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ECE 4450:427/527 - Computer Networks Spring 2015 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 9.1: Network Security Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 1

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Goals Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 2 Understand principles of network security: – Cryptography and its many uses beyond “confidentiality” Confidentiality (encryption) authentication message integrity Access and availability Example Systems: – Transport Layer security – IP security – Wireless security

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Goals Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 3 If time permits: Physical Layer Security

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Confidentiality Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 4 Consider some threats to secure use of, for example, the World Wide Web. – Suppose you are a customer using a credit card to order an item from a website. An obvious threat is that an adversary would eavesdrop on your network communication, reading your messages to obtain your credit card information. It is possible and practical, however, to encrypt messages so as to prevent an adversary from understanding the message contents. A protocol that does so is said to provide confidentiality. Taking the concept a step farther, concealing the quantity or destination of communication is called traffic confidentiality

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Integrity Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 5 Even with confidentiality there still remain threats for the website customer. – An adversary who can’t read the contents of your encrypted message might still be able to change a few bits in it, resulting in a valid order for, say, a completely different item or perhaps 1000 units of the item. – There are techniques to detect, if not prevent, such tampering. – A protocol that detects such message tampering provides data integrity. – The adversary could alternatively transmit an extra copy of your message in a replay attack.

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Authentication Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 6 Another threat to the customer is unknowingly being directed to a false website. – False information is entered in a Domain Name Server or the name service cache of the customer’s computer. – This leads to translating a correct URL (uniform resource locator) into an incorrect IP address—the address of a false website. – A protocol that ensures that you really are talking to whom you think you’re talking is said to provide authentication. – Authentication entails integrity since it is meaningless to say that a message came from a certain participant if it is no longer the same message.

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Availability Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 7 The owner of the website can be attacked as well. Some websites have been defaced; the files that make up the website content have been remotely accessed and modified without authorization. That is an issue of access control: enforcing the rules regarding who is allowed to do what. Websites have also been subject to Denial of Service (DoS) attacks, during which would-be customers are unable to access the website because it is being overwhelmed by bogus requests. Ensuring a degree of access is called availability.

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Friends and Enemies: Alice, Bob, & Trudy Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 8 well-known in network security world Bob, Alice (lovers!) want to communicate “securely” Trudy (intruder) may intercept, delete, add messages secure sender secure receiver channel data, control messages data Alice Bob Trudy

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Who might Alice and Bob be? Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 9 … well, real-life Bobs and Alices!

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There are bad guys (and girls)!! Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 10 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)

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Principles of Cryptography Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 11 m plaintext message K A (m) ciphertext, encrypted with key K A m = K B (K A (m)) plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B

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Simple encryption scheme Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 12 substitution cipher: substituting one thing for another – monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.: Key?

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Polyalphabetic Encryption Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 13 n monoalphabetic ciphers, M 1,M 2,…,M n 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 ; For each new plaintext symbol, use subsequent monoalphabetic pattern in cyclic pattern – dog: d from M 1, o from M 3, g from M 4 Key?

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Breaking Encryption Scheme Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 14 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 – e.g., in monoalphabetic cipher, Trudy determines pairings for a,l,i,c,e,b,o, Chosen-plaintext attack: Trudy can get the ciphertext for some chosen plaintext

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Types of Cryptography Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 15 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 keys – Nothing secret: How can this be useful?

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Symmetric Key Cryptography Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 16 symmetric key crypto: Bob and Alice share same (symmetric) key: K e.g., key is knowing substitution pattern in mono alphabetic substitution cipher Q: how do Bob and Alice agree on key value? plaintext ciphertext K S encryption algorithm decryption algorithm K S plaintext message, m K (m) S m = K S (K S (m))

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Two types of Symmetric Cipher Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 17 Stream ciphers – encrypt one bit at time Block ciphers – Break plaintext message in equal-size blocks – Encrypt each block as a unit

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Stream Ciphers Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 18 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) How can we decode? keystream generator key keystream pseudo random

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RC4 Stream Cipher Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 19 RC4 is a popular stream cipher – Extensively analyzed and considered good – Key can be from 1 to 256 bytes – Used in WEP (Wired Equivalent Privacy) (also WPA) for – Can be used in SSL (Secure Sockets Layer) – We will talk in further detail later on when examining WEP

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Block Ciphers Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 20 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 ciphertext Example with k=3: input output input output What is the ciphertext for ?

