8-1 chapter 8 roadmap 8.1 what is network security? 8.2 principles of cryptography 8.3 message...

8-1

Chapter 8 roadmap

8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity & end point

authentication8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS

8-2

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 that any message that is altered (in transit, or afterwards) will be detected

Access control and Availability: services accessible and available to authorized users

8-3

Friends and enemies: Alice, Bob, Trudy well-known in network security world Bob, Alice want to communicate “securely” Trudy (intruder) may intercept, delete, insert,

modify messages

securesender

securereceiver

channel data, control messages

data data

Alice Bob

Trudy

8-4

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?

8-5

What can a “bad” guy do?

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 itself in its place, or inserting itself in the middle

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

…

8-6

Chapter 8 roadmap



8-7

The language of cryptography

symmetric key crypto: sender, receiver keys identicalpublic-key crypto: encryption key is public, decryption

key is private (also called asymmetric key crypto)

plaintext plaintextciphertext

KA

encryptionalgorithm

decryption algorithm

Alice’s encryptionkey

Bob’s decryptionkey

KB

8-8

Symmetric key cryptography

symmetric key crypto: Bob and Alice share (know) the same key

standards: DES, AES, RC4, IDEA, …

plaintextciphertext

KA-B

encryptionalgorithm


KA-B

plaintextmessage, m

K (m)A-B

K (m)A-Bm = K ( )

A-B

8-9

Public Key Cryptography

symmetric key crypto requires sender and

receiver to share a secret key

Q: how to agree on key in first place (particularly if never “met”)?

public key cryptography radically different

approach [Diffie-Hellman76, RSA78]

sender, receiver do not share secret key

a public key for encrypting (or verifying)

a private key for decrypting (or signing)

8-10

Public key cryptography

plaintextmessage, m

ciphertextencryptionalgorithm


Bob’s public key

plaintextmessageK (m)

B+

K B+

Bob’s privatekey

K B-

m = K (K (m))B+

B-

8-11

Public key encryption algorithms

need K ( ) and K ( ) such thatB B. .

given public key K , it should be impossible to compute private key K

B

B

Requirements:

1

2

RSA: Rivest, Shamir, Adleman algorithm

+ -

K (K (m)) = m BB

- +

+

-

8-12

RSA: another important property

The following property will be very useful later:

K (K (m)) = m BB

- +K (K (m))

BB+ -

=

use public key first, followed

by private key

use private key first,

followed by public key

Result is the same!

8-13

Session keys

RSA requires exponentiation which is computationally intensive

e.g., DES is at least 100 times faster than RSA Session key, KS

Bob and Alice use RSA to exchange a symmetric key KS

Once both have KS, they use symmetric key crypto for confidential communication

8-14

Chapter 8 roadmap



8-15

Message Integrity Allows communicating parties to verify

that received messages are authentic. Content of message has not been altered Source of message is who/what you think it

is Message has not been replayed Sequence of messages is maintained

Let’s first consider message digests

8-16

Message Digests

Function H( ) that takes as input an arbitrary length message and outputs a fixed-length string Note that H( ) is a many-

to-1 function H( ) is often called a

“hash function”

Desirable properties of crypto hash function: Easy to calculate Irreversible: Can’t

determine m from H(m) Collision resistant:

computationally difficult to produce m and m’ such that

H(m) = H(m’) Seemingly random

output

large message

m

H: HashFunction

H(m)

8-17

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-oneBut for a message with given hash value, it is easy to find another message with same hash value. Simple checksum example:

I O U 10 0 . 99 B O B

49 4F 55 3130 30 2E 3939 42 4F 42

message ASCII format

B2 C1 D2 AC

49 4F 55 3930 30 2E 3139 42 4F 42

message ASCII format

B2 C1 D2 ACdifferent messagesbut identical checksums!

I O U 90 0 . 19 B O B

8-18

Message Authentication Code (MAC)

m

s(shared secret)

(message)

H(.)H(m+s)

publicInternetappend

m H(m+s)

s

compare

m

H(m+s)

H(.)

