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1、,1,网络信息安全原理简介,2,Outline,Foundations:what is security?cryptographyauthenticationmessage integritykey distribution and certificationSecurity in practice:application layer:secure e-mailtransport layer:Internet commerce,SSL,SETnetwork layer:IP security,3,Friends and enemies:Alice,Bob,Trudy,well-known in
2、 network security worldBob,Alice(lovers!)want to communicate“securely”Trudy,the“intruder”may intercept,delete,add messages,Figure 7.1 goes here,4,What is network security?,Secrecy:only sender,intended receiver should“understand”msg contentssender encrypts msgreceiver decrypts msgAuthentication:sende
3、r,receiver want to confirm identity of each other Message Integrity:sender,receiver want to ensure message not altered(in transit,or afterwards)without detection,5,Internet security threats,Packet sniffing:broadcast mediapromiscuous NIC reads all packets passing bycan read all unencrypted data(e.g.p
4、asswords)e.g.:C sniffs Bs packets,6,Internet security threats,IP Spoofing:can generate“raw”IP packets directly from application,putting any value into IP source address fieldreceiver cant tell if source is spoofede.g.:C pretends to be B,A,B,C,7,Internet security threats,Denial of service(DOS):flood
5、of maliciously generated packets“swamp”receiverDistributed DOS(DDOS):multiple coordinated sources swamp receivere.g.,C and remote host SYN-attack A,A,B,C,8,The language of cryptography,symmetric key crypto:sender,receiver keys identicalpublic-key crypto:encrypt key public,decrypt key secret,Figure 7
6、.3 goes here,plaintext,plaintext,ciphertext,9,Symmetric key cryptography,substitution cipher:substituting one thing for anothermonoalphabetic cipher:substitute one letter for another,plaintext:abcdefghijklmnopqrstuvwxyz,ciphertext:mnbvcxzasdfghjklpoiuytrewq,Plaintext:bob.i love you.alice,ciphertext:
7、nkn.s gktc wky.mgsbc,E.g.:,Q:How hard to break this simple cipher?:brute force(how hard?)other?,10,Symmetric key crypto:DES,DES:Data Encryption StandardUS encryption standard NIST 199356-bit symmetric key,64 bit plaintext inputHow secure is DES?DES Challenge:56-bit-key-encrypted phrase(“Strong crypt
8、ography makes the world a safer place”)decrypted(brute force)in 4 monthsno known“backdoor”decryption approachmaking DES more secureuse three keys sequentially(3-DES)on each datumuse cipher-block chaining,11,Symmetric key crypto:DES,initial permutation 16 identical“rounds”of function application,each
9、 using different 48 bits of keyfinal permutation,12,Public Key Cryptography,symmetric key cryptorequires sender,receiver know shared secret keyQ:how to agree on key in first place(particularly if never“met”)?,public key cryptographyradically different approach Diffie-Hellman76,RSA78sender,receiver d
10、o not share secret keyencryption key public(known to all)decryption key private(known only to receiver),13,Public key cryptography,Figure 7.7 goes here,14,Public key encryption algorithms,need d()and e()such that,B,B,.,.,Two inter-related requirements:,RSA:Rivest,Shamir,Adelson algorithm,15,RSA:Choo
11、sing 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 en)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)
12、.,16,RSA:Encryption,decryption,0.Given(n,e)and(n,d)as computed above,2.To decrypt received bit pattern,c,compute,(i.e.,remainder when c is divided by n),d,17,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,m,e,l,12,1524832,1
13、7,c,17,481968572106750915091411825223072000,12,letter,l,encrypt:,decrypt:,18,Authentication,Goal:Bob wants Alice to“prove”her identity to him,Protocol ap1.0:Alice says“I am Alice”,Failure scenario?,19,Authentication:another try,Protocol ap2.0:Alice says“I am Alice”and sends her IP address along to“p
14、rove”it.,Failure scenario?,20,Authentication:another try,Protocol ap3.0:Alice says“I am Alice”and sends her secret password to“prove”it.,Failure scenario?,21,Authentication:yet another try,Protocol ap3.1:Alice says“I am Alice”and sends her encrypted secret password to“prove”it.,Failure scenario?,I a
15、m Aliceencrypt(password),22,Authentication:yet another try,Goal:avoid playback attack,Failures,drawbacks?,Figure 7.11 goes here,Nonce:number(R)used onlyonce in a lifetime,ap4.0:to prove Alice“live”,Bob sends Alice nonce,R.Alicemust return R,encrypted with shared secret key,23,Figure 7.12 goes here,A
16、uthentication:ap5.0,ap4.0 requires shared symmetric keyproblem:how do Bob,Alice agree on keycan we authenticate using public key techniques?ap5.0:use nonce,public key cryptography,24,Figure 7.14 goes here,ap5.0:security hole,Man(woman)in the middle attack:Trudy poses as Alice(to Bob)and as Bob(to Al
