07 Chapter7 Multimedia Networking.pptx

第1页：第8章：网络安全

第8章：网络安全

• understand principles of network security:
  • cryptography and itsmanyuses beyond“confidentiality”
  • authentication
  • message integrity
• security in practice:
  • firewalls and intrusion detection systems
  • security in application, transport, network, linklayers

第2页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第3页：What network security?Confidentiality

What network security?Confidentiality

• 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 accessibleand available to users

第4页：Friends and enemies: Alice, Bob, Trudy

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 secure sender secure receiver channeldata, controlmessagesdatadata Alice Bob Trudy

第5页：Who might Bob, Alice

Who might Bob, Alice

• … 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?

第6页：There bad guys (and girls)

There bad guys (and girls)

• eavesdrop:intercept messages
• actively insert messages into connection
• impersonation:can fake (spoof) source addressin packet (or any field in packet)
• hijacking:“take over” ongoing connection byremoving sender or receiver, inserting himselfin place
• denial of service: prevent service from beingused by others (e.g., by overloading resources)more on this later ……

第7页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第8页：language cryptography symmetric

language cryptography symmetric

The language of cryptography symmetric keycrypto: sender, receiver keys identical public-keycrypto: encryption keypublic, decryption keysecret (private)plaintext plaintext ciphertext KAencryption algorithm decryption algorithm Alice’sencryptionkey Bob’sdecryptionkey KB

第9页：Symmetric key cryptography

Symmetric key cryptography

• monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyzciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.:Q:How hard to break this simple cipher?:
• brute force (how hard?)
• other?

第10页：对称密钥密码学对称

对称密钥密码学对称

• e.g., key is knowing substitution pattern in monoalphabetic substitution cipher
• Q:how do Bob and Alice agree on key value?plaintext ciphertext KA-Bencryption algorithm decryption algorithm A-BKA-Bplaintext message, m K (m)A-BK (m)A-Bm = K()A-B

第11页：对称密钥密码 DESDES：数据加密

对称密钥密码 DESDES：数据加密

• 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 saferplace”) 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

第12页：对称密钥密码学

对称密钥密码学

Symmetric keycrypto: DESinitial permutation 16 identical “rounds” offunction application,each using different 48 bits of keyfinal permutation DES operation

第13页：AES 高级加密标准

AES 高级加密标准

• new (Nov. 2001) symmetric-key NISTstandard, 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 trillionyears for AES

第14页：分组密码

分组密码

• one passthrough: oneinput bitaffects eight output bits 64-bit input T18 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 64-bit scrambler 64-bit outputloop forn rounds T2T3T4T6T5T7T8
• multiple passes: each input bit afects all output bits
• block ciphers: DES, 3DES, AES

第15页：Cipher Block Chain

Cipher Block Chain

• cipher block: if inputblock repeated, willproduce same ciphertext:t=1 m(1)= “HTTP/1.1”block cipherc(1)= “k 329 a M02”…
• cipher block chaining:XOR ith input block,m(i), with previous block of cipher text,c(i-1)
  • c(0) transmitted toreceiver in clear
  • what happens in“HTTP/1.1” scenariofrom above?+m(i)c(i)t=17 m(17)= “HTTP/1.1”block cipherc(17)= “k 329 a M02”block cipherc(i-1)

第16页：Public key cryptography symmetrickey

Public key cryptography symmetrickey

• requires sender,receiver know shared secret key
• Q: how to agree on keyin first place(particularly if never“met”)?publickey cryptography
• radically different approach [Diffie-Hellman 76, RSA78]
• sender, receiver donotshare secret key
• public encryption keyknown toall
• private decryptionkey known only toreceiver

第17页：公钥密码学明文报文

公钥密码学明文报文

Public key cryptography plaintext message, mciphertext encryption algorithm decryption algorithm Bob’spublickey plaintext message K (m)B+KB+Bob’sprivatekey KB-m = K(K (m))B+B-

第18页：公钥加密算法

公钥加密算法

Public key encryption algorithmsneed K ( ) and K ( ) such that BB..given public key K , it should beimpossible to compute private key KBBRequirements:12RSA:Rivest, Shamir, Adleman algorithm+-K (K (m)) = m BB-++-

