7: Network Security 1 Chapter 7 Network Security Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross.

7: Network Security 1

Chapter 7 Network Security

Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith RossAddison-Wesley, July 2002.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK / KWR

All material copyright 1996-2002J.F Kurose and K.W. Ross, All Rights Reserved


Chapter 7: Network security

Foundations: what is security? cryptography authentication message integrity key distribution and certification

Security in practice: application layer: secure e-mail transport layer: Internet commerce, SSL, SET network layer: IP security Firewalls

8: Network 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


Internet security threatsPacket sniffing:

broadcast media promiscuous NIC reads all packets passing by can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets

A

B

C

src:B dest:A payload


Internet security threatsIP Spoofing:

can generate “raw” IP packets directly from application, putting any value into IP source address field

receiver can’t tell if source is spoofed e.g.: C pretends to be B

A

B

C

src:B dest:A payload


Internet security threatsDenial of service (DOS):

flood of maliciously generated packets “swamp” receiver Distributed DOS (DDOS): multiple coordinated sources swamp

receiver e.g., C and remote host SYN-attack A

A

B

C

SYN

SYNSYNSYN

SYN

SYN

SYN

Cryptography Principles



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

securesender

securereceiver

channel data, control messages

data data

Alice Bob

Trudy


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?


The language of cryptography

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

key secret (private)

plaintext plaintextciphertext

KA

encryptionalgorithm

decryption algorithm

Alice’s encryptionkey

Bob’s decryptionkey

KB

Symmetric Key

Cryptography7: Network Security 11


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?


Perfect cipher

Definition: Let C = E[M] Pr[C=c] = Pr[C=c | M]

Example: one time pad Generate random bits b1 ... bn

E[M1 ... Mn] = (M1 b1 ... Mn bn ) Cons: size Pseudo Random Generator

G(R) = b1 ... bn

Indistinguishable from random (efficiently)


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


Symmetric key crypto: DES

initial permutation 16 identical “rounds” of

function application, each using different 48 bits of key

final permutation

DES operation


Block Cipher chaining

How do we encode a large message Would like to guarantee integrity

Encoding: Ci = E[Mi Ci-1]

Decoding: Mi = D[Ci] Ci-1

Malfunctions: Loss Reorder/ integrity


Break plaintext into blocks XOR previous round ciphertext with new plaintext

Block1

IV

DES

Cipher 1

Block2

DES

Block3

DES

Block4

DES

+

Cipher 2 Cipher 3 Cipher 4

+++

Cipher block chaining (CBC)


Cipher Block Chaining Mode

Cipher block chaining. (a) Encryption. (b) Decryption.


DES performance

DES relies on confusion and diffusion not mathematically proven to be secure

More secure: 3DES (triple DES) with cipher block chaining (CBC)

3DES products for IP security*

~10 Mb/s in software ~100 Mb/s in hardware

Both parties must know secret key, and keep it secret

* source: http://www.ietf.org/proceedings/01dec/slides/ipsec-11.pdf


Diffie-Hellman key exchange protocol Goal: Allow strangers establish a shared secret

key for later communication Assume two parties (Alice and Bob) want to

establish a secret key. Alice and Bob agree on two large numbers, n and

g usually, these are publicly known, and have some

additional conditions applied (e.g., n must be prime)


Diffie-Hellman Key Exchange Alice picks large x, Bob picks large y (e.g., 512 bits)


Man in the middle attack

Eavesdropper can’t determine secret key (gxy mod n) from (gx mod n) or (gy mod n)

However, how does Alice and Bob know if there is a third party adversary in between?


Exponentiation

Compute gx mod nExpg,n (x) Assume x = 2y + b Let z = Expg,n (y) R=z2

If (b=1) R = g R mod n Return R

Complexity: logarithmic in x

Public Key Cryptograph

y7: Network Security 25


Public Key Cryptography

symmetric key crypto

requires sender, receiver know shared 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

encryption key public (known to all)

decryption key private (known only to receiver)


Public key cryptography

plaintextmessage, m

ciphertextencryptionalgorithm

decryption algorithm

Bob’s public key

plaintextmessageK (m)

B+

K B+

Bob’s privatekey

K B-

m = K (K (m))B+

B-


Public key encryption algorithms

need d ( ) and e ( ) such that

d (e (m)) = m BB

B B. .

need public and private keysfor d ( ) and e ( ). .