H(m+s)

(shared secret)

8-19

Crypto hash functions in practice

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

• recent (2005) attacks on MD5, may not be collision resistant

SHA-1 is also used US standard [NIST, FIPS PUB 180-1] 160-bit message digest more robust algorithms such as SHA–224, SHA–256,

SHA–384 and SHA–512

8-20

Digital Signatures objective

Like hand-written signatures sender (Bob) digitally signs document with his

own private key, 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

8-21

Digital Signatures

simple digital signature for message m: Bob “signs” m by encrypting with his private

key KB, creating “signed” message, KB(m)--

Dear Alice

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

Bob

Bob’s message, m

public keyencryptionalgorithm

Bob’s privatekey

K B-

Bob’s message, m, signed

(encrypted) with his private key

K B-(m)

8-22

Digital Signatures (more) suppose Alice receives msg m, digital signature KB(m)

Alice verifies m by using Bob’s public key KB to check

KB(KB(m) ) = m.

if verified, 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 KB(m) to court and

prove that Bob signed m or he let someone else use his private key.

-

8-23

large message

mH: hashfunction H(m)

digitalsignature(encrypt)

Bob’s private

key K B-

+

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:

KB(H(m))-

encrypted msg digest

KB(H(m))-

encrypted msg digest

large message

m

H: hashfunction

H(m)

digitalsignature(decrypt)

H(m)

Bob’s public

key K B+

equal ?

Digital signature

8-24

Public Key Certification

public key problem: When Alice obtains Bob’s public key (from web

site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s?

solution: trusted certification authority (CA)

8-25

Certification Authorities

Certification Authority (CA): binds public key to a particular entity, E.

E 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 informatio

n

digitalsignature(encrypt)

CA private

key K CA-

K B+

certificate for Bob’s public

key, signed by CA

-K CA(K ) B+

8-26

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

Bob’s public

key K B+

digitalsignature(decrypt)

CA public

key K CA+

K B+

-K CA(K ) B+

Chain of authorities, root

certificate

8-27

Chapter 8 roadmap



8-28

Authentication: challenge-response

Goal: avoid playback attack

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

To prove that Alice is “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key

“I am Alice”

R

K (R)A-B

Alice is live, and only Alice knows key to encrypt

nonce, so it must be Alice!

8-29

Authentication using nonce and public key crypto ?

“I am Alice”

RBob computes

K (R)A-

“send me your public key”

K A+

(K (R)) = RA

-K A

+

and knows only Alice could have the private key to encrypt R such

that(K (R)) = RA

-K A

+

8-30

Security holeMan (woman) in the middle attack: Trudy poses

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

I am Alice I am Alice

R

TK (R)

-

Send me your public key

TK

+A

K (R)-

Send me your public key

AK

+

TK (m)+

Tm = K (K (m))+

T-

Trudy gets

sends m to Alice encrypted

with Alice’s public key

AK (m)+

Am = K (K (m))+

A-

R

8-31

Security hole (cont.)

Difficult to detect - Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet later and recall conversation)

problem is that Trudy receives all messages as well public key needs to come from a trustworthy source, such as, a public key certificate

8-32

Chapter 8 roadmap


authentication8.4 Securing TCP connections: SSL8.5 Network layer security: IPsec8.6 Securing wireless LANs8.7 Operational security: firewalls and IDS

8-33

Chapter 8 roadmap


authentication8.4 Securing TCP connections: SSL8.5 Network layer security: IPsec8.6 Securing wireless LANs8.7 Operational security: firewalls and IDS

8-34

Secure sockets layer

provides transport layer security to any TCP-based application e.g., between Web browsers, servers for e-commerce

(https) two variants: SSL 3.0, TLS 1.2

security services: server authentication, confidentiality, integrity, client

authentication (optional)

TCP

IP

TCP enhanced with SSL

TCP socket

Application

TCP

IP

TCP API

SSL/TLS

Application

securesocket

8-35

SSL: handshake

Bob (client) establishes TCP connection to Alice (server)

Bob authenticates Alice from her CA signed certificate

Bob creates, encrypts (using Alice’s public key), sends pre-master secret to Alice

(nonces not shown in messages)

SSL hello

certificate

KA+(PMS)

