Network Security. Abusayeed Saifullah. CS 5600 Computer Networks. These slides are adapted from Kurose and Ross 81


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1 Network Security Abusayeed Saifullah CS 5600 Computer Networks These slides are adapted from Kurose and Ross 81
2 Goals v understand principles of network security: cryptography and its many uses beyond confidentiality authentication message integrity v security in practice: firewalls and intrusion detection systems security in application, transport, network, link layers 82
3 Roadmap 1 What is network security? 2 Principles of cryptography 3 Message integrity, authentication 4 Securing 5 Securing TCP connections: SSL 6 Network layer security: Ipsec 7 Securing wireless LANs 8 Operational security: firewalls and IDS 83
4 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 84
5 Friends and enemies: Alice, Bob, Trudy v wellknown in network security world v Bob, Alice (lovers!) want to communicate securely v Trudy (intruder) may intercept, delete, add messages Alice channel data, control messages Bob data secure sender s secure receiver data Trudy 85
6 Who might Bob, Alice be? v well, reallife Bobs and Alices! v Web browser/server for electronic transactions (e.g., online purchases) v online banking client/server v DNS servers v routers exchanging routing table updates v other examples? 86
7 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) 87
8 Roadmap 1 What is network security? 2 Principles of cryptography 3 Message integrity, authentication 4 Securing 5 Securing TCP connections: SSL 6 Network layer security: Ipsec 7 Securing wireless LANs 8 Operational security: firewalls and IDS 88
9 The language of cryptography K A Alice s encryption key Bob s K decryption B key plaintext encryption algorithm ciphertext decryption algorithm plaintext m plaintext message K A (m) ciphertext, encrypted with key K A m = K B (K A (m)) 89
10 Breaking an encryption scheme v ciphertext only attack: Trudy has ciphertext she can analyze two approaches: brute force: search through all keys statistical analysis: e.g. birthday attack v knownplaintext attack: Trudy has some (plaintext, ciphertext) pair which he did not choose (say, stole after the receiver decrypted) v chosenplaintext attack: Trudy can select any plaintext and obtain the corresponding ciphertext à can thus find the key Finding mapping for The quick brown fox jumps over the lazy dog à disaster for monoalphabetic cipher 810
11 Symmetric key cryptography K S K S plaintext message, m encryption algorithm ciphertext K S (m) decryption algorithm plaintext m = K S (K S (m)) symmetric key crypto: Bob and Alice share same (symmetric) key: K S v e.g., key is knowing substitution pattern in mono alphabetic substitution cipher Q: how do Bob and Alice agree on key value? 811
12 Simple encryption scheme 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 Encryption key: mapping from set of 26 letters to set of 26 letters 812
13 A more sophisticated encryption approach v n substitution ciphers, M 1,M 2,,M n v cycling pattern: e.g., n=4: M 1,M 3,M 4,M 3,M 2 ; M 1,M 3,M 4,M 3,M 2 ;.. v for each new plaintext symbol, use subsequent subsitution pattern in cyclic pattern book: b from M 1, o from M 3, o from M 4, k from M 3 Encryption key: n substitution ciphers, and cyclic pattern Why is this better? 813
14 Symmetric key crypto: DES DES: Data Encryption Standard v US encryption standard [NIST 1993] developed by IBM v 56bit symmetric key, 64bit plaintext input v block cipher with cipher block chaining (CBC) CBC: a block is XORed with previous block before encryption v how secure is DES? DES Challenge: 56bitkeyencrypted phrase decrypted (brute force) in less than a day there are analytical results showing weakness (although infeasible in practice) v making DES more secure: 3DES: encrypt 3 times with 3 different keys 814
15 Figure 87. The Data Encryption Standard. (a) General outline. (b) Detail of one iteration. The circled + means exclusive OR. SEC. 8.2 SYMMETRICKEY ALGORITHMS 781 Symmetric key crypto: DES 64Bit plaintext L i1 R i1 Initial transposition 56Bit key Iteration 1 Iteration 2 Iteration 16 L i1 f(r i1,k i ) Feistel function 32Bit swap Inverse transposition 64Bit ciphertext (a) 32 bits 32 bits L i R i Details of (b) an iteration 815
16 Symmetric key crypto: DES v Plaintext is encrypted in blocks of 64 bits, yielding 64 bits of ciphertext. v The algorithm has 19 distinct stages. v The first stage is a keyindependent transposition on the 64bit plaintext. The last stage is the exact inverse of this transposition. v The stage prior to the last one exchanges the leftmost 32 bits with the right most 32 bits. v The remaining 16 stages (iterations) are identical but are parameterized by different functions of the key. 816