第19页：密码学原理（示意图）

密码学原理（示意图）

RSA: Choosing keys 1. Choose two large prime numbersp, q.(e.g., 1024 bits each)2. Computen= pq, z = (p-1)(q-1)3. Choosee(withe<n)that has no common factorswith z. (e, zare “relatively prime”).4. Choosedsuch thated-1 is exactly divisible byz.(in other words:edmodz = 1).5. Publickey is(n,e). Privatekey is(n,d). KB+KB-

第20页：RSA: Encryption, decryption 0. Given

RSA: Encryption, decryption 0. Given

RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 1. To encrypt bit pattern,m, computec = mmodne(i.e., remainder whenmis divided byn)e 2. To decrypt received bit pattern,c, computem = cmodnd(i.e., remainder whencis divided byn)dm = (mmodn)emodnd Magic happens!c

第21页：RSA example

RSA example

RSA example:Bob choosesp=5, q=7. Thenn=35, z=24.e=5(soe, zrelatively prime).d=29(soed-1 exactly divisible by z.lettermmec = m mod nel 12152483217 cm = c mod nd 1748196857210675091509141182522307169712 cdletterl encrypt:decrypt:

第22页：RSA:Why thatm =

RSA:Why thatm =

RSA:Why is thatm = (mmodn)emodnd(mmodn)emodn = mmodnded Useful number theory result:Ifp,qprime andn = pq,then:xmodn = xmodnyymod(p-1)(q-1)= mmodnedmod(p-1)(q-1)= mmodn 1= m(using number theory result above)(since wechoseedto be divisible by(p-1)(q-1)with remainder 1 )

第23页：RSA: another important property

RSA: another important property

RSA: another important property The following property will bevery useful later:K(K (m))= m BB-+K(K (m))BB+-=use public keyfirst, followedby private keyuse private keyfirst, followedby public key Result is the same!

第24页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第25页：Message Integrity Bob receives msg from

Message Integrity Bob receives msg from

• message originally came from Alice
• message not changed since sent by Alice Cryptographic Hash:
• takes input m, produces fixed length value, H(m)
  • e.g., as in Internet checksum
• computationally infeasible to find two different messages, x, y such that H(x) = H(y)
  • equivalently: given m = H(x), (x unknown), can not determinex.
  • note: Internet checksumfailsthis requirement!

第26页：因特网检验和 poor 密码 hash

因特网检验和 poor 密码 hash

• produces fixed length digest (16-bit sum) of message
• is many-to-one But given message with given hash value, it is easy to findanother message with same hash value:I O U 10 0 . 99 B O B49 4F 55 3130 30 2E 3939 42 4F 42 message ASCII format B2 C1 D2 ACI O U90 0 .19 B O B49 4F 553930 30 2E3139 42 4F 42 message ASCII format B2 C1 D2 ACdifferent messagesbut identical checksums!

第27页：Message Authentication Codem

Message Authentication Codem

Message Authentication Codems(shared secret)(message)H(.)H(m+s)public Internet appendm H(m+s)scomparem H(m+s)H(.)H(m+s)(shared secret)

第28页：MACs practice

MACs practice

• MD5 hash function widely used (RFC 1321)
  • computes 128-bit MAC in 4-step process.
  • arbitrary 128-bit string x, appears difficult toconstruct msg m whose MD5 hash is equal to x
• recent (2005) attacks on MD5
• SHA-1 is also used
  • US standard [NIST, FIPS PUB 180-1]
  • 160-bit MAC

第29页：Digital Signatures cryptographic

Digital Signatures cryptographic

• sender (Bob) digitally signs document,establishing he is document owner/creator.
• verifiable, nonforgeable:recipient (Alice) canprove to someone that Bob, and no one else(including Alice), must have signed document

第30页：Digital Signatures simple digital

Digital Signatures simple digital

• Bob “signs” m by encrypting with his private key KB, creating “signed” message, KB(m)--Dear Alice Oh, how I have missedyou. I think of you all thetime! …(blah blah blah)Bob Bob’s message, mpublic keyencryption algorithm Bob’s privatekey KB-Bob’s message,m, signed(encrypted) withhis private key KB-(m)

