Security 14-Oct-15CPSC558 Advanced Computer Networks Chapter 8: Network Security Chapter goals: understand principles of network security: –cryptography and its many uses beyond “confidentiality” –authentication –message integrity –key distribution security in practice: –firewalls –security in application, transport, network, link layers

Security 14-Oct-15CPSC558 Advanced Computer Networks Roadmap What is network security? Principles of cryptography Authentication Integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks 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

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 channel data, control messages data Alice Bob Trudy

Security 14-Oct-15CPSC558 Advanced Computer Networks 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

Security 14-Oct-15CPSC558 Advanced Computer Networks There are bad guys (and girls) out there! Q: What can a “bad guy” do? A: a lot! –intercept messages –actively insert messages into connection –impersonation: can fake (spoof) source address in packet (or any field in packet) –hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place –denial of service: prevent service from being used by others (e.g., by overloading resources) more on this later ……

Security 14-Oct-15CPSC558 Advanced Computer Networks roadmap What is network security? Principles of cryptography Authentication Integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B

Security 14-Oct-15CPSC558 Advanced Computer Networks 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. alice ciphertext: nkn. s gktc wky. mgsbc E.g.: Q: How hard to break this simple cipher?:  brute force  other?

Security 14-Oct-15CPSC558 Advanced Computer Networks Symmetric key cryptography symmetric key crypto: Bob and Alice share know same (symmetric) key: K e.g., key is knowing substitution pattern in mono alphabetic substitution cipher Q: how do Bob and Alice agree on key value? plaintext ciphertext K A-B encryption algorithm decryption algorithm A-B K plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B

Security 14-Oct-15CPSC558 Advanced Computer Networks 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

Security 14-Oct-15CPSC558 Advanced Computer Networks Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation

Security 14-Oct-15CPSC558 Advanced Computer Networks AES: Advanced Encryption Standard new (Nov. 2001) symmetric-key NIST standard, 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 trillion years for AES

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 public encryption key known to all private decryption key known only to receiver

Security 14-Oct-15CPSC558 Advanced Computer Networks Public key cryptography plaintext message, m ciphertext encryption algorithm decryption algorithm Bob’s public key plaintext message K (m) B + K B + Bob’s private key K B - m = K ( K (m) ) B + B -

Security 14-Oct-15CPSC558 Advanced Computer Networks Public key encryption algorithms need K ( ) and K ( ) such that B B.. given public key K, it should be impossible to compute private key K B B Requirements: 1 2 RSA: Rivest, Shamir, Adelson algorithm + - K (K (m)) = m B B

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 -

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 d Magic happens! c

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 c = m mod n e l c m = c mod n d c d letter l encrypt: decrypt:

Security 14-Oct-15CPSC558 Advanced Computer Networks RSA: Why is that m = (m mod n) e mod n d (m mod n) e mod n = m mod n d ed Useful number theory result: If p,q prime and n = pq, then: x mod n = x mod n yy mod (p-1)(q-1) = m mod n ed mod (p-1)(q-1) = m mod n 1 = m (using number theory result above) (since we chose ed to be divisible by (p-1)(q-1) with remainder 1 )

Security 14-Oct-15CPSC558 Advanced Computer Networks RSA: another important property The following property will be very useful later: K ( K (m) ) = m B B - + K ( K (m) ) B B + - = use public key first, followed by private key use private key first, followed by public key Result is the same!

Security 14-Oct-15CPSC558 Advanced Computer Networks roadmap What is network security? Principles of cryptography Authentication Integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” Failure scenario?? “I am Alice”

Security 14-Oct-15CPSC558 Advanced Computer Networks 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”

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario?? “I am Alice” Alice’s IP address

Security 14-Oct-15CPSC558 Advanced Computer Networks 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

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication: another try 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 OK Alice’s IP addr

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. playback attack: Trudy records Alice’s packet and later plays it back to Bob “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I’m Alice” Alice’s IP addr Alice’s password

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication: another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. record and playback still works! “I’m Alice” Alice’s IP addr encryppted password OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password

Security 14-Oct-15CPSC558 Advanced Computer Networks 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!