BB

Two inter-related requirements:

1

2

RSA: Rivest, Shamir, Adelson algorithm


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

-


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!


RSA operation

Encryption & Decryption E(P) = Pemod nD(C) = Cdmod n

Security is based on difficulty of factoring large numbers (can’t factor n to obtain p and q)

Sender Plaintext (P)

Public key eE(P, e)

Ciphertext (C)

Recv plaintext (P)

Private key d D(C, d)


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 481968572106750915091411825223072000 12

cdletter

l

encrypt:

decrypt:


RSA example

An example of the RSA algorithm. p = 3, q = 11, n = 33, z = 20, d = 7 e found by solving 7e = 1 (mod 20)--> 3


RSA: Why m = (m mod n)

e mod n

d

Number theory result:

• IF pq = n, p and q primes then:

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

• (m e)d mod n = m (ed mod (p-1)(q-1)) mod n

•But ed – 1 divisible by (p-1)(q-1) i.e., ed mod (p-1)(q-1) = 1

• = m 1 mod n = m


modified Diffie-Hellman Key Exchange

Encrypt 1 with Bob’s public key, 2 with Alice’s public key Prevents man-in-the-middle attack Actually, nonces and a third message are needed to

fully complete this exchange (in a few slides)

Authentication



Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

Failure scenario??“I am Alice”


Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

in a network,Bob can not “see”

Alice, so Trudy simply declares

herself to be Alice“I am Alice”


Authentication: another try

Protocol ap2.0: Alice says “I am Alice” in an IP packetcontaining her source IP address

Failure scenario??

“I am Alice”Alice’s

IP address



Protocol ap2.0: Alice says “I am Alice” in an IP packetcontaining her source IP address

Trudy can createa packet

“spoofing”Alice’s address“I am Alice”

Alice’s IP address



Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.

Failure scenario??

“I’m Alice”Alice’s IP addr

Alice’s password

OKAlice’s IP addr



Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.

playback attack: Trudy records Alice’s

packetand later

plays it back to Bob


Alice’s password

OKAlice’s IP addr


Alice’s password


Authentication: yet another try

Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

Failure scenario??


encrypted password

OKAlice’s IP addr



Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

recordand

playbackstill works!


encryptedpassword

OKAlice’s IP addr


encryptedpassword


Authentication: yet another try

Goal: avoid playback attack

Failures, drawbacks?

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

K (R)A-B

Alice is live, and only Alice knows key to encrypt

nonce, so it must be Alice!


Authentication: ap5.0

ap4.0 requires shared symmetric key can we authenticate using public key techniques?ap5.0: use nonce, public key cryptography

“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, that encrypted R such that

(K (R)) = RA-

K A+


ap5.0: 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


ap5.0: security holeMan (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!

Message Integrity

(Signatures etc)



Digital Signatures

Cryptographic technique analogous to hand-written signatures.

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

Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document.

Assumption: eB(dB(m)) = dB(eB(m)) RSA

Simple digital signature for message m:

Bob decrypts m with his private key dB, creating signed message, dB(m).

Bob sends m and dB(m) to Alice.


Digital Signatures (more)

Suppose Alice receives msg m, and digital signature dB(m)

Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m.

If eB(dB(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 dB(m) to court and prove that Bob signed m.


Message Digests

Computationally expensive to public-key-encrypt long messages

Goal: fixed-length,easy to compute digital signature, “fingerprint”

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

computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).


Hash Function Algorithms

Internet checksum would make a poor message digest. Too easy to find

two messages with same checksum.

