with CA.

E provides “proof of identity” to CA.

CA creates certificate binding E to its public key.

– certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key”

Bob’s public key

K

+

B

Bob’s identifying information digital signature

(encrypt)

CA private key

K

-

CA certificate for

Bob’s public key, signed by CA

Security

8-63

Certification authorities

• when Alice wants Bob’s public key:

– gets Bob’s certificate (Bob or elsewhere).

– apply CA’s public key to Bob’s certificate, get Bob’s public key digital signature

(decrypt)

CA public key

K

+

CA

K

+

B

Bob’s public key

Security

8-64

Chapter 8 roadmap

8.1

What is network security?

8.2

Principles of cryptography

8.3

Message integrity, authentication

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-65

Secure e-mail

Alice wants to send confidential e-mail, m, to Bob.

K

S m

K

S

( )

.

K

S

(m )

K

S

(m )

K

S

( )

.

+

Internet

-

K

S

K

+

B

( )

.

K

+

B

(K

S

)

K

+

B

(K

S

)

K

+

B

Alice:

 generates random symmetric private key, K

S

 encrypts message with K

 also encrypts K

 sends both K

S with Bob’s public key

S

(m) and K

S

(for efficiency)

B

(K

S

) to Bob

Security

K

-

B

K

S

K

-

B

( )

.

m

8-66

11

Secure e-mail

Alice wants to send confidential e-mail, m, to Bob

.

K

S m

K

S

( )

.

K

S

(m ) K

S

(m )

K

S

( )

.

+

-

K

S

K

+

B

( )

.

K

+

B

K

+

B

(K

S

)

Internet

K

+

B

(K

S

)

K

-

K

S

K

-

B

( )

.

Bob:

 uses his private key to decrypt and recover K

S

 uses K

S to decrypt K

S

(m) to recover m

Security m

8-67 m

Secure e-mail

(continued)

Alice wants to provide sender authentication message integrity

H( )

.

K

-

A

K

-

A

( )

.

K

-

A

(H(m))

K

-

A

(H(m))

K

+

A

K

+

A

( )

.

H(m )

+

m

Internet

H( )

.

compare

H(m ) m

Alice digitally signs message

 sends both message (in the clear) and digital signature

Security

8-68

Secure e-mail

(continued)

Alice wants to provide secrecy, sender authentication, message integrity.

m

H( )

.

K

A

-

K

-

A

( )

.

K

-

A

(H(m))

+

K

S

( )

.

K

S m

K

S

+

K

+

B

(K

S

)

Internet

K

+

B

( )

.

K

+

B

Alice uses three keys:

her private key, Bob’s public key, newly created symmetric key

Security

8-69

Chapter 8 roadmap

8.1

What is network security?

8.2

Principles of cryptography

8.3

Message integrity

8.4 Securing e-mail

8.5 Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-70

SSL: Secure Sockets Layer

• widely deployed security protocol

– supported by almost all browsers, web servers

– https

– billions $/year over SSL

• mechanisms: [Woo 1994], implementation: Netscape

• variation -TLS: transport layer security, RFC 2246

• provides

confidentiality

integrity

authentication

• original goals:

Web e-commerce transactions

– encryption (especially creditcard numbers)

Web-server authentication

– optional client authentication

– minimum hassle in doing business with new merchant

• available to all TCP applications

– secure socket interface

Security

8-71

Application

SSL and TCP/IP

TCP

IP

normal application

Application

SSL

TCP

IP

application with SSL

 SSL provides application programming interface

(API) to applications

 C and Java SSL libraries/classes readily available

Security

8-72

12

Toy SSL: a simple secure channel

handshake:

Alice and Bob use their certificates, private keys to authenticate each other and exchange shared secret

key derivation :

Alice and Bob use shared secret to derive set of keys

data transfer:

data to be transferred is broken up into series of records

connection closure :

special messages to securely close connection

Security

8-73

Toy: a simple handshake

MS:

master secret

EMS:

encrypted master secret

Security

8-74

Toy: key derivation

• considered bad to use same key for more than one cryptographic operation

– use different keys for message authentication code (MAC) and encryption

• four keys:

K c

– M c

= encryption key for data sent from client to server

= MAC key for data sent from client to server

– K s

M s

= encryption key for data sent from server to client

= MAC key for data sent from server to client

• keys derived from key derivation function (KDF)

– takes master secret and (possibly) some additional random data and creates the keys

Security

8-75

Toy: data records

• why not encrypt data in constant stream as we write it to

TCP?