Security 14-Oct-15CPSC558 Advanced Computer Networks 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” R Bob computes K (R) A - “send me your public key” K A + (K (R)) = R A - K A + and knows only Alice could have the private key, that encrypted R such that (K (R)) = R A - K A + Is K A + really Alice’s public key?

Security 14-Oct-15CPSC558 Advanced Computer Networks ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) I am Alice R T K (R) - Send me your public key T K + A K (R) - Send me your public key A K + T K (m) + T m = K (K (m)) + T - Trudy gets sends m to Alice ennrypted with Alice’s public key A K (m) + A m = K (K (m)) + A - R

Security 14-Oct-15CPSC558 Advanced Computer Networks 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!

Security 14-Oct-15CPSC558 Advanced Computer Networks Chapter 7 roadmap What is network security? Principles of cryptography Authentication Message integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 prove to someone that Bob, and no one else (including Alice), must have signed document

Security 14-Oct-15CPSC558 Advanced Computer Networks Digital Signatures Simple digital signature for message m: Bob signs m by encrypting with his private key K B, creating “signed” message, K B (m) - - Dear Alice Oh, how I have missed you. I think of you all the time! …(blah blah blah) Bob Bob’s message, m Public key encryption algorithm Bob’s private key K B - Bob’s message, m, signed (encrypted) with his private key K B - (m)

Security 14-Oct-15CPSC558 Advanced Computer Networks Digital Signatures (more) Suppose Alice receives msg m, digital signature K B (m) Alice verifies m signed by Bob by applying Bob’s public key K B to K B (m) then checks K B (K B (m) ) = m. If K B (K B (m) ) = m, whoever signed m must have used Bob’s private key Alice thus verifies that: üBob signed m. üNo one else signed m. üBob signed m and not m’. Non-repudiation: Alice can take m, and signature K B (m) to court and prove that Bob signed m. -

Security 14-Oct-15CPSC558 Advanced Computer Networks Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easy- to- compute digital “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) large message m H: Hash Function H(m)

Security 14-Oct-15CPSC558 Advanced Computer Networks Internet checksum: poor crypto hash function Internet checksum has some properties of hash function: üproduces fixed length digest (16-bit sum) of message üis many-to-one But given message with given hash value, it is easy to find another message with same hash value: I O U B O B 49 4F E D2 42 message ASCII format B2 C1 D2 AC I O U B O B 49 4F E D2 42 message ASCII format B2 C1 D2 AC different messages but identical checksums!

Security 14-Oct-15CPSC558 Advanced Computer Networks large message m H: Hash function H(m) digital signature (encrypt) Bob’s private key K B - + Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: K B (H(m)) - encrypted msg digest K B (H(m)) - encrypted msg digest large message m H: Hash function H(m) digital signature (decrypt) H(m) Bob’s public key K B + equal ? Digital signature = signed message digest

Security 14-Oct-15CPSC558 Advanced Computer Networks Hash Function Algorithms MD5 hash function widely used (RFC 1321) –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 [ NIST, FIPS PUB 180-1] –160-bit message digest

Security 14-Oct-15CPSC558 Advanced Computer Networks Chapter 7 roadmap What is network security? Principles of cryptography Authentication Integrity Key distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks Trusted Intermediaries Symmetric key problem: How do two entities establish shared secret key over network? Solution: trusted key distribution center (KDC) acting as intermediary between entities Public key problem: When Alice obtains Bob’s public key (from web site, , diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: trusted certification authority (CA)

Security 14-Oct-15CPSC558 Advanced Computer Networks Key Distribution Center (KDC) Alice, Bob need shared symmetric key. KDC: server shares different secret key with each registered user (many users) Alice, Bob know own symmetric keys, K A-KDC K B-KDC, for communicating with KDC. K B-KDC K X-KDC K Y-KDC K Z-KDC K P-KDC K B-KDC K A-KDC K P-KDC KDC

Security 14-Oct-15CPSC558 Advanced Computer Networks Key Distribution Center (KDC) Alice knows R1 Bob knows to use R1 to communicate with Alice Alice and Bob communicate: using R1 as session key for shared symmetric encryption Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R1 K B-KDC (A,R1) K A-KDC (A,B) K A-KDC (R1, K B-KDC (A,R1) )