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Block Ciphers Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 21 How many possible mappings are there for k=3? In general, 2 k ! mappings; huge for k=64 Problem? – Table approach requires table with 2 64 entries, each entry with 64 bits Table too big: instead use function that simulates a randomly permuted table

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Prototype Function Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks bit input S1S1 8bits S2S2 S3S3 S4S4 S7S7 S6S6 S5S5 S8S8 64-bit intermediate 64-bit output Loop for n rounds 8-bit to 8-bit mapping

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Why rounds? Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 23 If only a single round, then one bit of input affects at most 8 bits of output. In 2 nd round, the 8 affected bits get scattered and inputted into multiple substitution boxes. How many rounds? – How many times do you need to shuffle cards – Becomes less efficient as n increases

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Encrypting Large Message Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 24 Why not just break message in 64-bit blocks, encrypt each block separately? – If same block of plaintext appears twice, will give same ciphertext. How about: – Generate random 64-bit number r(i) for each plaintext block m(i) – Calculate c(i) = K S ( m(i) r(i) ) – Transmit c(i), r(i), i=1,2,… – At receiver, how to decode? m(i) = K S (c(i)) r(i) – Problem? inefficient, need to send c(i) and r(i)

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Cipher Block Chaining (CBC) Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 25 CBC generates its own random numbers – Have encryption of current block depend on result of previous block – c(i) = K S ( m(i) c(i-1) ) – m(i) = K S ( c(i)) c(i-1) How do we encrypt first block? – Initialization vector (IV): random block = c(0) – IV does not have to be secret Change IV for each message (or session) – Guarantees that even if the same message is sent repeatedly, the ciphertext will be completely different each time

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Cipher Block Chaining Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 26 cipher block: if input block repeated, will produce same cipher text: t=1 m(1) = “HTTP/1.1” block cipher c(1) = “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) t=17 m(17) = “HTTP/1.1” block cipher c(17) = “k329aM02” block cipher c(i-1)

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Symmetric key crypto: DES Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 27 DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64-bit plaintext input Block cipher with cipher block chaining

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Symmetric key crypto: DES Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 28 initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation

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AES: Advanced Encryption Standard Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 29 How secure is DES? – DES Challenge: 56-bit-key-encrypted phrase decrypted (brute force) in less than a day – Has been withdrawn as a standard making DES more secure: – 3DES: encrypt 3 times with 3 different keys (actually encrypt, decrypt, encrypt) – Practically secured, although there are theoretical attacks

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AES: Advanced Encryption Standard Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 30 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

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Public Key Cryptography Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 31 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 [Diffie- Hellman76, RSA78] sender, receiver do not share secret key public encryption key known to all private decryption key known only to receiver

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Public Key Cryptography Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 32 plaintext message, m ciphertext encryption algorithm decryption algorithm Bob’s public key plaintext message K (m) B + K B + Bob’s private key K B - m = K ( K (m) ) B + B -

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Public key encryption algorithms Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 33 need K ( ) and K ( ) such that B B.. given public key K, it should be impossible to compute private key K B B Requirements: 1 2 RSA: Rivest, Shamir, Adelson algorithm + - K (K (m)) = m B B

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Prerequisite: modular arithmetic Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 34 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 = (a-b) 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 = 6

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RSA: getting ready Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 35 A message is a bit pattern. A bit pattern can be uniquely represented by an integer number. Thus encrypting a message is equivalent to encrypting a decima number. Example m= This message is uniquely represented by the decimal number 145. To encrypt m, we encrypt the corresponding number, which gives a new number (the ciphertext). RSA: two interrelated components – 1) Choice of public and private keys; 2) Encryption and decryption algorithm

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RSA: Creating public/private key pair Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 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

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RSA: Encryption, decryption Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks Given (n,e) and (n,d) as computed above 1. To encrypt message m (

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RSA example: Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 38 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). bit pattern m m e c = m mod n e 0000l c m = c mod n d c d encrypt: decrypt: Encrypting 8-bit messages.

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Why does RSA work? Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 39 Must show that c d mod n = m where c = m e mod n Fact: for any x and y: x y mod n = x (y mod z) mod n – where n= pq and z = (p-1)(q-1) 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

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RSA: another important property Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 40 The following property will be very useful later: K ( K (m) ) = m B B - + K ( K (m) ) B B + - = use public key first, followed by private key use private key first, followed by public key Result is the same!

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Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 41 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 K ( K (m) ) = m B B - + K ( K (m) ) B B + - = Why ?