– where would we put the MAC? If at end, no message integrity until all data processed.

– e.g., with instant messaging, how can we do integrity check over all bytes sent before displaying?

• instead, break stream in series of records

– each record carries a MAC

– receiver can act on each record as it arrives

• issue: in record, receiver needs to distinguish MAC from data

– want to use variable-length records length data MAC

Security

8-76

Toy: sequence numbers

problem: attacker can capture and replay record or reorder records

solution: put sequence number into MAC:

 MAC = MAC(M x

, sequence||data)

 note: no sequence number field

problem: attacker could replay all records

solution: use nonce

Security

8-77

Toy: control information

problem:

truncation attack:

– attacker forges TCP connection close segment

– one or both sides thinks there is less data than there actually is.

solution:

record types, with one type for closure

– type 0 for data; type 1 for closure

MAC = MAC(M x

, sequence||type||data) length type data

Security

MAC

8-78

13

Toy SSL: summary

Security bob.com

8-79

Toy SSL isn’t complete

• how long are fields?

• which encryption protocols?

• want negotiation?

– allow client and server to support different encryption algorithms

– allow client and server to choose together specific algorithm before data transfer

Security

8-80

SSL cipher suite

• cipher suite

– public-key algorithm

– symmetric encryption algorithm

MAC algorithm

SSL supports several cipher suites

• negotiation: client, server agree on cipher suite

– client offers choice

– server picks one common SSL symmetric ciphers

 DES – Data Encryption

Standard: block

 3DES – Triple strength: block

 RC2 – Rivest Cipher 2: block

 RC4 – Rivest Cipher 4: stream

SSL Public key encryption

 RSA

Security

8-81

Real SSL: handshake (1)

Purpose

1. server authentication

2. negotiation: agree on crypto algorithms

3. establish keys

4. client authentication (optional)

Security

8-82

Real SSL: handshake (2)

1.

client sends list of algorithms it supports, along with client nonce

2.

server chooses algorithms from list; sends back: choice + certificate + server nonce

3.

client verifies certificate, extracts server’s public key, generates pre_master_secret, encrypts with server’s public key, sends to server

4.

client and server independently compute encryption and

MAC keys from pre_master_secret and nonces

5.

client sends a MAC of all the handshake messages

6.

server sends a MAC of all the handshake messages

Security

8-83

Real SSL: handshaking (3)

last 2 steps protect handshake from tampering

• client typically offers range of algorithms, some strong, some weak

• man-in-the middle could delete stronger algorithms from list

• last 2 steps prevent this

– last two messages are encrypted

Security

8-84

14

Real SSL: handshaking (4)

• why two random nonces?

• suppose Trudy sniffs all messages between Alice & Bob

• next day, Trudy sets up TCP connection with Bob, sends exact same sequence of records

– Bob (Amazon) thinks Alice made two separate orders for the same thing

– solution: Bob sends different random nonce for each connection. This causes encryption keys to be different on the two days

Trudy’s messages will fail Bob’s integrity check

Security

8-85

SSL record protocol

data data fragment

MAC data fragment

MAC record header encrypted data and MAC record header encrypted data and MAC

record header:

content type; version; length

MAC:

includes sequence number, MAC key M x

fragment:

each SSL fragment 2 14 bytes (~16 Kbytes)

Security

8-86

1 byte content type

SSL record format

2 bytes 3 bytes

SSL version length data

MAC data and MAC encrypted (symmetric algorithm)

Security

8-87

Real SSL connection

everything henceforth is encrypted

TCP FIN follows

8-88

Chapter 8 roadmap

8.1

What is network security?

8.2

Principles of cryptography

8.3

Message integrity

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6 Network layer security: IPsec

8.7

Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-89

What is network-layer confidentiality ?

between two network entities:

• sending entity encrypts datagram payload, payload could be:

TCP or UDP segment, ICMP message, OSPF message ….