第31页：Digital Signatures (more)

Digital Signatures (more)

• suppose Alice receives msg m, digital signature KB(m)
• Alice verifies m signed by Bob by applying Bob’spublic key KBto KB(m) then checks KB(KB(m) ) = m.
• if KB(KB(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 KB(m) tocourt and prove that Bob signed m.-

第32页：large messagem H: hash

large messagem H: hash

large messagem H: hashfunction H(m)digital signature(encrypt)Bob’sprivatekey KB-+Bob sends digitally signed message:Alice verifies signature andintegrity of digitally signed message:KB(H(m))-encryptedmsg digest KB(H(m))-encryptedmsg digest large messagem H: hashfunction H(m)digital signature(decrypt)H(m)Bob’spublickey KB+equal?Digital signature = signed MAC

第33页：Public Key Certification public key

Public Key Certification public key

• When Alice obtains Bob’s public key (from web site,e-mail, diskette), how does sheknowit is Bob’spublic key, not Trudy’s?solution:
• trusted certification authority (CA)

第34页：Certification Author

Certification Author

• Certification Authority (CA):binds public key toparticular 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’spublickey KB+Bob’sidentifying information digital signature(encrypt)CAprivatekey KCA-KB+certificate for Bob’s public key,signed by CA-KCA(K )B+

第35页：Certification Author

Certification Author

• 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’spublickey KB+digital signature(decrypt)CApublickey KCA+KB+-KCA(K )B+

第36页：certificate contains

certificate contains

• Serial number (unique to issuer)
• info about certificate owner, including algorithmand key value itself (not shown)
• info about certificate issuer
• valid dates
• digital signature byissuer

第37页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第38页：Authentication Goal:Bob wants Alice

Authentication Goal:Bob wants Alice

Authentication Goal:Bob wants Alice to “prove” her identityto him Protocol ap 1.0:Alice says “I am Alice”Failure scenario??“I am Alice”

第39页：Authentication Goal:Bob wants Alice

Authentication Goal:Bob wants Alice

Authentication Goal:Bob wants Alice to “prove” her identityto him Protocol ap 1.0:Alice says “I am Alice”in a network, Bob can not “see”Alice, so Trudy simply declares herself to be Alice“I am Alice”

第40页：鉴别另一种 try 协议 ap 2.0

鉴别另一种 try 协议 ap 2.0

Authentication: another try Protocol ap 2.0:Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario??“I am Alice”Alice’s IP address

第41页：鉴别另一种 try 协议 ap 2.0

鉴别另一种 try 协议 ap 2.0

Authentication: another try Protocol ap 2.0:Alice says “I am Alice” in an IP packet containing her source IP address Trudy can createa packet“spoofing”Alice’s address“I am Alice”Alice’s IP address

第42页：鉴别另一种 try 协议 ap 3.0

鉴别另一种 try 协议 ap 3.0

Authentication: another try Protocol ap 3.0:Alice says “I am Alice” and sends hersecret password to “prove” it. Failure scenario??“I’m Alice”Alice’s IP addr Alice’spassword OKAlice’s IP addr

第43页：鉴别另一种 try 协议 ap 3.0

鉴别另一种 try 协议 ap 3.0

Authentication: another try Protocol ap 3.0:Alice says “I am Alice” and sends hersecret password to “prove” it.playback attack:Trudy records Alice’s packetand laterplays it back to Bob“I’m Alice”Alice’s IP addr Alice’spassword OKAlice’s IP addr“I’m Alice”Alice’s IP addr Alice’spassword

第44页：鉴别 yet 另一种 try 协议 ap

鉴别 yet 另一种 try 协议 ap

Authentication: yet another try Protocol ap 3.1:Alice says “I am Alice” and sends herencrypted secret password to “prove” it. Failure scenario??“I’m Alice”Alice’s IP addrencryptedpassword OKAlice’s IP addr

第45页：鉴别另一种 try 协议 ap 3.1

鉴别另一种 try 协议 ap 3.1

Authentication: another try Protocol ap 3.1:Alice says “I am Alice” and sends herencrypted secret password to “prove” it.recordand playback still works!“I’m Alice”Alice’s IP addrencryptedpassword OKAlice’s IP addr“I’m Alice”Alice’s IP addrencryptedpassword

第46页：鉴别 yet 另一种 try 目标

鉴别 yet 另一种 try 目标

• in-a-lifetimeap 4.0:to prove Alice “live”, Bob sends Alice nonce, R. Alicemust return R, encrypted with shared secret key“I am Alice”RK (R)A-BAlice is live, andonly Alice knowskey to encrypt nonce, so it mustbe Alice!

