חישוב ציון תרגילי בית: מי שלא יגיש את הפרויקט בזמן (PA3): ממוצע של כל 7 תרגילי הבית מי שיגיש את הפרויקט בזמן (PA3): ממוצע של 6 תרגילי בית: הפרויקט (PA3) חמשת הטובים (מתוך ששת) התרגילים האחרים משקל כל תרגיל (מתוך השישה) יהיה זהה (1/6) הערה: חייבים להקפיד על הגשה בזמן!

Chapter 7: Network security January 13, 2010 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 Lecture 13: Network Security

What is network security? 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 Lecture 13: Network Security

Internet security threats January 13, 2010 Internet security threats Packet 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 C src:B dest:A payload B Lecture 13: Network Security

Internet security threats January 13, 2010 Internet security threats IP 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 C src:B dest:A payload B Lecture 13: Network Security

Internet security threats January 13, 2010 Internet security threats Denial 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 C SYN SYN SYN SYN SYN B SYN SYN Lecture 13: Network Security

Cryptography Principles January 13, 2010 Cryptography Principles Lecture 13: Network Security

Friends and enemies: Alice, Bob, Trudy 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 Alice Bob channel data, control messages secure sender secure receiver data data Trudy Lecture 13: Network Security

Who might Bob, Alice be? … well, real-life Bobs and Alices! 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? Lecture 13: Network Security

The language of cryptography January 13, 2010 The language of cryptography Alice’s encryption key Bob’s decryption key K A K B encryption algorithm plaintext ciphertext decryption algorithm plaintext symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) Lecture 13: Network Security

Symmetric Key Cryptography January 13, 2010 Symmetric Key Cryptography Lecture 13: Network Security

Symmetric key cryptography January 13, 2010 Symmetric key cryptography substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq E.g.: Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard to break this simple cipher?: brute force (how hard?) other? Lecture 13: Network Security

Perfect cipher [Shannon 1948] 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) Lecture 13: Network Security

Symmetric key crypto: DES 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 Advanced Encryption Standard (AES) Lecture 13: Network Security

Symmetric key crypto: DES January 13, 2010 Symmetric key crypto: DES DES operation initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation Lecture 13: Network Security

Block Cipher chaining How do we encode a large message Encoding: 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 Lecture 13: Network Security

Cipher Block Chaining Mode Cipher block chaining. (a) Encryption. (b) Decryption. 7: Network Security 17

Diffie-Hellman key exchange protocol January 13, 2010 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) Lecture 13: Network Security

Diffie-Hellman Key Exchange January 13, 2010 Diffie-Hellman Key Exchange Alice picks large x, Bob picks large y (e.g., 512 bits) Lecture 13: Network Security

Man in the middle attack 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? Lecture 13: Network Security

Exponentiation Compute gx mod n Expg,n (x) Assume x = 2y + b January 13, 2010 Exponentiation Compute gx mod n Expg,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 Lecture 13: Network Security

Public Key Cryptography January 13, 2010 Public Key Cryptography Lecture 13: Network Security

Public Key Cryptography 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) Lecture 13: Network Security

Public key cryptography January 13, 2010 Public key cryptography + Bob’s public key K B - Bob’s private key K B plaintext message, m encryption algorithm ciphertext decryption algorithm plaintext message K (m) B + m = K (K (m)) B + - Lecture 13: Network Security

Public key encryption algorithms January 13, 2010 Public key encryption algorithms Two inter-related requirements: . . 1 need d ( ) and e ( ) such that B B d (e (m)) = m B 2 need public and private keys for d ( ) and e ( ) . B RSA: Rivest, Shamir, Adelson algorithm Lecture 13: Network Security

RSA: Choosing keys 1. Choose two large prime numbers p, q. 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 - Lecture 13: Network Security

RSA: Encryption, decryption 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) 2. To decrypt received bit pattern, c, compute m = c mod n d d (i.e., remainder when c is divided by n) m = (m mod n) e mod n d Magic happens! Lecture 13: Network Security

Bob chooses p=5, q=7. Then n=35, z=24. 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). e c = m mod n e letter m m encrypt: l 12 1524832 17 c d m = c mod n d c letter decrypt: 17 12 481968572106750915091411825223072000 l Lecture 13: Network Security

RSA: Why x y mod n = x (y mod (p-1)(q-1) ) mod n m = (m mod n) e mod n January 13, 2010 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 Always there is a solution, and satisfies (Chinese Remainder) Lecture 13: Network Security

modified Diffie-Hellman Key Exchange 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) Lecture 13: Network Security

January 13, 2010 Authentication Lecture 13: Network Security

Protocol ap1.0: Alice says “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” “I am Alice” Failure scenario?? Lecture 13: Network Security

Authentication Goal: Bob wants Alice to “prove” her identity to him 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” Lecture 13: Network Security

Authentication: another try January 13, 2010 Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address “I am Alice” Alice’s IP address Failure scenario?? Lecture 13: Network Security

Authentication: another try January 13, 2010 Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Trudy can create a packet “spoofing” Alice’s address “I am Alice” Alice’s IP address Lecture 13: Network Security

Authentication: another try January 13, 2010 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. “I’m Alice” Alice’s IP addr password Failure scenario?? OK Alice’s IP addr Lecture 13: Network Security

