חישוב ציון תרגילי בית:

בזמןמי שלא יגיש את הפרויקט( PA3:) תרגילי הבית7ממוצע של כל

בזמןמי שיגיש את הפרויקט( PA3:) תרגילי בית:6ממוצע של ( הפרויקטPA3)חמשת הטובים )מתוך ששת( התרגילים האחרים( יהיה זהה )(1/6משקל כל תרגיל )מתוך השישה

!הערה: חייבים להקפיד על הגשה בזמן

Lecture 13: Network Security

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

January 13, 2010

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

January 13, 2010

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

src:B dest:A payload

January 13, 2010

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

src:B dest:A payload

January 13, 2010

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

SYNSYNSYN

January 13, 2010

Cryptography Principles

January 13, 2010

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

January 13, 2010

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?

January 13, 2010

The language of cryptography

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

key secret (private)

plaintext plaintextciphertext

encryptionalgorithm

decryption algorithm

Alice’s encryptionkey

Bob’s decryptionkey

January 13, 2010

Symmetric Key

CryptographyLecture 13: Network Security

January 13, 2010

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

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

January 13, 2010

Perfect cipher [Shannon 1948]

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)

January 13, 2010

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

January 13, 2010

Symmetric key crypto: DES

initial permutation 16 identical “rounds” of

function application, each using different 48 bits of key

final permutation

DES operation

January 13, 2010

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

January 13, 2010

7: Network Security 17

Cipher Block Chaining Mode

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

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)

January 13, 2010

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

January 13, 2010

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?

January 13, 2010

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

January 13, 2010

Public Key Cryptograph

yLecture 13: Network Security

January 13, 2010

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)

January 13, 2010

Public key cryptography

plaintextmessage, m

ciphertextencryptionalgorithm

decryption algorithm

Bob’s public key

plaintextmessageK (m)

Bob’s privatekey

m = K (K (m))B+

January 13, 2010

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 ( ). .

Two inter-related requirements:

RSA: Rivest, Shamir, Adelson algorithm

January 13, 2010

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

January 13, 2010

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!

January 13, 2010

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

encrypt:

decrypt:

January 13, 2010

RSA: Why m = (m mod n)

e mod n

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

January 13, 2010

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)

January 13, 2010

Authentication

January 13, 2010

Authentication

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

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

Failure scenario??“I am Alice”

January 13, 2010

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”

January 13, 2010

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

January 13, 2010

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

January 13, 2010

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

January 13, 2010

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

January 13, 2010

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

January 13, 2010

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

January 13, 2010

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”

K (R)A-B

Alice is live, and only Alice knows key to encrypt

nonce, so it must be Alice!

January 13, 2010

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 (R)) = RA

and knows only Alice could have the

private key, that encrypted R such that

(K (R)) = RA-

January 13, 2010

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

TK (R)

Send me your public key

K (R)-

Send me your public key

TK (m)+

Tm = K (K (m))+

Trudy gets

sends m to Alice encrypted

with Alice’s public key

AK (m)+

Am = K (K (m))+

January 13, 2010

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!

January 13, 2010

Message Integrity

(Signatures etc)

January 13, 2010

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.

January 13, 2010

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.

January 13, 2010

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’).

January 13, 2010

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

January 13, 2010

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

H: Hashfunction

digitalsignature(decrypt)

Bob’s public

key K B+

equal ?

Digital signature = signed message digest

January 13, 2010

Key Distribution

CentersLecture 13: Network Security

January 13, 2010

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

encryption.

January 13, 2010

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)

January 13, 2010

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.

January 13, 2010

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

January 13, 2010

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

January 13, 2010

Public-Key Infrastructures

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

certificates.

January 13, 2010

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!

January 13, 2010

Secure email

January 13, 2010

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( ).+

KB(KS )+

Internet

KS( ).

KB( ).-

KB(KS )+

January 13, 2010

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( ).+

KB(KS )+

Internet

KS( ).

KB( ).-

KB(KS )+

January 13, 2010

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))-

Internet

KA( ).+

KA(H(m))-

mH( ). H(m )

compare

January 13, 2010

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:

January 13, 2010

Secure Socket layer

(SSl)Lecture 13: Network Security

January 13, 2010

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 E-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.

January 13, 2010

Internet Explorer:Tools Internet options Content Certificates

January 13, 2010

Internet Explorer: Error Message

January 13, 2010

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.

January 13, 2010

SSL basics

January 13, 2010

Lecture 13: Network Security 68

Access Control in

the NetworkLecture 13: Network Security

January 13, 2010

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

January 13, 2010

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.

January 13, 2010

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.

January 13, 2010

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.

January 13, 2010

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.

January 13, 2010

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

January 13, 2010

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