第47页：Authentication: ap 5.0 ap 4.0 requires

Authentication: ap 5.0 ap 4.0 requires

• can we authenticate using public key techniques?ap 5.0:use nonce, public key cryptography“I am Alice”RBob computes K (R)A-“send me your public key”KA+(K (R)) = RA-KA+and knows only Alice could have the privatekey, that encrypted Rsuch that(K (R)) = RA-KA+

第48页：ap 5.0: security hole Man (woman)

ap 5.0: security hole Man (woman)

ap 5.0: security hole Man (woman) in the middle attack:Trudy poses as Alice (to Bob) and as Bob (to Alice)I am Alice I am Alice RTK (R)-Send me your public key TK+AK (R)-Send me your public key AK+TK (m)+Tm = K (K (m))+T-Trudy getssends m to Alice encrypted with Alice’s public key AK (m)+Am = K (K (m))+A-R

第49页：ap 5.0: security hole Man (woman)

ap 5.0: security hole Man (woman)

• Bob receives everything that Alice sends, and viceversa. (e.g., so Bob, Alice can meet one week later andrecall conversation)
• problem is that Trudy receives all messages as well!

第50页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第51页：安全电子邮件 Alice

安全电子邮件 Alice

• generates random symmetric private key, KS.
• encrypts message with KS(for efficiency)
• also encrypts KSwith 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 KSKSKB+Internet KS( ). KB( ).-KB-KSm KS(m )KB(KS)+

第52页：安全电子邮件 Bob

安全电子邮件 Bob

• uses his private key to decrypt and recover KS
• uses KSto decrypt KS(m) to recover m
• Alice wants to send confidential e-mail, m, to Bob. KS( ). KB( ).++-KS(m )KB(KS)+m KSKSKB+Internet KS( ). KB( ).-KB-KSm KS(m )KB(KS)+

第53页：安全电子邮件续

安全电子邮件续

• 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-Internetm KA( ).+KA+KA(H(m))-m H( ). H(m )compare

第54页：安全电子邮件续

安全电子邮件续

• Alice wants to provide secrecy, sender authentication,message integrity. Alice uses three keys:her private key, Bob’s publickey, newly created symmetric key H( ). KA( ).-+KA(H(m))-m KA-m KS( ). KB( ).++KB(KS)+KSKB+Internet KS

第55页：Pretty good privacy (PGP)

Pretty good privacy (PGP)

• Internet e-mail encryption scheme, de-facto standard.
• uses symmetric keycryptography, public keycryptography, hashfunction, and digital signature as described.
• provides secrecy, sender authentication, integrity.
• inventor, Phil Zimmerman,was target of 3-yearfederal investigation.---BEGIN PGP SIGNED MESSAGE---Hash: SHA1 Bob:My husband is out of towntonight. Passionately yours, Alice---BEGIN PGP SIGNATURE---Version: PGP 5.0 Charset: noconvyh HJRHh GJGhgg/12 Ep J+lo 8 g E4 v B3 mq Jh FEv ZP9 t 6 n 7G6 m 5 Gw 2---END PGP SIGNATURE---A PGP signed message:

第56页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第57页：Secure sockets layer (SSL)

Secure sockets layer (SSL)

• provides transport layer security to any TCP-basedapplication using SSL services.
  • e.g., between Web browsers, servers for e-commerce (shttp)
• security services:
  • server authentication, data encryption, client authentication(optional)TCPIPTCP enhanced with SSLTCPsocket Application TCPIPTCP APISSL sublayer Application SSLsocket

第58页：SSL: three phases 1. Handshake

SSL: three phases 1. Handshake

• Bob establishes TCPconnection to Alice
• authenticates Alicevia CA signed certificate
• creates, encrypts(using Alice’s publickey), sends master secret key to Alice
  • nonce exchange notshown SSL hello certificate KA+(MS)TCP SYNTCP SYNACKTCP ACKdecrypt using KA-to get MScreate Master Secret(MS)

第59页：SSL: three phases 2. Key Derivation

SSL: three phases 2. Key Derivation

• Alice, Bob use shared secret (MS) to generate 4 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
• why 4 keys?