TCP SYN

TCP SYNACK

TCP ACK

decrypt using KA

-

to get PMS

create Pre-mastersecret (PMS)

8-36

SSL: key derivations

Master secret generated from pre-master secret, server and client nonces

Alice, Bob use shared master secret to generate 4 symmetric keys: EB: Bob->Alice data encryption key

EA: Alice->Bob data encryption key

MB: Bob->Alice MAC key

MA: Alice->Bob MAC key

encryption and MAC algorithms negotiable between Bob, Alice

Actually two “finished” messages (encrypted), before transfer of application data, to detect any tampering of handshake messages :

1. client sends a MAC of all handshake messages

2. server sends a MAC of all handshake messages

More detail

8-38

SSL record protocol

SSL record protocol provides transport service to SSL Handshake protocol, application data, and other protocols (e.g., Alert protocol for ending session) Type is for demultiplexing

Application data undergo Fragmentation into blocks Compression ( Optionally ) Computing MAC ( Message Authentication Code )

• using entire record + MAC key + value of sequence counter Encryption

• Type, Version, and Length fields are left in the clear

8-39

Chapter 8 roadmap



8: Network SecuritySSL (8/09)

8-40

What is confidentiality at the network-layer?

Between two network entities: Sending entity encrypts the payloads of

datagrams. Payload could be: TCP segment, UDP segment, ICMP message,

OSPF message, and so on. All data sent from one network entity to

the other would be hidden: Web pages, e-mail, P2P file transfers, TCP

SYN packets, and so on. That is, “blanket coverage”.


8-41

IPheader

IPsecheader

Securepayload

IPhe

ader

IPse

che

ader

Sec

ure

payl

oad

IP

header

IPsec

header

Secure

payload

IPhe

ader

payl

oad

IPheader

payload

headquartersbranch office

salespersonin hotel

PublicInternet

laptop w/ IPsec

Router w/IPv4 and IPsec

Router w/IPv4 and IPsec

Virtual Private Network (VPN)


8-42

IPsec services

Origin authentication Data integrity Replay attack prevention Confidentiality

Two protocols providing different service models: authentication header (AH) protocol encapsulation security payload (ESP)

protocol


8-43

IPsec Transport Mode

IPsec datagrams emitted and received by two end systems.

Protects upper level protocols of these end systems

IPsec IPsec


8-44

IPsec – tunneling mode (1)

End routers are IPsec aware. Hosts need not be.

IPsec IPsec


8-45

IPsec – tunneling mode (2)

Also tunneling mode.

IPsecIPsec


8-46

Two protocols

Authentication Header (AH) protocol provides source authentication & data

integrity but not confidentiality 51 in protocol field of IP header

Encapsulation Security Protocol (ESP) provides source authentication, data

integrity, and confidentiality 50 in protocol field of IP header


8-47

Four combinations are possible

Transport mode with AH

Transport mode with ESP

Tunnel modewith AH

Tunnel modewith ESP

Most common andmost important


8-48

Security association Before sending data, a logical connection is

established from sending entity to receiving entity, called security association (SA) SAs are unidirectional

Both sending and receiving entities maintain state information for the SA Recall that TCP endpoints also maintain state

information. IP is connectionless; IPsec is connection-

oriented How many SAs in VPN for headquarters,

branch office, and n traveling salesperson?


8-49

193.68.2.23200.168.1.100

172.16.1/24172.16.2/24

SA

InternetHeadquartersBranch Office

R1R2

Example SA from R1 to R2

R1 stores SA state info 32-bit identifier for SA: Security Parameter Index (SPI) the origin interface of the SA (200.168.1.100) destination interface of the SA (193.68.2.23) type of encryption to be used (for example, 3DES with

CBC) encryption key type of integrity check (for example, HMAC with MD5) authentication key


8-50

Security Association Database (SAD)

Endpoint holds state of its SAs in a SAD

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.