• all data sent from one entity to other would be hidden:

– web pages, e-mail, P2P file transfers, TCP SYN packets …

• “ blanket coverage”

Security

8-90

15

Virtual Private Networks (VPNs)

motivation:

• institutions often want private networks for security.

• costly: separate routers, links, DNS infrastructure.

• VPN: institution’s inter-office traffic is sent over public

Internet instead

– encrypted before entering public Internet

– logically separate from other traffic

Security

8-91

Virtual Private Networks (VPNs)

public

Internet laptop w/ IPsec router w/

IPv4 and IPsec router w/

IPv4 and IPsec salesperson in hotel headquarters branch office

8-92

IPsec services

• data integrity

• origin authentication

• replay attack prevention

• confidentiality

• two protocols providing different service models:

– AH

– ESP

Security

8-93

IPsec transport mode

IPsec

IPsec

• IPsec datagram emitted and received by end-system

• protects upper level protocols

Security

8-94

IPsec – tunneling mode

IPsec

IPsec

• edge routers IPsec-aware

IPsec IPsec

 hosts IPsec-aware

Security

8-95

Two IPsec protocols

• Authentication Header (AH) protocol

– provides source authentication & data integrity but not confidentiality

• Encapsulation Security Protocol (ESP)

– provides source authentication, data integrity, and

confidentiality

– more widely used than AH

Security

8-96

16

Four combinations are possible!

Host mode with AH

Host mode with ESP

Tunnel mode with AH

Tunnel mode with ESP most common and most important

Security

8-97

Security associations (SAs)

• before sending data,

“ security association (SA)” established from sending to receiving entity

SAs are simplex: for only one direction

• ending, receiving entitles maintain state information about SA

– recall: TCP endpoints also maintain state info

IP is connectionless; IPsec is connection-oriented!

Security

8-98

Example SA from R1 to R2

headquarters

Internet branch office

R1

200.168.1.100

193.68.2.23

security association

R2

172.16.1/24

172.16.2/24

R1 stores for SA:

• 32-bit SA identifier: Security Parameter Index (SPI)

• origin SA interface (200.168.1.100)

• destination SA interface (193.68.2.23)

• type of encryption used (e.g., 3DES with CBC)

• encryption key

• type of integrity check used (e.g., HMAC with MD5)

• authentication key

Security

8-99

Security Association Database (SAD)

 endpoint holds SA state in

security association database

(SAD)

, where it can locate them during processing.

 with n salespersons, 2 + 2n SAs in R1’s SAD

 when sending IPsec datagram, R1 accesses SAD to determine how to process datagram.

 when IPsec datagram arrives to R2, R2 examines SPI in

IPsec datagram, indexes SAD with SPI, and processes datagram accordingly.

Security

8-100

IPsec datagram

focus for now on tunnel mode with ESP new IP header

ESP hdr

“ enchilada” authenticated encrypted original

IP hdr

Original IP datagram payload

ESP trl

ESP auth

SPI

Seq

# padding pad length next header

Security

8-101 headquarters

What happens?

Internet

R1

200.168.1.100

193.68.2.23

security association

R2

172.16.1/24 branch office

172.16.2/24 new IP header

ESP hdr

“ enchilada” authenticated encrypted original

IP hdr

Original IP datagram payload

ESP trl

ESP auth

SPI

Seq

# padding

Security pad length next header

8-102

17

R1:

convert original datagram to IPsec datagram

• appends to back of original datagram (which includes original header fields!) an “ESP trailer” field.

• encrypts result using algorithm & key specified by SA.

• appends to front of this encrypted quantity the “ESP header, creating “enchilada”.