17、ice),Need“certified”public keys(more later),25,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.,Si
18、mple digital signature for message m:Bob encrypts m with his private key dB,creating signed message,dB(m).Bob sends m and dB(m)to Alice.,26,Digital Signatures(more),Suppose Alice receives msg m,and digital signature dB(m)Alice verifies m signed by Bob by applying Bobs public key eB to dB(m)then chec
19、ks eB(dB(m)=m.If eB(dB(m)=m,whoever signed m must have used Bobs 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.,27,Message Digests,Computationally expensive to pu
20、blic-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-1Produces fixed-size msg digest(fingerprint)Given message digest x,computationally infeasible to find m such
21、 that x=H(m)computationally infeasible to find any two messages m and m such that H(m)=H(m).,28,Digital signature=Signed message digest,Bob sends digitally signed message:,Alice verifies signature and integrity of digitally signed message:,29,Hash Function Algorithms,Internet checksum would make a p
22、oor 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 standard160-bit message digest,30,Tr
23、usted 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 Bobs public key(from web site,e-mail,diskette),how does she know it is Bobs public key,not Trudys?
24、Solution:trusted certification authority(CA),31,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 R
25、1,and KB-KDC(A,R1)Alice sends Bob KB-KDC(A,R1),Bob extracts R1Alice,Bob now share the symmetric key R1.,32,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.
26、CA creates certificate binding entity to public key.Certificate digitally signed by CA.,When Alice wants Bobs public key:gets Bobs certificate(Bob or elsewhere).Apply CAs public key to Bobs certificate,get Bobs public key,33,Secure e-mail,generates random symmetric private key,KS.encrypts message wi
27、th KS also encrypts KS with Bobs public key.sends both KS(m)and eB(KS)to Bob.,Alice wants to send secret e-mail message,m,to Bob.,34,Secure e-mail(continued),Alice wants to provide sender authentication message integrity.,Alice digitally signs message.sends both message(in the clear)and digital sign
28、ature.,35,Secure e-mail(continued),Alice wants to provide secrecy,sender authentication,message integrity.,Note:Alice uses both her private key,Bobs public key.,36,Pretty good privacy(PGP),Internet e-mail encryption scheme,a de-facto standard.Uses symmetric key cryptography,public key cryptography,h
29、ash 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:SHA1Bob:My husband is out of town tonight.Passionately yours,Alice-BEGIN PGP SIGNATURE-Version:PGP 5.0Ch
30、arset:noconvyhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2-END PGP SIGNATURE-,A PGP signed message:,37,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,s
31、ervers for I-commerce(shttp).SSL security services:server authenticationdata 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 CAs public key to extract serve
32、rs public key from certificate.Visit your browsers security menu to see its trusted CAs.,38,SSL(continued),Encrypted SSL session:Browser generates symmetric session key,encrypts it with servers public key,sends encrypted key to server.Using its private key,server decrypts session key.Browser,server
33、agree that future msgs will be encrypted.All data sent into TCP socket(by client or server)i 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.,39,Ipsec:Network Lay
34、er Security,Network-layer secrecy:sending host encrypts the data in IP datagramTCP and UDP segments;ICMP and SNMP messages.Network-layer authenticationdestination host can authenticate source IP addressTwo principle protocols:authentication header(AH)protocolencapsulation security payload(ESP)protoc
35、ol,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 address32-bit connection ID,40,ESP Protocol,Provides secrecy,host authentication,data integrity.D
36、ata,ESP trailer encrypted.Next header field is in ESP trailer.,ESP authentication field is similar to AH authentication field.Protocol=50.,41,Authentication Header(AH)Protocol,Provides source host authentication,data integrity,but not secrecy.AH header inserted between IP header and IP data field.Pr
37、otocol field=51.Intermediate routers process datagrams as usual.,AH header includes:connection identifierauthentication 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.),42,Network Security(summary),Basic techniques.cryptography(symmetric and public)authenticationmessage integrity.used in many different security scenariossecure emailsecure transport(SSL)IP sec,See also:firewalls,in network management,