8-51

IPsec datagram – tunnel mode, ESP

193.68.2.23200.168.1.100

172.16.1/24172.16.2/24

SA

InternetHeadquartersBranch Office

R1R2

new IPheader

ESPhdr

originalIP hdr

Original IPdatagram payload

ESPtrl

ESPMAC

encrypted

“enchilada” authenticated

paddingpad

lengthnext

headerSPISeq

#


8-52

Inside the enchilada:

ESP header: SPI, so receiving entity knows what to do Sequence number, to thwart replay attacks

ESP trailer: Padding for block ciphers next header identifies protocol in payload field

Use shared keys to encrypt and create ESP MAC field

new IPheader

ESPhdr

originalIP hdr


ESPtrl

ESPMAC

encrypted


paddingpad

lengthnext

headerSPISeq

#


8-53

Summary: IPsec services

Suppose Trudy sits somewhere between R1 and R2. She doesn’t know the keys. Will Trudy be able to see contents of

original datagram? How about source, destination IP addresses, transport protocol, application port?

Flip bits without detection? Masquerade as R1 using R1’s IP address? Replay a datagram?


8-54

Athentication methods

PSK (pre-shared key)

PKI (public key infrastructure)pubic/private keys and certificates


8-55

IKE: Key Management in IPsec In previous examples, we manually

established IPsec SAs in IPsec endpoints:Example SA

SPI: 12345Source IP: 200.168.1.100Dest IP: 193.68.2.23 Protocol: ESPEncryption algorithm: 3DES-cbcHMAC algorithm: MD5Encryption key: 0x7aeaca…HMAC key:0xc0291f…

Such manual keying is impractical for large VPN with, say, hundreds of people.

Instead use IPsec IKE (Internet Key Exchange)


8-56

Two Phases of IKEPhase I: Establish bi-directional IKE

SA IKE SA is different from IPsec SA, also called ISAKMP security associationEach endpoint has private and public

keysUse Diffie-Hellman algorithm to

establish a shared secret

(Mutual authentication is not done in phase I)


8-57

Diffie-Hellman algorithm Publicly known large prime number p and

number g that is prime root mod p Alice and Bob independently choose private

keys, and , respectively and exchange their public keys public key of Alice is g mod p public key of Bob is g mod p

Shared secret known only to Alice and Bob is g mod p = (g mod p) mod p = (g mod p)

mod p

Note that in phase I, public keys of Alice and Bob have not been authenticated


8-58

Two Phases of IKE (cont.)Phase I: Establish bi-directional IKE

SAPhase II: The endpoints reveal their

identities to each other and mutually authenticate using public key certificates Identities are not revealed to passive

sniffers since messages are sent over encrypted IKE SA channel between the endpoints


8-59

Chapter 8 roadmap




8-60

IEEE 802.11 security

first attempt at Wi Fi security: Wired Equivalent Privacy (WEP) Many weaknesses Pre-shared key only for authentication

Most recent standard 802.11i (WPA2)


8-61

WEP weaknesses RC4 stream encryption, based on XOR of

plaintext and key, is subject to “known plaintext attack” Append 24-bit initialization vector (IV) to 40-

bit (or 104-bit) secret to generate key for each frame

There are only 224 unique keys 24-bit IV, one per frame, is transmitted in

plaintext -> reuse occurs quickly other weaknesses, e.g., no message

integrity protection


8-62

802.11i (WPA2)

Provides AES block encryption and message integrity

Allows key distribution in addition to pre-shared key use of an authentication server separate

from access point, in addition to local authentication by access point


8-63

wirednetwork

EAP TLSEAP

EAP over LAN (EAPoL)

IEEE 802.11

RADIUS

UDP/IP

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)


8-64

AP: access point AS:Authentication

server

wirednetwork

STA:client station

1 Discovery ofsecurity capabilities

3

STA and AS mutually authenticate, togethergenerate Master Key (MK). AP serves as a relay2

3 STA derivesPairwise Master

Key (PMK)

AS derivessame PMK, sends to AP

4 STA, AP use PMK to derive Temporal Key (TK) used for message

encryption, integrity

802.11i: four phases of operation


8-65

Chapter 8 roadmap




8-66

Firewalls

isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others (i.e., access control)

firewall

administerednetwork

publicInternet

firewall


8-67

Three types of firewalls

stateless packet filters

stateful packet filters

application gateways


8-68

Stateless packet filtering

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

Should arriving packet be allowed

in? Departing packet let out?