• creates authentication MAC over the whole enchilada, using algorithm and key specified in SA;

• appends MAC to back of enchilada, forming payload;

• creates brand new IP header, with all the classic IPv4 header fields, which it appends before payload

Security

8-103 new IP header

ESP hdr

Inside the enchilada:

“ enchilada” authenticated original

IP hdr encrypted

Original IP datagram payload

ESP trl

ESP auth

SPI

Seq

# padding pad length next header

ESP trailer: Padding for block ciphers

• ESP header:

– SPI, so receiving entity knows what to do

Sequence number, to thwart replay attacks

• MAC in ESP auth field is created with shared secret key

Security

8-104

IPsec sequence numbers

• for new SA, sender initializes seq. # to 0

• each time datagram is sent on SA:

– sender increments seq # counter

– places value in seq # field

• goal:

– prevent attacker from sniffing and replaying a packet

– receipt of duplicate, authenticated IP packets may disrupt service

• method:

– destination checks for duplicates

– doesn’t keep track of all received packets; instead uses a window

Security

8-105

Security Policy Database (SPD)

• policy: For a given datagram, sending entity needs to know if it should use IPsec

• needs also to know which SA to use

– may use: source and destination IP address; protocol number

• info in SPD indicates “what” to do with arriving datagram

• info in SAD indicates “how” to do it

Security

8-106

Summary: IPsec services

• suppose Trudy sits somewhere between R1 and R2. she doesn’t know the keys.

– will Trudy be able to see original contents of datagram?

How about source, dest IP address, transport protocol, application port?

– flip bits without detection?

– masquerade as R1 using R1’s IP address?

– replay a datagram?

Security

8-107

IKE: Internet Key Exchange

previous examples:

manual establishment of IPsec SAs in IPsec endpoints:

Example SA

SPI: 12345

Source IP: 200.168.1.100

Dest IP: 193.68.2.23

Protocol: ESP

Encryption algorithm: 3DES-cbc

HMAC algorithm: MD5

Encryption key: 0x7aeaca…

HMAC key:0xc0291f…

• manual keying is impractical for VPN with 100s of endpoints

• instead use

IPsec IKE (Internet Key Exchange )

Security

8-108

18

IKE: PSK and PKI

• authentication (prove who you are) with either

– pre-shared secret (PSK) or

– with PKI (pubic/private keys and certificates).

PSK: both sides start with secret

– run IKE to authenticate each other and to generate IPsec

SAs (one in each direction), including encryption, authentication keys

PKI: both sides start with public/private key pair, certificate

– run IKE to authenticate each other, obtain IPsec SAs (one in each direction).

– similar with handshake in SSL.

Security

8-109

IKE phases

IKE has two phases

phase 1:

establish bi-directional IKE SA

• note: IKE SA different from IPsec SA

• aka ISAKMP security association

phase 2:

ISAKMP is used to securely negotiate IPsec pair of

SAs

• phase 1 has two modes: aggressive mode and main mode

– aggressive mode uses fewer messages

– main mode provides identity protection and is more flexible

Security

8-110

Diffie-Hellman Key Exchange

111

IPsec summary

IKE message exchange for algorithms, secret keys, SPI numbers

• either AH or ESP protocol (or both)

– AH provides integrity, source authentication

– ESP protocol (with AH) additionally provides encryption

IPsec peers can be two end systems, two routers/firewalls, or a router/firewall and an end system

Security

8-112

Chapter 8 roadmap

8.1 What is network security?

8.2

Principles of cryptography

8.3

Message integrity

8.4

Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7 Securing wireless LANs

8.8

Operational security: firewalls and IDS

Security

8-113

WEP design goals

• symmetric key crypto

– confidentiality

– end host authorization

– data integrity

• self-synchronizing: each packet separately encrypted

– given encrypted packet and key, can decrypt; can continue to decrypt packets when preceding packet was lost (unlike Cipher Block Chaining

(CBC) in block ciphers)

• Efficient

– implementable in hardware or software

Security

8-114

19

Review: symmetric stream ciphers

key keystream generator keystream

combine each byte of keystream with byte of plaintext to get ciphertext:

– m(i) = ith unit of message

– ks(i) = ith unit of keystream

– c(i) = ith unit of ciphertext

– c(i) = ks(i) ⊕ m(i) (⊕ = exclusive or)

– m(i) = ks(i) ⊕ c(i)

WEP uses RC4

Security

8-115

Stream cipher and packet independence

• recall design goal: each packet separately encrypted

• if for frame n+1, use keystream from where we left off for frame n, then each frame is not separately encrypted