Authentication: another try January 13, 2010 Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Alice’s IP addr Alice’s password “I’m Alice” playback attack: Trudy records Alice’s packet and later plays it back to Bob OK Alice’s IP addr “I’m Alice” Alice’s IP addr password Lecture 13: Network Security

Authentication: yet another try January 13, 2010 Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. “I’m Alice” Alice’s IP addr encrypted password Failure scenario?? OK Alice’s IP addr Lecture 13: Network Security

Authentication: another try January 13, 2010 Authentication: another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Alice’s IP addr encrypted password “I’m Alice” record and playback still works! OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password Lecture 13: Network Security

Authentication: yet another try January 13, 2010 Authentication: yet another try Goal: avoid playback attack 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! Failures, drawbacks? Lecture 13: Network Security

“send me your public key” 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” Bob computes R (K (R)) = R A - K + K (R) A - and knows only Alice could have the private key, that encrypted R such that “send me your public key” K A + (K (R)) = R A - K + Lecture 13: Network Security

sends m to Alice encrypted with Alice’s public key January 13, 2010 ap5.0: 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 R T K (R) - R A K (R) - Send me your public key T K + Send me your public key A K + T K (m) + Trudy gets T m = K (K (m)) + - A K (m) + sends m to Alice encrypted with Alice’s public key A m = K (K (m)) + - Lecture 13: Network Security

January 13, 2010 ap5.0: security hole Man (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! Lecture 13: Network Security

Message Integrity (Signatures etc) January 13, 2010 Lecture 13: Network Security

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. Lecture 13: Network Security

Digital Signatures (more) January 13, 2010 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. Lecture 13: Network Security

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’). Lecture 13: Network Security

Hash Function Algorithms January 13, 2010 Hash Function Algorithms 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 Internet checksum would make a poor message digest. Too easy to find two messages with same checksum. Lecture 13: Network Security

Digital signature = signed message digest January 13, 2010 Alice verifies signature and integrity of digitally signed message: Bob sends digitally signed message: large message m H: Hash function KB(H(m)) - encrypted msg digest H(m) digital signature (encrypt) Bob’s private key large message m K B - Bob’s public key digital signature (decrypt) K B + KB(H(m)) - encrypted msg digest H: Hash function + H(m) H(m) equal ? Lecture 13: Network Security

Key Distribution Centers January 13, 2010 Key Distribution Centers Lecture 13: Network Security

Problems with Public-Key Encryption January 13, 2010 Problems with Public-Key Encryption A way for Trudy to subvert public-key encryption. Lecture 13: Network Security

Trusted Intermediaries 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) Lecture 13: Network Security

Key Distribution Center (KDC) 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. Lecture 13: Network Security

Certification Authorities 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 Lecture 13: Network Security

X.509 is the standard for certificates January 13, 2010 X.509 is the standard for certificates The basic fields of an X.509 certificate. Lecture 13: Network Security

Public-Key Infrastructures January 13, 2010 Public-Key Infrastructures PKIs are a way to structure certificates (a) A hierarchical PKI. (b) A chain of certificates. Lecture 13: Network Security

Example revisited (solved with 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! Lecture 13: Network Security

January 13, 2010 Secure email Lecture 13: Network Security

January 13, 2010 Secure e-mail Alice wants to send confidential e-mail, m, to Bob. KS( ) . KB( ) + - KS(m ) KB(KS ) m KS KB Internet 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. Lecture 13: Network Security

January 13, 2010 Secure e-mail Alice wants to send confidential e-mail, m, to Bob. KS( ) . KB( ) + - KS(m ) KB(KS ) m KS KB Internet Bob: uses his private key to decrypt and recover KS uses KS to decrypt KS(m) to recover m Lecture 13: Network Security

Secure e-mail (continued) January 13, 2010 Secure e-mail (continued) Alice wants to provide sender authentication message integrity. H( ) . KA( ) - + H(m ) KA(H(m)) m KA Internet compare Alice digitally signs message. sends both message (in the clear) and digital signature. Lecture 13: Network Security

Pretty good privacy (PGP) 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. A PGP signed message: ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- Lecture 13: Network Security

Secure Socket layer (SSl) January 13, 2010 Secure Socket layer (SSl) Lecture 13: Network Security

Secure sockets layer (SSL) January 13, 2010 Secure sockets layer (SSL) 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. 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) Lecture 13: Network Security

Tools  Internet options  Content  Certificates January 13, 2010 Internet Explorer: Tools  Internet options  Content  Certificates Lecture 13: Network Security

Internet Explorer: Error Message January 13, 2010 Internet Explorer: Error Message Lecture 13: Network Security

SSL (continued) Encrypted SSL session: January 13, 2010 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. Lecture 13: Network Security

January 13, 2010 SSL basics Lecture 13: Network Security

Access Control in the Network January 13, 2010 Access Control in the Network Lecture 13: Network Security

Firewalls Two firewall types: packet filter application gateways January 13, 2010 Firewalls 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. firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. Two firewall types: packet filter application gateways Lecture 13: Network Security

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. Lecture 13: Network Security

Virtual Private Networks 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. Lecture 13: Network Security

January 13, 2010 Application gateways host-to-gateway telnet session gateway-to-remote host telnet session application gateway router and filter Filters packets on application data as well as on IP/TCP/UDP fields. Example: allow select internal users to telnet outside. 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. Lecture 13: Network Security

Limitations of firewalls and gateways 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. Lecture 13: Network Security

Network Security (summary) 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 Lecture 13: Network Security