Security 14-Oct-15CPSC558 Advanced Computer Networks Certification Authorities Certification authority (CA): binds public key to particular entity, E. E (person, router) 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’s public key K B + Bob’s identifying information digital signature (encrypt) CA private key K CA - K B + certificate for Bob’s public key, signed by CA

Security 14-Oct-15CPSC558 Advanced Computer Networks Certification Authorities 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’s public key K B + digital signature (decrypt) CA public key K CA + K B +

Security 14-Oct-15CPSC558 Advanced Computer Networks A certificate contains: Serial number (unique to issuer) info about certificate owner, including algorithm and key value itself (not shown) info about certificate issuer valid dates digital signature by issuer

Security 14-Oct-15CPSC558 Advanced Computer Networks Chapter 7 roadmap What is network security? Principles of cryptography Authentication Integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks Firewalls isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. firewall

Security 14-Oct-15CPSC558 Advanced Computer Networks Firewalls: Why prevent denial of service attacks: –SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections. prevent illegal modification/access of internal data. –e.g., attacker replaces CIA’s homepage with something else allow only authorized access to inside network (set of authenticated users/hosts) two types of firewalls: –application-level –packet-filtering

Security 14-Oct-15CPSC558 Advanced Computer Networks Packet Filtering internal network connected to Internet via router firewall router filters packet-by-packet, decision to forward/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 arriving packet be allowed in? Departing packet let out?

Security 14-Oct-15CPSC558 Advanced Computer Networks Packet Filtering 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.

Security 14-Oct-15CPSC558 Advanced Computer Networks 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-gateway telnet session gateway-to-remote host 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 to dest host. Gateway relays data between 2 connections 3. Router filter blocks all telnet connections not originating from gateway.

Security 14-Oct-15CPSC558 Advanced Computer Networks 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.

Security 14-Oct-15CPSC558 Advanced Computer Networks Chapter 7 roadmap What is network security? Principles of cryptography Authentication Integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers

Security 14-Oct-15CPSC558 Advanced Computer Networks Internet security threats Mapping: –before attacking: “case the joint” – find out what services are implemented on network –Use ping to determine what hosts have addresses on network –Port-scanning: try to establish TCP connection to each port in sequence (see what happens) –nmap ( mapper: “network exploration and security auditing” Countermeasures?

Security 14-Oct-15CPSC558 Advanced Computer Networks Internet security threats Mapping: countermeasures –record traffic entering network –look for suspicious activity (IP addresses, pots being scanned sequentially)

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 B C src:B dest:A payload Countermeasures?

Security 14-Oct-15CPSC558 Advanced Computer Networks Internet security threats Packet sniffing: countermeasures –all hosts in orgnization run software that checks periodically if host interface in promiscuous mode. –one host per segment of broadcast media (switched Ethernet at hub) A B C src:B dest:A payload

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 B C src:B dest:A payload Countermeasures?

Security 14-Oct-15CPSC558 Advanced Computer Networks Internet security threats IP Spoofing: ingress filtering –routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network) –great, but ingress filtering can not be mandated for all networks A B C src:B dest:A payload

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 B C SYN Countermeasures?

Security 14-Oct-15CPSC558 Advanced Computer Networks Internet security threats Denial of service (DOS): countermeasures –filter out flooded packets (e.g., SYN) before reaaching host: throw out good with bad –traceback to source of floods (most likely an innocent, compromised machine) A B C SYN

Security 14-Oct-15CPSC558 Advanced Computer Networks Chapter 7 roadmap What is network security? Principles of cryptography Authentication Integrity Key Distribution and certification Access control: firewalls Attacks and counter measures Security in many layers a. Secure b. Secure sockets c. IPsec d WEP

Security 14-Oct-15CPSC558 Advanced Computer Networks Secure Alice:  generates random symmetric private key, K S.  encrypts message with K S (for efficiency)  also encrypts K S with Bob’s public key.  sends both K S (m) and K B (K S ) to Bob.  Alice wants to send confidential , m, to Bob. K S ( ). K B ( ) K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +

Security 14-Oct-15CPSC558 Advanced Computer Networks Secure Bob:  uses his private key to decrypt and recover K S  uses K S to decrypt K S (m) to recover m  Alice wants to send confidential , m, to Bob. K S ( ). K B ( ) K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +

Security 14-Oct-15CPSC558 Advanced Computer Networks Secure (continued) Alice wants to provide sender authentication message integrity. Alice digitally signs message. sends both message (in the clear) and digital signature. H( ). K A ( ) H(m ) K A (H(m)) - m KAKA - Internet m K A ( ). + KAKA + K A (H(m)) - m H( ). H(m ) compare

Security 14-Oct-15CPSC558 Advanced Computer Networks Secure (continued) Alice wants to provide secrecy, sender authentication, message integrity. Alice uses three keys: her private key, Bob’s public key, newly created symmetric key H( ). K A ( ). - + K A (H(m)) - m KAKA - m K S ( ). K B ( ). + + K B (K S ) + KSKS KBKB + Internet KSKS

Security 14-Oct-15CPSC558 Advanced Computer Networks Pretty good privacy (PGP) Internet encryption scheme, 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 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ hFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- A PGP signed message:

Security 14-Oct-15CPSC558 Advanced Computer Networks Secure sockets layer (SSL) transport layer security to any TCP-based app using SSL services. used between Web browsers, servers for e- commerce (shttp). 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. check your browser’s security menu to see its trusted CAs.

Security 14-Oct-15CPSC558 Advanced Computer Networks SSL (continued) Encrypted SSL session: Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. Using private key, server decrypts session key. Browser, server know session key –All data sent into TCP socket (by client or server) 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.

Security 14-Oct-15CPSC558 Advanced Computer Networks 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 security association (SA) Each SA unidirectional. Uniquely determined by: –security protocol (AH or ESP) –source IP address –32-bit connection ID

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication Header (AH) Protocol provides source authentication, data integrity, no confidentiality AH header inserted between IP header, data field. protocol field: 51 intermediate routers process datagrams as usual AH header includes: connection identifier authentication data: source- signed message digest calculated over original IP datagram. next header field: specifies type of data (e.g., TCP, UDP, ICMP) IP headerdata (e.g., TCP, UDP segment) AH header

Security 14-Oct-15CPSC558 Advanced Computer Networks 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. IP header TCP/UDP segment ESP header ESP trailer ESP authent. encrypted authenticated

Security 14-Oct-15CPSC558 Advanced Computer Networks IEEE security War-driving: drive around Bay area, see what networks available? –More than 9000 accessible from public roadways –85% use no encryption/authentication –packet-sniffing and various attacks easy! Wired Equivalent Privacy (WEP): authentication as in protocol ap4.0 –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

Security 14-Oct-15CPSC558 Advanced Computer Networks IEEE security Wired Equivalent Privacy (WEP): data encryption –Host/AP share 40 bit symmetric key (semi-permanent) –Host appends 24-bit initialization vector (IV) to create 64-bit key –64 bit key used to generate stream of keys, k i IV –k i IV used to encrypt ith byte, d i, in frame: c i = d i XOR k i IV –IV and encrypted bytes, c i sent in frame

Security 14-Oct-15CPSC558 Advanced Computer Networks WEP encryption Sender-side WEP encryption

Security 14-Oct-15CPSC558 Advanced Computer Networks Breaking WEP encryption Security hole: 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: c i = d i XOR k i IV –Trudy knows c i d i, so can compute k i IV –Trudy knows encrypting key sequence k 1 IV k 2 IV k 3 IV … –Next time IV is used, Trudy can decrypt!

Security 14-Oct-15CPSC558 Advanced Computer Networks Network Security (summary) Basic techniques…... –cryptography (symmetric and public) –authentication –message integrity –key distribution …. used in many different security scenarios –secure –secure transport (SSL) –IP sec – WEP

Security 14-Oct-15CPSC558 Advanced Computer Networks Security Summary Review Outline Encryption Algorithms Authentication Protocols Message Integrity Protocols Key Distribution Firewalls

Security 14-Oct-15CPSC558 Advanced Computer Networks Overview Cryptography functions –Secret key (e.g., DES) –Public key (e.g., RSA) –Message digest (e.g., MD5) Security services –Privacy: preventing unauthorized release of information –Authentication: verifying identity of the remote participant –Integrity: making sure message has not been altered

Security 14-Oct-15CPSC558 Advanced Computer Networks Secret Key (DES)

Security 14-Oct-15CPSC558 Advanced Computer Networks 64-bit key (56-bits + 8-bit parity) 16 rounds Each Round + F L i─ 1 R i─ 1 R i K i L i

Security 14-Oct-15CPSC558 Advanced Computer Networks Repeat for larger messages

Security 14-Oct-15CPSC558 Advanced Computer Networks Public Key (RSA) Encryption & Decryption c = m e mod n m = c d mod n

Security 14-Oct-15CPSC558 Advanced Computer Networks RSA (cont) Choose two large prime numbers p and q (each 256 bits) Multiply p and q together to get n Choose the encryption key e, such that e and (p - 1) x (q - 1) are relatively prime. Two numbers are relatively prime if they have no common factor greater than one Compute decryption key d such that d = e -1 mod ((p - 1) x (q - 1)) Construct public key as (e, n) Construct public key as (d, n) Discard (do not disclose) original primes p and q

Security 14-Oct-15CPSC558 Advanced Computer Networks Message Digest Cryptographic checksum –just as a regular checksum protects the receiver from accidental changes to the message, a cryptographic checksum protects the receiver from malicious changes to the message. One-way function –given a cryptographic checksum for a message, it is virtually impossible to figure out what message produced that checksum; it is not computationally feasible to find two messages that hash to the same cryptographic checksum. Relevance –if you are given a checksum for a message and you are able to compute exactly the same checksum for that message, then it is highly likely this message produced the checksum you were given.

Security 14-Oct-15CPSC558 Advanced Computer Networks Authentication Protocols Three-way handshake ClientServer ClientId, E (, CHK) E( y +, CHK) E(SK, SHK) Y

Security 14-Oct-15CPSC558 Advanced Computer Networks Trusted third party (Kerberos)

Security 14-Oct-15CPSC558 Advanced Computer Networks Public key authentication AB E ( x, Public B ) x

Security 14-Oct-15CPSC558 Advanced Computer Networks Message Integrity Protocols Digital signature using RSA –special case of a message integrity where the code can only have been generated by one participant –compute signature with private key and verify with public key Keyed MD5 –sender: m + MD5(m + k) + E(k, private) –receiver recovers random key using the sender’s public key applies MD5 to the concatenation of this random key message MD5 with RSA signature –sender: m + E(MD5(m), private) –receiver decrypts signature with sender’s public key compares result with MD5 checksum sent with message

Security 14-Oct-15CPSC558 Advanced Computer Networks Message Integrity Protocols Digital signature using RSA –special case of a message integrity where the code can only have been generated by one participant –compute signature with private key and verify with public key Keyed MD5 –sender: m + MD5(m + k) + E(E(k, rcv-pub), private) –receiver recovers random key using the sender’s public key applies MD5 to the concatenation of this random key message MD5 with RSA signature –sender: m + E(MD5(m), private) –receiver decrypts signature with sender’s public key compares result with MD5 checksum sent with message

Security 14-Oct-15CPSC558 Advanced Computer Networks Key Distribution Certificate –special type of digitally signed document: “I certify that the public key in this document belongs to the entity named in this document, signed X.” –the name of the entity being certified –the public key of the entity –the name of the certified authority –a digital signature Certified Authority (CA) –administrative entity that issues certificates –useful only to someone that already holds the CA’s public key.

Security 14-Oct-15CPSC558 Advanced Computer Networks Key Distribution (cont) Chain of Trust –if X certifies that a certain public key belongs to Y, and Y certifies that another public key belongs to Z, then there exists a chain of certificates from X to Z –someone that wants to verify Z’s public key has to know X’s public key and follow the chain Certificate Revocation List

Security 14-Oct-15CPSC558 Advanced Computer Networks Firewalls Filter-Based Solution –example ( , 1234, , 80 ) (*,*, , 80 ) –default: forward or not forward? –how dynamic?

Security 14-Oct-15CPSC558 Advanced Computer Networks Proxy-Based Firewalls Problem: complex policy Example: web server Solution: proxy Design: transparent vs. classical Limitations: attacks from within

Security 14-Oct-15CPSC558 Advanced Computer Networks Denial of Service Attacks on end hosts –SYN attack Attacks on routers –Christmas tree packets –pollute route cache Authentication attacks Distributed DoS attacks