– need to know where we left off for packet n

• WEP approach: initialize keystream with key + new IV for each packet:

Key+IV packet keystream generator keystream packet

Security

8-116

WEP encryption (1)

• sender calculates Integrity Check Value (ICV, four-byte hash/CRC over data

• each side has 104-bit shared key

• sender creates 24-bit initialization vector (IV), appends to key: gives 128-bit key

• sender also appends keyID (in 8-bit field)

• 128-bit key inputted into pseudo random number generator to get keystream

• data in frame + ICV is encrypted with RC4:

– bytes of keystream are XORed with bytes of data & ICV

– IV & keyID are appended to encrypted data to create payload

– payload inserted into 802.11 frame encrypted

IV

Key

ID data ICV

MAC payload

Security

8-117

WEP encryption (2)

IV

(per frame)

K

S

: 104-bit secret symmetric plaintext

frame data plus CRC key sequence generator

( for given K

S

, IV) k

1

IV

k

2

IV

k

3

IV

… k

N

IV

k

N+1

IV

… k

N+1

IV d

1

d

2

d

3

… d

N

CRC

1

… CRC

4 c

1

c

2

c

3

… c

N c

N+1

… c

N+4

new IV for each frame

Figure 7.8-new1: 802.11 WEP protocol

802.11 header IV

WEP-encrypted data plus ICV

Security

8-118

WEP decryption overview

encrypted

IV

Key

ID data ICV

MAC payload

• receiver extracts IV

• inputs IV, shared secret key into pseudo random generator, gets keystream

• XORs keystream with encrypted data to decrypt data + ICV

• verifies integrity of data with ICV

– note: message integrity approach used here is different from MAC

(message authentication code) and signatures (using PKI).

Security

8-119

End-point authentication w/ nonce

Nonce: number (R) used only once –in-a-lifetime

How to prove Alice

live

:

Bob sends Alice

nonce

, R. Alice must return R, encrypted with shared secret key

I am Alice”

R

Security

Alice is live, and only

Alice knows key to encrypt nonce, so it must be Alice!

8-120

20

WEP authentication authentication request nonce (128 bytes) nonce encrypted shared key success if decrypted value equals nonce

Notes:

 not all APs do it, even if WEP is being used

AP indicates if authentication is necessary in beacon frame

 done before association

Security

8-121

Breaking 802.11 WEP encryption

security hole:

• 24-bit IV, one IV per frame, -> IV’s eventually reused

• IV transmitted in plaintext -> IV reuse detected

attack:

– Trudy causes Alice to encrypt known plaintext d

1

– d

3 d

4

Trudy sees: c i

Trudy knows c

= d i d i i

XOR k i

IV

, so can compute k i

IV

Trudy knows encrypting key sequence k

Next time IV is used, Trudy can decrypt!

1

IV

k d

2

2

IV

k

3

IV

Security

8-122

802.11i: improved security

• numerous (stronger) forms of encryption possible

• provides key distribution

• uses authentication server separate from access point

Security

8-123

802.11i: four phases of operation

AP:

access point

STA:

client station wired network

AS:

Authentication server

1 Discovery of security capabilities

2 STA and AS mutually authenticate, together generate Master Key (MK)

. AP serves as pass through

3 STA derives

Pairwise Master

Key (PMK)

3

AS derives same PMK, sends to AP

4 STA, AP use PMK to derive

Temporal Key (TK) used for message encryption, integrity

Security

8-124

EAP: extensible authentication protocol

• EAP: end-end client (mobile) to authentication server protocol

EAP sent over separate “links”

– mobile-to-AP (EAP over LAN)

AP to authentication server (RADIUS over UDP) wired network

EAP over LAN (EAPoL)

IEEE 802.11

EAP TLS

EAP

RADIUS

UDP/IP

Security

8-125

Chapter 8 roadmap

8.1 What is network security?

8.2

Principles of cryptography

8.3

Message integrity

8.4 Securing e-mail

8.5

Securing TCP connections: SSL

8.6

Network layer security: IPsec

8.7

Securing wireless LANs

8.8 Operational security: firewalls and IDS

Security

8-126

21

Firewalls

firewall

isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others administered network

trusted “good guys” firewall

public

Inter net

untrusted “bad guys”

Security

8-127

Firewalls: why

prevent denial of service attacks:

SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections prevent illegal modification/access of internal data

 e.g., attacker replaces CIA’s homepage with something else allow only authorized access to inside network

 set of authenticated users/hosts three types of firewalls:

 stateless packet filters

 stateful packet filters

 application gateways

Security

8-128

Stateless packet filtering

Should arriving packet be allowed in?

Departing packet let out?

• internal network connected to Internet via

router firewall

• router

filters packet-by-packet,

decision to forward/drop packet based on:

– source IP address, destination IP address

– TCP/UDP source and destination port numbers

– ICMP message type

TCP SYN and ACK bits

Security

8-129

Stateless packet filtering: example

example 1:

block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23

result:

all incoming, outgoing UDP flows and telnet connections are blocked

example 2:

block inbound TCP segments with ACK=0.

result:

prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.

Security

8-130

Stateless packet filtering

: more examples

Policy

No outside Web access.

No incoming TCP connections, except those for institution’s public Web server only.

Prevent Web-radios from eating up the available bandwidth.

Firewall Setting

Drop all outgoing packets to any IP address, port 80

Drop all incoming TCP SYN packets to any IP except 130.207.244.203, port 80

Drop all incoming UDP packets except DNS and router broadcasts.

Prevent your network from being used for a smurf DoS attack.

Prevent your network from being tracerouted

Drop all ICMP packets going to a

“ broadcast” address (e.g.

130.207.255.255).

Drop all outgoing ICMP TTL expired traffic

Security

8-131

Access Control Lists

ACL:

table of rules, applied top to bottom to incoming packets:

(action, condition) pairs: looks like OpenFlow forwarding (Ch. 4)!

action allow allow allow allow deny source address

222.22/16 dest address outside of

222.22/16

222.22/16 outside of

222.22/16

222.22/16 outside of

222.22/16 all outside of

222.22/16

222.22/16 all protocol

TCP

TCP

UDP source port

> 1023

80

> 1023

UDP all

53

Security all dest port

80

> 1023

53

> 1023 all flag bit any

ACK

---

---all

8-132

22

Stateful packet filtering

stateless packet filter:

heavy handed tool

– admits packets that “make no sense,” e.g., dest port = 80, ACK bit set, even though no TCP connection established:

action source address dest address protocol source port dest port flag bit

allow outside of

222.22/16

222.22/16

TCP 80 > 1023 ACK

stateful packet filter:

track status of every TCP connection

• track connection setup (SYN), teardown (FIN): determine whether incoming, outgoing packets “makes sense”

• timeout inactive connections at firewall: no longer admit packets

Security

8-133

Stateful packet filtering

ACL augmented to indicate need to check connection state table before admitting packet action allow allow allow allow deny source address

222.22/16 outside of

222.22/16

222.22/16 outside of

222.22/16 all dest address outside of

222.22/16

222.22/16 outside of

222.22/16

222.22/16 all proto

TCP

TCP

UDP

UDP all source port

> 1023

80

> 1023

53 dest port

80

> 1023

53

> 1023 all

Security all flag bit any

ACK

---

---all check conxion x x

8-134

Application gateways

• filter packets on application data as well as on IP/TCP/UDP fields.

host-to-gateway telnet session application gateway router and filter

example:

allow select internal users to telnet outside

1.

require all telnet users to telnet through gateway.

gateway-to-remote host telnet session

2.

for authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections

3.

router filter blocks all telnet connections not originating from gateway.

Security

8-135

Intrusion detection systems

• packet filtering:

– operates on TCP/IP headers only

– no correlation check among sessions

IDS: intrusion detection system

deep packet inspection:

look at packet contents (e.g., check character strings in packet against database of known virus, attack strings)

– examine correlation among multiple packets

• port scanning

• network mapping

• DoS attack

Security

8-136

Intrusion detection systems

multiple IDSs: different types of checking at different locations firewall internal network

Internet

IDS sensors

Web server FTP server

DNS server demilitarized zone

Security

8-137

IDS

• Detect if attacks are being attempted, or if system has been compromised

• Desirable features

– Accuracy

– Fast

– Flexible, general

– Results easy to understand

138

23

Measuring Accuracy

Events are actions occurring in the system (file accesses, login attempts, etc.)