第60页：SSL: three phases 3. Data transfer

SSL: three phases 3. Data transfer

SSL: three phases 3. Data transfer H( ). MBb 1 b 2 b 3… bndd H(d)d H(d)H( ). EBTCP byte stream block n bytes together compute MACencrypt d, MAC, SSLseq. #SSLseq. #d H(d)Type Ver Len SSL record format encrypted using EBunencrypted

第61页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第62页：IPsec 网络层安全

IPsec 网络层安全

• network-layer secrecy:
  • sending host encrypts thedata in IP datagram
  • TCP and UDP segments;ICMP and SNMPmessages.
• network-layer authentication
  • destination host canauthenticate source IPaddress
• two principal protocols:
  • authentication header(AH) protocol
  • encapsulation security payload (ESP) protocol
• for both AH and ESP, source,destination handshake:
  • create network-layer logical channel called asecurity association (SA)
• each SA unidirectional.
• uniquely determined by:
  • security protocol (AH or ESP)
  • source IP address
  • 32-bit connection ID

第63页：鉴别首部 (AH) 协议

鉴别首部 (AH) 协议

• provides source authentication, dataintegrity, noconfidentiality
• AH header inserted between IP header,data field.
• protocol field: 51
• intermediate routers process datagrams asusual AH header includes:
• connection identifier
• authentication data:source-signed message digest calculated overoriginal IP datagram.
• next header field:specifies type of data(e.g., TCP, UDP, ICMP)IP headerdata (e.g., TCP, UDP segment)AH header

第64页：ESP Protocol

ESP Protocol

• provides secrecy, host authentication, dataintegrity.
• data, ESP trailer encrypted.
• next header field is in ESPtrailer.
• ESP authentication field is similar to AHauthentication field.
• Protocol = 50. IP header TCP/UDP segment ESPheader ESPtrailer ESPauthent.encrypted authenticated

第65页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第66页：IEEE 802.11 security

IEEE 802.11 security

• war-driving:drive around Bay area, see what 802.11 networks available?
  • 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 802.11 security: Wired Equivalent Privacy (WEP): a failure
  • current attempt: 802.11 i

第67页：Wired Equivalent Privacy (WEP)

Wired Equivalent Privacy (WEP)

• authentication as in protocolap 4.0
  • 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

第68页：WEP 数据加密

WEP 数据加密

• host/AP share 40 bit symmetric key (semi-permanent)
• host appends 24-bit initialization vector (IV) tocreate 64-bit key
• 64 bit key used to generate stream of keys, ki IV
• ki IVused to encrypt ith byte, di, in frame:ci=di XORki IV
• IV and encrypted bytes, cisent in frame

第69页：11 WEP encryption Sender-side

11 WEP encryption Sender-side

.11 WEP encryption Sender-side WEP encryption

第70页：WEP 加密机制详解完整性校验和，32 bits 初始化向量，24 bits 公共密

WEP 加密机制详解完整性校验和，32 bits 初始化向量，24 bits 公共密

WEP加密机制详解完整性校验和，32 bits初始化向量，24 bits公共密钥，40 bits WEP data encryption

第71页：破解802.11 WEP加密

破解802.11 WEP加密

• 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 2 d 3 d 4…
  • Trudy sees: ci=di XORki IV
  • Trudy knows cidi, so can computeki IV
  • Trudy knows encrypting key sequence k 1IVk 2IVk 3IV…
  • Next time IV is used, Trudy can decrypt!