17 4step Feistel Function v E (48 bit) is created by expanding 32bit R i 1 using a transposition and duplication rule. v E and K i are XORed v This output is split into 8 groups of 6 bits each; each is mapped onto a 4 bit output. v These 8 4 bits are passed through a permutation box. K i 817
18 AES: Advanced Encryption Standard v symmetrickey NIST standard, replacied DES (Nov 2001) v processes data in 128 bit blocks v 128, 192, or 256 bit keys v brute force decryption (try each key): a machine taking 1 sec on DES, takes 149 trillion years for AES 818
19 AES: Advanced Encryption Standard v Rijndael proposal (Joan Daemen, Vincent Rijmen) v Rijndael is based on Galois theory, which gives it some provable security properties. v Like DES, Rijndael uses substitution and permutations, and it also uses multiple rounds. v The number of rounds depends on the key size and block size, being 10 for 128bit keys with 128bit blocks and moving up to 14 max. v Unlike DES, all operations involve entire bytes for efficient implementation in hardware and software. 819
20 Public Key Cryptography symmetric key crypto v requires sender, receiver know shared secret key v Q: how to agree on key in first place (particularly if never met )? public key crypto v radically different approach [Diffie Hellman76, RSA78] v sender, receiver do not share secret key v public encryption key known to all v private decryption key known only to receiver 820
21 Public key cryptography + K B  K B Bob s public key Bob s private key plaintext message, m encryption algorithm ciphertext + K (m) B decryption algorithm plaintext message  + m = K (K (m)) B B 821
22 Public key encryption algorithms requirements: B  + B need K ( ) and K ( ) such that K (K (m)) = m B B given public key K B, it should be impossible to compute private  key K B + RSA: Rivest, Shamir, Adelson algorithm 822
23 Prerequisite: modular arithmetic v v v v x mod n = remainder of x when divide by n facts: [(a mod n) + (b mod n)] mod n = (a+b) mod n [(a mod n)  (b mod n)] mod n = (ab) mod n [(a mod n) * (b mod n)] mod n = (a*b) mod n thus (a mod n) d mod n = a d mod n example: x=14, n=10, d=2: (x mod n) d mod n = 4 2 mod 10 = 6 x d = 14 2 = 196 x d mod 10 =
24 RSA: getting ready v message: just a bit pattern v bit pattern can be uniquely represented by an integer number v thus, encrypting a message is equivalent to encrypting a number. example: v m= This message is uniquely represented by the decimal number 145. v to encrypt m, we encrypt the corresponding number, which gives a new number (the ciphertext). 824
25 RSA: Creating public/private key pair 1. choose two large prime numbers p, q. (e.g., 1024 bits each) 2. compute n = pq, z = (p1)(q1) 3. choose e (with e<n) that has no common factors with z (e, z are relatively prime ). 4. choose d such that ed1 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 825
26 RSA: encryption, decryption 0. given (n,e) and (n,d) as computed above 1. to encrypt message m (<n), compute c = m e mod n 2. to decrypt received bit pattern, c, compute m = c d mod n magic happens! m = (m e mod n) c d mod n 826
27 RSA example: Bob chooses p=5, q=7. Then n=35, z=24. e=5 (so e, z relatively prime). d=29 (so ed1 exactly divisible by z). encrypting 8bit messages. encrypt: decrypt: bit pattern m m e c = m e mod n 0000l c c d m = c d mod n
28 Why does RSA work? v must show that c d mod n = m where c = m e mod n v fact: for any x and y: x y mod n = x (y mod z) mod n where n= pq and z = (p1)(q1) due to Kaufman 1995 v thus, c d mod n = (m e mod n) d mod n = m ed mod n = m (ed mod z) mod n = m 1 mod n = m 828
29 RSA: another important property The following property will be very useful later:  + K (K (m)) = m B B use public key first, followed by private key = +  B B K (K (m)) use private key first, followed by public key result is the same! 829
30  + Why K (K (m)) = m = K (K (m))? B B +  B B follows directly from modular arithmetic: (m e mod n) d mod n = m ed mod n = m de mod n = (m d mod n) e mod n 830
31 Why is RSA secure? v suppose you know Bob s public key (n,e). How hard is it to determine d? v essentially need to find factors of n without knowing the two factors p and q fact: factoring a big number is hard 831
32 RSA in practice: session keys v exponentiation in RSA is computationally intensive v DES is at least 100 times faster than RSA v use public key cryto to establish secure connection, then establish second key symmetric session key for encrypting data session key, K S v Bob and Alice use RSA to exchange a symmetric key K S v once both have K S, they use symmetric key cryptography 832
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