MD5 hash function widely used. 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 160-bit message digest


MD5 for message integrity

Keyed MD5 assume sender, receiver share secret key k sender: m + MD5(m + k)

(+ means concatenation in this notation) receiver computes MD5(m + k) and sees if

it matches


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 = signed message digest


MD5 with RSA signature

MD5 with RSA signature sender: m + E(MD5(m), private)

receiver: compare MD5(m) with D(checksum, public)

checksum

Key Distribution

Centers7: Network Security 58


Problems with Public-Key Encryption A way for Trudy to subvert public-key

encryption.


Trusted Intermediaries

Problem: How do two

entities establish shared secret key over network?

Solution: trusted key

distribution center (KDC) acting as intermediary between entities

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)


Key Distribution Center (KDC)

Alice,Bob need shared symmetric key.

KDC: server shares different secret key with each registered user.

Alice, Bob know own symmetric keys, KA-

KDC KB-KDC , for communicating with KDC.

Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1)

Alice sends Bob KB-KDC(A,R1), Bob extracts R1

Alice, Bob now share the symmetric key R1.


Certification Authorities

Certification authority (CA) binds public key to particular entity.

Entity (person, router, etc.) can register its public key with CA. Entity provides “proof

of identity” to CA. CA creates certificate

binding entity to public key.

Certificate digitally signed by CA.

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


X.509 is the standard for certificates The basic fields of an X.509 certificate.


Public-Key Infrastructures

PKIs are a way to structure certificates (a) A hierarchical PKI. (b) A chain of

certificates.


Example revisited (solved with certificates) Trudy presents Alice a certificate,

purporting to be Bob, but Alice is unable to trace Trudy’s certificate back to a trusted root

Be wary if your browser warns about certs!

Secure email



Secure e-mail

Alice: generates random symmetric private key, KS. encrypts message with KS (for efficiency) also encrypts KS with Bob’s public key. sends both KS(m) and KB(KS) to Bob.

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

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+


Secure e-mail

Bob: uses his private key to decrypt and recover KS

uses KS to decrypt KS(m) to recover m

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

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+

7; Network Security 8-69

Secure e-mail (continued)

• Alice wants to provide sender authentication message integrity.

• Alice digitally signs message.• sends both message (in the clear) and digital signature.

H( ). KA( ).-

+ -

H(m )KA(H(m))-

m

KA-

Internet

m

KA( ).+

KA+

KA(H(m))-

mH( ). H(m )

compare


Pretty good privacy (PGP)

Internet e-mail encryption scheme, a de-facto standard.

Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.

Provides secrecy, sender authentication, integrity.

Inventor, Phil Zimmerman, was target of 3-year federal investigation.

---BEGIN PGP SIGNED MESSAGE---Hash: SHA1

Bob:My husband is out of town tonight.Passionately yours, Alice

---BEGIN PGP SIGNATURE---Version: PGP 5.0Charset: noconvyhHJRHhGJGhgg/

12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2

---END PGP SIGNATURE---

A PGP signed message:

Secure Socket layer

(SSl)7: Network Security 71


Secure sockets layer (SSL)

PGP provides security for a specific network app.

SSL works at transport layer. Provides security to any TCP-based app using SSL services.

SSL: used between WWW browsers, servers for I-commerce (https).

SSL security services: server authentication data encryption client authentication

(optional)

Server authentication: SSL-enabled browser

includes public keys for trusted CAs.

Browser requests server certificate, issued by trusted CA.

Browser uses CA’s public key to extract server’s public key from certificate.

Visit your browser’s security menu to see its trusted CAs.


Internet Explorer:Tools Internet options Content Certificates


Internet Explorer: Error Message


SSL (continued)

Encrypted SSL session: Browser generates

symmetric session key, encrypts it with server’s public key, sends encrypted key to server.

Using its private key, server decrypts session key.

Browser, server agree that future msgs will be encrypted.

All data sent into TCP socket (by client or server) is encrypted with session key.

SSL: basis of IETF Transport Layer Security (TLS).

SSL can be used for non-Web applications, e.g., IMAP.

Client authentication can be done with client certificates.