第72页：11 i: improved security

11 i: improved security

• numerous (stronger) forms of encryption possible
• provides key distribution
• uses authentication server separate fromaccess point

第73页：AP: access point AS:Authentication

AP: access point AS:Authentication

AP: access point AS:Authentication server wired network STA:client station 1 Discovery ofsecurity capabilities 3STA and AS mutually authenticate, together generate Master Key (MK). AP servers as “pass through”23STA derives Pairwise Master Key (PMK)AS derivessame PMK,sends to AP4STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity 802.11 i: four phases of operation

第74页：wired network EAP TLS

wired network EAP TLS

• 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)

第75页：第8章路线图

第8章路线图

Chapter 8 roadmap 8.1 What is network security?8.2 Principles of cryptography 8.3 Message integrity 8.4 End point authentication 8.5 Securing e-mail 8.6 Securing TCP connections: SSL8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS

第76页：Firewalls isolates organization’s

Firewalls isolates organization’s

Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blockingothers.firewall administered network public Internet firewall

第77页：防火墙概述

防火墙概述

• SYN flooding: attacker establishes many bogus TCPconnections, no resources left for “real” connections prevent illegal modification/access of internal data.
• e.g., attacker replaces CIA’s homepage withsomething elseallow only authorized access to inside network(set ofauthenticated users/hosts)three types of firewalls:
• stateless packet filters
• stateful packet filters
• application gateways

第78页：无状态分组 filter

无状态分组 filter

• internal network connected to Internet viarouter firewall
• router filters packet-by-packet,decision toforward/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 arrivingpacket be allowedin? Departing packetlet out?

第79页：Stateless packet filteri

Stateless packet filteri

• example 1: block incoming and outgoing datagrams with IP protocol field = 17 and witheither 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 TCPconnections with internal clients, but allows internal clients to connect to outside.

第80页：Firewall Setting Policy Drop all outgo

Firewall Setting Policy Drop all outgo

Firewall Setting Policy Drop all outgoing packets to any IPaddress, port 80 No outside Web access. Drop all incoming TCP SYN packets toany IP except 130.207.244.203, port 80 No incoming TCP connections,except those for institution’spublic Web server only. Drop all incoming UDP packets-except DNS and router broadcasts. Prevent Web-radios from eatingup the available bandwidth. Drop all ICMP packets going to a“broadcast” address (eg 130.207.255.255). Prevent your network from beingused for a smurf Do S attack. Drop all outgoing ICMP TTL expired traffic Prevent your network from being trace routed Stateless packet filtering: more examples

第81页：访问控制列表示例

访问控制列表示例

• ACL:table of rules, applied top to bottom toincoming packets: (action, condition) pairs

第82页：有状态分组过滤

有状态分组过滤

• 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:flagbitdestportsourceportprotocoldestaddresssourceaddressaction ACK> 102380TCP222.22/16 outside of 222.22/16 allow
• stateful packet filter:track status of every TCPconnection
  • track connection setup (SYN), teardown (FIN): candetermine whether incoming, outgoing packets “makes sense”
  • timeout inactive connections at firewall: no longer admit packets

第83页：有状态分组过滤（续）

有状态分组过滤（续）

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

第84页：Application gateways

Application gateways

• filters packets onapplication data as wellas on IP/TCP/UDP fields.
• example:allow select internal users to telnet outside.host-to-gateway telnet session gateway-to-remotehost telnet session application gateway router and filter 1.require all telnet users to telnet through gateway.2.for authorized users, gateway sets up telnet connection todest host. Gateway relays data between 2 connections 3.router filter blocks all telnet connections not originatingfrom gateway.

第85页：Limitations firewalls and gateways

Limitations firewalls and gateways

• IP spoofing:routercan’t know if data“really” comes fromclaimed source
• if multiple app’s. needspecial treatment, eachhas own app. gateway.
• client software mustknow how to contact gateway.
  • e.g., must set IP addressof proxy in Webbrowser
• filters often use all ornothing policy for UDP.
• tradeoff:degree ofcommunication withoutside world, level ofsecurity
• many highly protectedsites still suffer fromattacks.

第86页：Intrusion detection systems

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
• Do S attack

第87页：防火墙与入侵检测（示意图）

防火墙与入侵检测（示意图）

• multiple IDSs: different types of checkingat different locations

第88页：Network Security (summary)Basic

Network Security (summary)Basic

• 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