Ch 8. Security in computer networks Myungchul Kim

2 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

8-3 There are bad guys (and girls) out there! Q: What can a “ bad guy ” do? A: a lot!  eavesdrop: 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 ……

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5 Principles of cryptography – The encryption technique is known – published, standardized, and available to everyone. – Symmetric key systems – Public key systems o Symmetric key cryptography – Block ciphers  PGP, SSL, IPsec

6 Symmetric key crypto: DES DES: Data Encryption Standard o US encryption standard [NIST 1993] o 56-bit symmetric key, 64-bit plaintext input o 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 o making DES more secure: – use three keys sequentially (3-DES) on each datum – use cipher-block chaining

7 Symmetric key crypto: DES initial permutation 16 identical “ rounds ” of function application, each using different 48 bits of key final permutation DES operation

8 AES: Advanced Encryption Standard o new (Nov. 2001) symmetric-key NIST standard, replacing DES o processes data in 128 bit blocks o 128, 192, or 256 bit keys o brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES

9 o Public key encryption – Diffie and Hellman, 1976 – For encryption, authentication, digital signature – A public key available to every one and a private key that is known only to a person

10 Message integrity – Cryptographic hash function  Originated from …  Not tampered with on its way to …  A cryptograhic hash function: it is computationaly infeasible to find any two different messages x and y such that H(x) = H(y). – MD5 (128-bit hash)

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12 o Message authentication code

13 o Digital signatures – Verifiable and nonforgeable

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16 o Public key certification – Verify that you have the actual public key fo the entity – Certification Authority: binding a public key to a particular entity – ITU X.509

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18 A certificate contains: o Serial number (unique to issuer) o info about certificate owner, including algorithm and key value itself (not shown) o info about certificate issuer o valid dates o digital signature by issuer

19 End-point authentication – The process of proving one’s identity to someone else.

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22 - nonce: once in a lifetime

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26 Securing – Confidentiality, sender authentication, message integrity, receiver authentication

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28 Pretty Good Privacy (PGP): MD5 or SHA for message digest; CAST, triple-DES or IDEA for symmetric key encryption and RSA for the public key encryption

29 Securing TCP connections: SSL – Secure Sockets Layer (SSL) – Transport Layer Security (TLS)

30 – Handshake, key distribution, and data transfer

31 Network-layer security: IPsec – Virtual private networks (VPNs) – Authentication Header (AH) protocol: source host authentication and data integrity – Encapsulation Security Payload (ESP) protocol: … and confidentiality – AH header: next header, security parameter index, sequence number, authentication data

32 – The ESP protocol  Key distribution – Manual – Automated: Internet Key Exchange protocol using public-key cryptography

33 IEEE security o 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! o securing – encryption, authentication – first attempt at security: Wired Equivalent Privacy (WEP): a failure – current attempt: i

34 Securing wireless LANs o Wired equivalent privacy (WEP) – Authentication and data encryption – Symmetric shared key – No key distribution

35 o IEEE i

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37 Firewalls and Intrusion Detection Systems o The goals of firewall – All traffic from outside to inside, and vice versa, passes through the firewall – Only authorized traffic, as defined by the local security policy, will be allowed to pass. – The firewall itself is immune to penetration.

38 o Traditional packet filters – Filtering decision  IP source or destination address  Protocol type in IP datagram field: TCP, UDP, ICMP, OSPF, …  TCP or UDP source and destination port  TCP flag bits: SYN, ACK, …  ICMP message type  Different rules for datagrams leaving and entering the network  Different rules for the different router interfaces.

39 Policy Firewall Setting No outside Web access. Drop all outgoing packets to any IP address, port 80 No incoming TCP connections, except those for institution ’ s public Web server only. Drop all incoming TCP SYN packets to any IP except , port 80 Prevent Web-radios from eating up the available bandwidth. Drop all incoming UDP packets - except DNS and router broadcasts. Prevent your network from being used for a smurf DoS attack. Drop all ICMP packets going to a “ broadcast ” address (eg ). Prevent your network from being tracerouted Drop all outgoing ICMP TTL expired traffic Stateless packet filtering: more examples

40 - Access control list for /16

41 o stateful packet filters – Actually track TCP connections – Check connections o Application gateway – Policy decision based on application data – Disadvantages  A different application gateway for each application  Perfrance penalty  The client software must know how to contact the gateway

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43 o Intrusion detection systems – Deep packet inspection – A high-security region and a lower-security region (demilitarized zone(DMZ)) – Signature-based system: require previous knowledge of the attach to generate an accurate signature – Anomaly-based system: create a traffic profile – Example: snort

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