SSL basics

RA, RB are “nonces” used with Premaster key to create session keyAlice validatesBob’s key

EB is Bob’s public key

Other Security Layers



Secure electronic transactions (SET)

designed for payment-card transactions over Internet.

provides security services among 3 players: customer merchant merchant’s bankAll must have certificates.

SET specifies legal meanings of certificates. apportionment of

liabilities for transactions

Customer’s card number passed to merchant’s bank without merchant ever seeing number in plain text. Prevents merchants

from stealing, leaking payment card numbers.

Three software components: Browser wallet Merchant server Acquirer gateway

See book for description of SET transaction.


IPsec: Network Layer Security Network-layer secrecy:

sending host encrypts the data in IP datagram

TCP and UDP segments; ICMP and SNMP messages.

Network-layer authentication destination host can

authenticate source IP address

Two principle protocols: authentication header

(AH) protocol encapsulation security

payload (ESP) protocol

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

logical channel called a service agreement (SA)

Each SA unidirectional. Uniquely determined by:

security protocol (AH or ESP)

source IP address 32-bit connection ID


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.


Authentication Header (AH) Protocol

Provides source host authentication, data integrity, but not secrecy.

AH header inserted between IP header and IP data field.

Protocol field = 51. Intermediate routers

process datagrams as usual.

AH header includes: connection identifier authentication data: signed

message digest, calculated over original IP datagram, providing source authentication, data integrity.

Next header field: specifies type of data (TCP, UDP, ICMP, etc.)


IPsec authentication (AH)

The IPsec authentication header in transport mode for IPv4.

HMAC stands for Hashed Message Authentication Code


IPsec encryption (ESP)

(a) ESP in transport mode (for end-to-end IPsec). (b) ESP in tunnel mode (used in VPNs).

Access Control in

the Network7: Network Security 84


Firewalls

Two firewall types: packet filter application

gateways

To prevent denial of service attacks: SYN flooding: attacker

establishes many bogus TCP connections. Attacked host alloc’s TCP buffers for bogus connections, none left for “real” connections.

To prevent illegal modification of internal data. e.g., attacker replaces

CIA’s homepage with something else

To prevent intruders from obtaining secret info.

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

firewall


Packet Filtering

Internal network is connected to Internet through a router.

Router manufacturer provides options for filtering packets, based on: source IP address destination IP address TCP/UDP source and

destination port numbers

ICMP message type TCP SYN and ACK bits

Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. All incoming and 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.


Filter-based firewalls

Sit between site and rest of Internet, filter packets Enforce site policy in a manageable way e.g. pass (*,*, 128.7.6.5, 80 ), then drop (*, *, *, 80) Rules may be added dynamically to pass new connections

Sometimes called a “ level 4” switch Acts like a router (accepts and forwards packets) But looks at information up to TCP port numbers (layer

Rest of the Internet Local siteFirewall


Proxy-Based Firewalls

Solves more complex policy problems Example:

I want internal company to be able to access entire web server

I want outside world only to access public pages

Company netFirewall Internalserver

Randomexternaluser

Remotecompanyuser

Internet Proxy

Use proxies outside firewall

Firewall only lets proxyconnect to web server

Webserver


Proxy-based firewalls

Run proxies for Web, mail, etc. just outside firewall In the “ de-militarized zone” DMZ External requests go to proxies, only proxies

connect inside External user may (“classical model”) or may

not (“transparent model”) know this is happening

Proxies filter based on application semantics


Virtual Private Networks

Using firewalls and IPsec encryption to provide a “leased-line” like connection over the Internet

(a) A leased-line private network. (b) A virtual private network.


Application gateways

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

Example: allow select internal users to telnet outside.

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 filter blocks all telnet connections not originating from gateway.


Limitations of firewalls and gateways

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

If multiple app’s. need special treatment, each has own app. gateway.

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

address of proxy in Web browser

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.


Network Security (summary)

Basic techniques…... cryptography (symmetric and public) authentication message integrity…. used in many different security scenarios secure email secure transport (SSL) IP sec Firewalls

7: Network Security 1 Chapter 7 Network Security Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross.

Documents

7: Network Security 1 Chapter 7 Network Security Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross.