21 - SecuritySlide 2What is network security?Friends and enemies: Alice, Bob, TrudySlide 5Who might Bob, Alice be?There are bad guys (and girls) out there!The language of cryptographySymmetric key cryptographySlide 10Symmetric key crypto: DESSymmetric key crypto: DESAES: Advanced Encryption StandardPublic Key CryptographyPublic key cryptographyPublic key encryption algorithmsRSA: Choosing keysRSA: Encryption, decryptionRSA example:RSA: another important propertyAuthenticationAuthentication: another trySlide 23Authentication: yet another trySlide 25Authentication: ap5.0ap5.0: security holeSlide 288: Network Security8-121 - Security8: Network Security8-2Chapter 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 1996-2004J.F Kurose and K.W. Ross, All Rights ReservedComputer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.8: Network Security8-3What is network security?Confidentiality: - only sender, intended receiver should “understand” message contentssender encrypts messagereceiver decrypts message Message Integrity:-sender, receiver want to ensure message not altered (in transit, or afterwards) without detectionAuthentication:- sender, receiver want to confirm identity of each otherAccess and Availability:- services must be accessible and available to usersAchieved through a continuous cycle of protection, detection, and response.8: Network Security8-4Friends and enemies: Alice, Bob, Trudywell-known in network security worldBob, Alice (lovers!) want to communicate “securely”Trudy (intruder) may intercept, delete, add messagessecuresendersecurereceiverchanneldata, control messagesdatadataAliceBobTrudy8: Network Security8-5http://xkcd.com8: Network Security8-6Who 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/serverDNS serversrouters exchanging routing table updates8: Network Security8-7There are bad guys (and girls) out there!Q: What can a “bad guy” do?A: a lot!eavesdrop: intercept messagesactively insert messages 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)more on this later ……8: Network Security8-8The language of cryptographysymmetric key crypto: sender, receiver keys identicalpublic-key crypto: encryption key public, decryption key secret (private)plaintextplaintextciphertextKAencryptionalgorithmdecryption algorithmAlice’s encryptionkeyBob’s decryptionkeyKB8: Network Security8-9Symmetric key cryptographysubstitution cipher: substituting one thing for anothermonoalphabetic cipher: substitute one letter for anotherplaintext: abcdefghijklmnopqrstuvwxyzciphertext: mnbvcxzasdfghjklpoiuytrewqPlaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbcE.g.:Q: How hard to break this simple cipher?: brute force (how hard?) other?8: Network Security8-10Symmetric key cryptographysymmetric key crypto: Bob and Alice share same (symmetric) key: Ke.g., key is knowing substitution pattern in mono alphabetic substitution cipherplaintextciphertextKA-Bencryptionalgorithmdecryption algorithmA-BKA-Bplaintextmessage, mK (m)A-BK (m)A-Bm = K ( ) A-B8: Network Security8-11Symmetric key crypto: DESDES: Data Encryption StandardUS encryption standard [NIST 1993]56-bit symmetric key, 64-bit plaintext inputHow secure is DES?DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 monthsmaking DES more secure:use two keys sequentially (3-DES) on each datumuse cipher-block chaining8: Network Security8-12Symmetric key crypto: DESinitial permutation 16 identical “rounds” of function application, each using different 48 bits of keyfinal permutationDES operation8: Network Security8-13AES: Advanced Encryption Standardnew (Nov. 2001) symmetric-key NIST standard, replacing DESprocesses data in 128 bit blocks128, 192, or 256 bit keysbrute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES8: Network Security8-14Public Key Cryptographysymmetric 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, RSA78]sender, receiver do not share secret keypublic encryption key known to allprivate decryption key known only to receiver or sender8: Network Security8-15Public key cryptographyplaintextmessage, mciphertextencryptionalgorithmdecryption algorithmBob’s public key plaintextmessageK (m)B+K B+Bob’s privatekey K B-m = K (K (m))B+B-8: Network Security8-16Public key encryption algorithmsneed K ( ) and K ( ) such thatBB..given public key K , it should be impossible to compute private key K BBRequirements:12RSA: Rivest, Shamir, Adleman algorithm+-K (K (m)) = m BB-++-8: Network Security8-17RSA: Choosing keys1. 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-8: Network Security8-18RSA: Encryption, decryption0. Given (n,e) and (n,d) as computed above1. To encrypt bit pattern, m, computec = m mod ne(i.e., remainder when m
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