Part I: Introduction

Part I: Introduction

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 applications Internet spam, viruses, and worms Network Security 7-1 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 Virus email really from your friends? The website really belongs to the bank? Network Security 7-2 What is network security? Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection Digital signature Nonrepudiation: sender cannot deny later that messages received were not sent by him/her Access and Availability: services must be accessible and available to users upon demand Denial of service attacks Anonymity: identity of sender is hidden from

receiver (within a group of possible senders) Network Security 7-3 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 Alice channel data secure sender Bob data, control messages

secure receiver data Trudy Network Security 7-4 Who might Bob, Alice be? Web client/server (e.g., on-line purchases) DNS servers routers exchanging routing table updates Two computers in peer-to-peer networks Wireless laptop and wireless access point Cell phone and cell tower Cell phone and bluetooth earphone RFID tag and reader ....... Network Security 7-5

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 Network Security 7-6 The language of cryptography

plaintext Alices K encryptio A n key encryption ciphertext algorithm Bobs K decryptio Bn key decryption plaintext algorithm symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) Network Security

7-7 Classical Cryptography Transposition Cipher Substitution Cipher Simple substitution cipher (Caesar cipher) Vigenere cipher One-time pad Network Security 7-8 Transposition Cipher: rail fence Write plaintext in two rows Generate ciphertext in column order Example: HELLOWORLD HLOOL

ELWRD ciphertext: HLOOLELWRD Problem: does not affect the frequency of individual symbols Network Security 7-9 Simple substitution cipher substituting one thing for another Simplest one: monoalphabetic cipher: substitute one letter for another (Caesar Cipher) ABCDEFGHIJKLMNOPQRSTUVWXYZ DEFGHIJKLMNOPQRSTUVWXYZABC Example: encrypt I attack Network Security 7-10 Problem of simple substitution

cipher The key space for the English Alphabet is very large: 26! 4 x 1026 However: Previous example has a key with only 26 possible values English texts have statistical structure: the letter e is the most used letter. Hence, if one performs a frequency count on the ciphers, then the most frequent letter can be assumed to be e Network Security 7-11 Distribution of Letters in English Frequency analysis Network Security 7-12

Vigenere Cipher Idea: Uses Caesar's cipher with various different shifts, in order to hide the distribution of the letters. A key defines the shift used in each letter in the text A key word is repeated as many times as required to become the same length Plain text: I a t t a c k Key: 2342342 Cipher text: K d x v d g m (key is 234) Network Security 7-13 Problem of Vigenere Cipher Vigenere is easy to break (Kasiski, 1863): Assume we know the length of the key. We can organize the ciphertext in rows with the same

length of the key. Then, every column can be seen as encrypted using Caesar's cipher. The length of the key can be found using several methods: 1. If short, try 1, 2, 3, . . . . 2. Find repeated strings in the ciphertext. Their distance is expected to be a multiple of the length. Compute the gcd of (most) distances. 3. Use the index of coincidence. Network Security 7-14 One-time Pad Extended from Vigenere cipher Key is as long as the plaintext Key string is random chosen Pro: Proven to be perfect secure

Cons: How to generate Key? How to let bob/alice share the same key pad? Code book Network Security 7-15 Symmetric key cryptography KA-B KA-B plaintext encryption ciphertext message, m algorithm K (m) A-B decryption plaintext algorithm m=K (KA-B(m) A-B

symmetric key crypto: Bob and Alice share know same (symmetric) key: K A-B e.g., key is knowing substitution pattern in mono alphabetic substitution cipher Q: how do Bob and Alice agree on key value? Network Security 7-16 ) 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 (3DES): use three keys sequentially on each datum

use cipher-block chaining Network Security 7-17 Symmetric key crypto: DES DES operation initial permutation 16 identical rounds of function application, each using different 48 bits of key final permutation Network Security 7-18 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 Network Security 7-19 Block Cipher loop for n rounds 64-bit input 8bits 8bits 8bits 8bits 8bits 8bits

8bits 8bits T1 T2 T3 T4 T5 T6 T7 T8 8 bits

one pass through: one input bit affects eight output bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 64-bit scrambler 64-bit output multiple passes: each input bit affects most output bits block ciphers: DES, 3DES, AES Network Security 7-20 Cipher Block Chaining cipher block: if input

block repeated, will produce same cipher text: t=1 m(1) = HTTP/1.1 block c(1) cipher m(17) = HTTP/1.1 block c(17) cipher t=17 = k329aM02 = k329aM02

cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i-1) c(0) transmitted to receiver in clear what happens in HTTP/1.1 scenario from above? m(i) c(i-1) + block cipher

c(i) Network Security 7-21 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 [DiffieHellman76, RSA78] sender, receiver do not share secret key public encryption key known to all private decryption key known only to

receiver Network Security 7-22 Public key cryptography + Bobs public B key K K plaintext encryption ciphertext + message, m algorithm K (m) B - Bobs private B key decryption plaintext algorithm message - +

m = KB (K (m)) B Network Security 7-23 Public key encryption algorithms Requirements: + . . 1 need KB ( ) and -K ( ) such that B - + K (K (m)) = m B B 2 + given public keyBK , it should

be impossible to compute private key K B RSA: Rivest, Shamir, Adelson algorithm Network Security 7-24 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

KB Network Security 7-25 RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 1. To encrypt bit pattern, m, compute e c = me mod(i.e., remainder when m is divided by n) n 2. To decrypt received bit pattern, c, compute d m = cd mod(i.e., remainder when c is divided by n) n Magic e mod d mod m = (m happens! n) c n Network Security 7-26 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). encrypt: decrypt: letter m me l 12 1524832 c 17

c = me mod n 17 d c m = cd mod n letter 481968572106750915091411825223071697 12 l Computational extensive Network Security 7-27 RSA: e mod d mod m = (m n) n Useful number theory result: If p,q prime and n = pq, then: y y mod (p-1)(q-1) x mod n = x

mod n Why is that e (m mod d mod n = ed m n) mod n ed mod (p-1)(q= m 1) number theory result above) (using mod n 1 = m mod n (since we chose ed to be divisible by (p-1)(q-1) with remainder 1 ) = m Network Security 7-28

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

Network Security 7-29

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