– an intrusion (I) is an event that is part of an attack

– an alarm (A) is generated if an event is diagnosed as being an intrusion

Intrusion

Not an

Intrusion

Alarm

Generated

Alarm Not

Generated

True positive False positive

False negative True negative

139

Basic IDS Techniques

• Misuse detection

– use attack signatures (characteristics of real attacks, e.g., illegal sequences of system calls, invalid packets, etc.)

– can only detect already-known attacks

– false positive rate is low, but false negative rate is high

140

Basic IDS Techniques

• Anomaly detection

– uses a model of “normal” system behavior

– tries to detect deviations from this behavior, e.g., raises an alarm when a statistically rare event occurs

– can potentially detect new (not previouslyencountered) attacks

– low false negative rate, high false positive rate

• Which is better?

141

Example Signatures

• A sequence of connection attempts to a large number of ports

• A privileged program spawning a shell

• A network packet that has lots of NOP instruction bytes in it

• Program input containing a very long string (parameter value)

• A large number of TCP SYN packets sent, with no ACKs coming back

142

Conventional View

• Anomaly-based IDS by itself generates too many false positives

• Combination of anomaly-based and signature-based is best

143

Where Is the IDS Deployed?

• Host-based intrusion detection

– monitor activity on a single host

• Network-based intrusion detection (NIDS)

– monitor traffic, examine packet headers and payloads

144

24

Rootkit

• Rootkit is a set of “Trojan” system binaries

• Break into a host, download rootkit by FTP, unpack, compile and install

• Possibly turn off anti-virus / IDS

• Hides its own presence!

– installs hacked binaries for common system monitoring commands, e.g., netstat, ps, ls, du,

login

• “Sniff” user passwords

145

File Integrity Checking

• Tripwire

– Records hashes of critical files and binaries

– System periodically checks that files have not been modified by re-computing and comparing hash

• Ways to bypass?

146

Network-Based IDS

• Inspects network traffic

– passive (unlike packet-filtering firewalls)

– often handled by a router or firewall

• Monitors user activities

– e.g., protocol violations, unusual connection patterns, attack strings in packet payloads

• Advantage: single NIDS can protect many hosts and look for widespread patterns of activity

147

Popular NIDS : Snort

• Popular open-source tool

• Large (> 4000) ruleset for vulnerabilities; Ex.: http://www.snort.org/vrt/advisories/

148

• Backdoors

• Chat

• DDoS

• Finger

• FTP

• ICMP

• IMAP

Some Snort Rule Categories

Multimedia

POP

MySQL

NETBIOS

NNTP

Oracle

P2P

RPC

Scan

Shellcode

SMTP

SNMP

Telnet

TFTP

Virus

Web…

X11

SQL

Snort Rule Syntax

• Each snort rule has two logical sections: rule header and rule options

– rule header contains action, protocol, source (IP address/port), direction, destination (IP address/port)

– rule option contains alert messages, info on which parts of packet to be inspected

150

25

Snort Rule Examples

• alert icmp $EXTERNAL_NET any <> $HOME_NET any

(msg:"DDOS Stacheldraht agent->handler (skillz)"; content:"skillz"; itype:0; icmp_id:6666; reference:url,staff.washington.edu/dittrich/misc/stacheldraht.analysis; classtype:attempted-dos; sid:1855; rev:2;)

• alert any any -> 192.168.1.0/24 any

(flags:A; ack:0; msg: “NMAP TCP ping”;)

# nmap send TCP ACK pkt with ack field set to 0

• alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS

(msg:"WEB-IIS cmd.exe access"; flow:to_server,established; content:"cmd.exe"; nocase; classtype:web-application-attack; sid:1002; rev:5;)

151

Network Security (summary)

basic techniques…...

– cryptography (symmetric and public)

– message integrity

– end-point authentication

…. used in many different security scenarios

– secure email

– secure transport (SSL)

IP sec

– 802.11

operational security: firewalls and IDS

Security

8-152

26

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