8-69

Stateless packet filtering: example

example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. all incoming, outgoing UDP flows and

telnet connections are blocked. example 2: Block inbound TCP segments with

ACK=0. prevents external clients from making TCP

connections with internal clients, but allows internal clients to connect to outside.


8-70

Policy Firewall Setting

No outside Web access. Drop all outgoing packets to any IP address, port 80

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

Drop all incoming TCP SYN packets except those with destination IP 130.207.244.203, port 80

Prevent Web-radios from eating up the available bandwidth.

Drop all incoming UDP packets - except DNS and router broadcasts.

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

Drop all ICMP packets going to a “broadcast” address (e.g. 130.207.255.255).

Prevent your network from being tracerouted

Drop all outgoing ICMP TTL expired traffic

Stateless packet filtering: more examples


8-71

actionsourceaddress

destaddress

protocolsource

portdestport

flagbit

allow222.22/1

6outside of222.22/16

TCP > 1023 80any

allowoutside

of222.22/1

6

222.22/16TCP 80 > 1023 ACK

allow222.22/1

6outside of222.22/16

UDP > 1023 53 ---

allowoutside

of222.22/1

6

222.22/16UDP 53 > 1023 ----

deny all all all all all all

Access Control Lists ACL: table of rules, applied top to bottom to

incoming packets: (action, condition) pairs


8-72

Stateful packet filtering stateless packet filter: memoryless

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

actionsource

addressdest

addressprotocol

sourceport

destport

flagbit

allow outside of222.22/16

222.22/16TCP 80 > 1023 ACK

stateful packet filter: track status of every TCP connection track connection setup (SYN), teardown (FIN): can determine

whether incoming, outgoing packets “make sense” timeout inactive connections at firewall: no longer admit packets


8-73

actionsourceaddress

destaddress

protosource

portdestport

flagbit

check conxion

allow 222.22/16outside of222.22/16

TCP > 1023 80any


222.22/16TCP 80 > 1023 ACK

x

allow 222.22/16outside of222.22/16

UDP > 1023 53 ---


222.22/16UDP 53 > 1023 ----

x

deny all all all all all all

Stateful packet filtering

ACL augmented to indicate need to check connection state table before admitting packet


8-74

Application gateways

Make policy decisions based on application or application data

Example: allow select internal users to telnet outside.

common examples: mail server, web cache

host-to-gatewaytelnet session

gateway-to-remote host telnet session

applicationgateway

router and filter

1. Require all telnet users to telnet through gateway.2. For authorized users, gateway sets up telnet

connection to dest host. Gateway relays data between 2 connections

3. Router filters all telnet connections not originating from gateway.


8-75

Limitations of firewalls and gateways

If multiple app’s need special treatment, each needs its own gateway

client software must know how to contact gateway. e.g., must set IP

address of proxy in Web browser

IP spoofing: router can’t know if data “really” come from claimed source

filters often use all or nothing policy for UDP

tradeoff: degree of communication with outside world, level of security

many highly protected sites still suffer from attacks


8-76

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 viruses, attack strings)

examine correlation among multiple packets to detect

• port scanning• network mapping• DoS attack


8-77

Webserver

FTPserver

DNSserver

applicationgateway

Internet

demilitarized zone

internalnetwork

firewall

IDS sensors

Intrusion detection systems

multiple IDSs: different types of checking at different locations


8-78

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


8-79

End of Chapter 8


8-80

Unused slides


8-81

Chapter 8: Network Security

Chapter goals: understand principles of network security:

cryptography and its many uses beyond “confidentiality”

message integrity authentication

security in practice: security in application, transport, network, link

layers firewalls and intrusion detection systems


8-82

A certificate contains: Serial number (unique to issuer) info about certificate owner, including

algorithm and key value itself (not shown) info about

certificate issuer valid dates digital signature by

issuer


8-83

IPsec datagram

Focus for now on tunnel mode with ESP

new IPheader

ESPhdr

originalIP hdr


ESPtrl

ESPMAC

encrypted


paddingpad

lengthnext

headerSPISeq

#


8-84

Firewall: objectives

prevent denial of service attacks: SYN flooding: attacker establishes many bogus TCP

connections, no resources left for “real” connectionsprevent 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


