8-1Network SecurityChapter 8Network SecurityA note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2010J.F Kurose and K.W. Ross, All Rights ReservedComputer Networking: A Top Down Approach ,5thedition. Jim Kurose, Keith RossAddison-Wesley, April 2009.8-2Network SecurityChapter 8 roadmap8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS8-3Network SecurityWhat is network security?Confidentiality: only sender, intended receiver should “understand” message contents sender encrypts message receiver decrypts messageAuthentication: sender, receiver want to confirm identity of each other Message integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detectionAccess and availability: services must be accessible and available to users8-4Network SecurityFriends and enemies: Alice, Bob, Trudy well-known in network security world Bob, Alice (lovers!) want to communicate “securely” Trudy (intruder) may intercept, delete, add messagessecuresendersecurereceiverchanneldata, control messagesdatadataAliceBobTrudy8-5Network SecurityWho might Bob, Alice be? … well, real-lifeBobs and Alices! Web browser/server for electronic transactions (e.g., on-line purchases) on-line banking client/server DNS servers routers exchanging routing table updates8-6Network SecurityThere are bad guys (and girls) out there!Q: What can a “bad guy” do?A: A lot! See section 1.6eavesdrop:intercept messages actively insertmessages into connectionimpersonation: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 placedenial of service: prevent service from being used by others (e.g., by overloading resources)8-7Network SecurityChapter 8 roadmap8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS8-8Network SecurityThe language of cryptographym plaintext messageKA(m) ciphertext, encrypted with key KAm = KB(KA(m))plaintextplaintextciphertextKAencryptionalgorithmdecryption algorithmAlice’s encryptionkeyBob’s decryptionkeyKB8-9Network SecurityBreaking an encryption scheme Cipher-text only attack: Trudy has ciphertext that she can analyze Two approaches: Search through all keys: must be able to differentiate resulting plaintext from gibberish Statistical analysis Known-plaintext attack:Trudy has some plaintext corresponding to some ciphertext Chosen-plaintext attack:Trudy can get the ciphertext for some chosen plaintext8-10Network SecurityTypes of Cryptography Crypto often uses keys: Algorithm is known to everyone Only “keys” are secret Public key cryptography Involves the use of two keys Symmetric key cryptography Involves the use one key Hash functions Involves the use of no keys8-11Network SecuritySymmetric key cryptographysymmetric key crypto: Bob and Alice share same (symmetric) key: KQ: how do Bob and Alice agree on key value?plaintextciphertextKSencryptionalgorithmdecryption algorithmSKSplaintextmessage, mK (m)Sm = KS(KS(m))8-12Network SecurityTwo types of symmetric ciphers Stream ciphers encrypt one bit at time Block ciphers Break plaintext message in equal-size blocks Encrypt each block as a unit8-13Network SecurityStream Ciphers Combine each bit of keystream with bit of plaintext to get bit of ciphertext m(i) = ith bit of message ks(i) = ith bit of keystream c(i) = ith bit of ciphertext c(i) = ks(i) m(i) ( = exclusive or) m(i) = ks(i) c(i) keystreamgeneratorkeykeystreampseudo random8-14Network SecurityRC4 Stream Cipher RC4 is a popular stream cipher Fast, simple, can be vulnerable to attack Key can be from 1 to 256 bytes Used in WEP for 802.11 Can be used in SSL8-15Network SecurityBlock ciphers Message to be encrypted is processed in blocks of k bits (e.g., 64-bit blocks). 1-to-1 mapping is used to map k-bit block of plaintext to k-bit block of ciphertextExample with k=3:input output000 110001 111010 101011 100input output100 011101 010110 000111 001What is the ciphertext for 010110001111 ?8-16Network SecurityBlock ciphers How many possible mappings are there for k=3? How many 3-bit inputs? How many permutations of the 3-bit inputs? Answer: 40,320 ; not very many! In general, 2k! mappings; huge for k=64 Problem: Table approach requires table with 264entries, each entry with 64 bits Table too big: instead use function that simulates a randomly permuted table8-17Network SecurityPrototype function64-bit inputS18bits8 bitsS28bits8 bitsS38bits8 bitsS48bits8 bitsS78bits8 bitsS68bits8 bitsS58bits8 bitsS88bits8 bits64-bit intermediate64-bit outputLoop for n rounds8-bit to8-bitmappingFrom Kaufmanet al8-18Network SecurityWhy rounds in prototype? If only a single round, then one bit of input affects at most 8 bits of output. In 2ndround, the 8 affected bits get scattered and inputted into multiple substitution boxes. The purpose of the rounds is to make each input bit effect most (if not all) of the final output bits8-19P4 Suppose each block cipher Tisimply reverses the order of the eight input bits. Further suppose that the 64-bit scrambler does not modify any bits (so that the output value of the mth bit is equal to the input value of the mth bit). With n=3 and the
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