8-85

IEEE 802.11 security

War-driving: drove around Bay area with laptop and 802.11 card [Shipley 2001] more than 9000 accessible from public

roadways 85% use no encryption/authentication packet-sniffing and various attacks easy

Securing 802.11 encryption, authentication first attempt at Wi Fi security: Wired Equivalent

Privacy (WEP): has weaknesses recent standard 802.11i (WPA2)


8-86

Wired Equivalent Privacy (WEP):

authentication as in protocol ap4.0 host requests authentication from access point access point sends 128 bit nonce host encrypts nonce using shared symmetric

key access point decrypts nonce, authenticates

host no key distribution mechanism authentication: knowing the shared key is enough


8-87

WEP data encryption

Host/AP share 40 bit symmetric key (semi-permanent)

Host appends 24-bit initialization vector (IV) to create 64-bit key—a new IV for each frame

64 bit key used to generate (by RC4) a stream of keys, ki(IV)

ki(IV) used to encrypt ith byte, di, in frame:

ci = di XOR ki(IV)

Each frame contains its IV (in the clear) and encrypted data


8-88

802.11 WEP encryption

IV (per frame)

KS: 40-bit secret

symmetric key k1

IV k2IV k3

IV … kNIV kN+1

IV… kN+1IV

d1 d2 d3 … dN

CRC1 … CRC4

c1 c2 c3 … cN

cN+1 … cN+4

plaintext frame data

plus CRC

key sequence generator ( for given KS, IV)

802.11 header IV

WEP-encrypted data plus CRC

Figure 7.8-new1: 802.11 WEP protocol Sender-side WEP encryption


8-89

Breaking 802.11 WEP encryptionsecurity 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 d1 d2 d3 d4 …

Trudy sees: ci = di XOR ki(IV) Trudy knows ci di, so can compute ki(IV) Trudy knows encrypting key sequence k1(IV), k2(IV),

k3(IV) … Next time IV is used, Trudy can decrypt

plus other weaknesses (e.g., no message integrity protection)


8-90

R1 converts original datagraminto 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.


8-91

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 But doesn’t keep track of ALL received packets;

instead uses a window


8-92

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 the SAD indicates “how” to do it.


8-9393

IKE: PSK and PKI

Authentication (proof who you are) with either pre-shared secret (PSK) or with PKI (pubic/private keys and certificates).

With PSK, both sides start with secret: then run IKE to authenticate each other and to

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

With PKI, both sides start with public/private key pair and certificate. run IKE to authenticate each other and obtain

IPsec SAs (one in each direction). Similar with handshake in SSL.


8-94

Symmetric key crypto

DES: Data Encryption Standard U.S. encryption standard [NIST 1993] 64 bit plaintext input, 56-bit symmetric key

cipher-block chaining for sequence of 64-bit data units

• XOR encrypted nth data unit with n+1st data unit; the result is then encrypted

Triple-DES (3DES)• use three keys sequentially -- output of one key

operation as input of next key operation

Other symmetric key systems: RC4, IDEA, …, AES—recent (Nov. 2001) NIST standard to

succeed DES, 128-bit data, 128, 192, or 256 bit keys


8-95

Message integrity & authenticity

Bob receives msg from Alice, wants to ensure:

message originally came from Alice message not changed since sent by Alice

Cryptographic Hash: takes input m, produces fixed length value,

H(m), called message digest e.g., as in Internet checksum

computationally infeasible to find two different messages, x, y such that H(x) = H(y) Equivalently: given x = H(m), cannot find m’ such that

H(m’)=x note: Internet checksum fails this requirement


8-96

IPsec: Security Association

For both AH and ESP, source, destination handshake: create network-layer unidirectional logical

channel called a security association (SA) source and dest share a symmetric key

• Internet Key Exchange protocol (RFC 2409) allows the use of signatures, public keys, or a pre-shared key

Uniquely determined by: security protocol (AH or ESP) source IP address 32-bit connection ID (Security Parameter

Index)


8-97

Authentication Header (AH) Protocol provides source

authentication, data integrity, no confidentiality

AH header inserted between IP header, data field.

IP header protocol field: 51 intermediate routers

process datagrams as usual

AH header includes: connection identifier authentication data: MAC

calculated over original IP datagram, AH header fields and symmetric key (except IP TTL field)

next header field: specifies type of data (e.g., TCP, UDP, ICMP)

sequence number

IP header data (e.g., TCP, UDP segment)AH header


8-98

ESP Protocol

provides secrecy, host authentication, data integrity.

data, ESP trailer encrypted. next header field is in ESP

trailer.

ESP authentication field is similar to AH authentication field.

Protocol = 50 in IP header.

IP header TCP/UDP segmentESP

headerESP

trailerESP

authent.

encryptedauthenticated


8-99

Many more details

We have decribed the IPsec transport mode for security association between hosts for IPv4

IPsec tunnel model is for security association between routers

IPsec for IPv6


8-100

Symmetric key cryptography

substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another

plaintext: abcdefghijklmnopqrstuvwxyz

ciphertext: mnbvcxzasdfghjklpoiuytrewq

Plaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbc

E.g.:

Q: How hard to break this simple cipher?: brute force (how hard?) other?


8-101

Symmetric key crypto: DES

DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64-bit plaintext input How secure is DES?

DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months

no known “backdoor” decryption approach making DES more secure:

use three keys sequentially (3-DES) on each datum use cipher-block chaining


8-102

Symmetric key crypto: DES

initial permutation 16 identical “rounds” of

function application, each using different 48 bits of key

final permutation

DES operation


8-103

AES: Advanced Encryption Standard

new (Nov. 2001) symmetric-key NIST standard, replacing DES

processes data in 128 bit blocks 128, 192, or 256 bit keys brute force decryption (try each key)

taking 1 sec on DES, takes 149 trillion years for AES


8-104

Block Cipher

one pass through: one input bit affects eight output bits

64-bit input

T1

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

8bits

8 bits

64-bit scrambler

64-bit output

loop for n rounds

T2 T3 T4 T6T5T7

T8

multiple passes: each input bit afects all output bits

block ciphers: DES, 3DES, AES


8-105

Cipher Block Chaining cipher block: if input

block repeated, will produce same cipher text:

t=1m(1) = “HTTP/1.1” block

cipherc(1) = “k329aM02”

…

cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i-1) c(0) transmitted to receiver in clear what happens in “HTTP/1.1”

scenario from above? +

m(i)

c(i)

t=17m(17) = “HTTP/1.1” block

cipherc(17) = “k329aM02”

blockcipher

c(i-1)


8-106

RSA: Choosing keys

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

-


8-107

RSA: Encryption, decryption

0. Given (n,e) and (n,d) as computed above

1. To encrypt bit pattern, m, compute

c = m mod n

e (i.e., remainder when m is divided by n)e

2. To decrypt received bit pattern, c, compute

m = c mod n

d (i.e., remainder when c is divided by n)d

m = (m mod n)

e mod n

dMagichappens!

c


8-108

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.

letter m me c = m mod ne

l 12 1524832 17

c m = c mod nd

17 481968572106750915091411825223071697 12

cdletter

l

encrypt:

decrypt:


8-109

RSA: Why is that m = (m mod n)

e mod n

d

(m mod n)

e mod n = m mod n

d ed

Useful number theory result: If p,q prime and n = pq, then:

x mod n = x mod ny y mod (p-1)(q-1)

= m mod n

ed mod (p-1)(q-1)

= m mod n1

= m

(using number theory result above)

(since we chose ed to be divisible by(p-1)(q-1) with remainder 1 )


8-110

SSL: three phases3. Data transfer

H( ).MB

b1b2b3 … bn

d

d H(d)

d H(d)

H( ).EB

TCP byte stream

block n bytes together compute

MAC

encrypt d, MAC, SSL

seq. #

SSL seq. #

d H(d)Type Ver Len

SSL record format

encrypted using EBunencrypted

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