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Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 42 Why is RSA Secure? suppose you know Bob’s public key (n,e). How hard is it to determine d? essentially need to find factors of n without knowing the two factors p and q. fact: factoring a big number is hard. Generating RSA keys have to find big primes p and q approach: make good guess then apply testing rules (see Kaufman)

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Session keys Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 43 Exponentiation is computationally intensive DES is at least 100 times faster than RSA Session key, K S Bob and Alice use RSA to exchange a symmetric key K S Once both have K S, they use symmetric key cryptography

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Message Integrity Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 44 allows communicating parties to verify that received messages are authentic. – Content of message has not been altered – Source of message is who/what you think it is – Message has not been replayed – Sequence of messages is maintained let’s first talk about message digests and cryptographic hash functions

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Message Digests Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 45 function H( ) that takes as input an arbitrary length message and outputs a fixed-length string: “message signature” note that H( ) is a many- to-1 function H( ) is often called a “hash function” desirable properties: – easy to calculate – irreversibility: Can’t determine m from H(m) – collision resistance: computationally difficult to produce m and m’ such that H(m) = H(m’) – seemingly random output large message m H: Hash Function H(m)

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Internet checksum: poor message digest Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 46 Internet checksum has some properties of hash function: üproduces fixed length digest (16-bit sum) of input üis many-to-one but given message with given hash value, it is easy to find another message with same hash value. e.g.,: simplified checksum: add 4-byte chunks at a time: I O U B O B 49 4F E D2 42 message ASCII format B2 C1 D2 AC I O U B O B 49 4F E D2 42 message ASCII format B2 C1 D2 AC different messages but identical checksums!

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Hash Function Algorithms Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 47 MD5 hash function widely used (RFC 1321) – computes 128-bit message digest in 4-step process. SHA-1 is also used. – US standard [ NIST, FIPS PUB 180-1] – 160-bit message digest

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Message Authentication Code (MAC) Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 48 message H( ) s message s H( ) compare s = shared secret Authenticates sender Verifies message integrity No encryption ! Also called “keyed hash” Notation: MD m = H(s||m) ; send m||MD m

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HMAC Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 49 popular MAC standard addresses some subtle security flaws operation: – concatenates secret to front of message. – hashes concatenated message – concatenates secret to front of digest – hashes combination again

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Example: OSPF Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 50 Recall that OSPF is an intra-AS routing protocol Each router creates map of entire AS (or area) and runs shortest path algorithm over map. Router receives link-state advertisements (LSAs) from all other routers in AS. Attacks: Message insertion Message deletion Message modification How do we know if an OSPF message is authentic?

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OSPF Authentication Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 51 within an Autonomous System, routers send OSPF messages to each other. OSPF provides authentication choices – no authentication – shared password: inserted in clear in 64-bit authentication field in OSPF packet – cryptographic hash cryptographic hash with MD5 – 64-bit authentication field includes 32-bit sequence number – MD5 is run over a concatenation of the OSPF packet and shared secret key – MD5 hash then appended to OSPF packet; encapsulated in IP datagram

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End-point authentication Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 52 want to be sure of the originator of the message – end-point authentication assuming Alice and Bob have a shared secret, will MAC provide end-point authentication? – we do know that Alice created message. – … but did she send it?

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Playback attack Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 53 MAC Transfer $1M from Bill to Trudy MAC Transfer $1M from Bill to Trudy MAC = f(msg,s)

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Defending against playback attack: nonce Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 54 “I am Alice” R MAC Transfer $1M from Bill to Susan MAC = f(msg,s,R)

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Digital Signatures Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 55 cryptographic technique analogous to hand- written signatures. sender (Bob) digitally signs document, establishing he is document owner/creator. goal is similar to that of MAC, except now use public-key cryptography verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document

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Digital Signatures Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 56 simple digital signature for message m: Bob signs m by encrypting with his private key K B, creating “signed” message, K B (m) - - Dear Alice Oh, how I have missed you. I think of you all the time! …(blah blah blah) Bob Bob’s message, m Public key encryption algorithm Bob’s private key K B - Bob’s message, m, signed (encrypted) with his private key K B - (m)

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Digital signature = signed message digest Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 57 large message m H: Hash function H(m) digital signature (encrypt) Bob’s private key K B - + Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: K B (H(m)) - encrypted msg digest K B (H(m)) - encrypted msg digest large message m H: Hash function H(m) digital signature (decrypt) H(m) Bob’s public key K B + equal ?

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Digital Signatures (more) Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 58 suppose Alice receives msg m, digital signature K B (m) Alice verifies m signed by Bob by applying Bob’s public key K B to K B (m) then checks K B (K B (m) ) = m. if K B (K B (m) ) = m, whoever signed m must have used Bob’s private key Alice thus verifies that: üBob signed m. üno one else signed m. üBob signed m and not m’. Non-repudiation: Alice can take m, and signature K B (m) to court and prove that Bob signed m. -

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Public-key certification Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 59 motivation: Trudy plays pizza prank on Bob – Trudy creates order: Dear Pizza Store, Please deliver to me four pepperoni pizzas. Thank you, Bob – Trudy signs order with her private key – Trudy sends order to Pizza Store – Trudy sends to Pizza Store her public key, but says it’s Bob’s public key. – Pizza Store verifies signature; then delivers four pizzas to Bob. – Bob doesn’t even like Pepperoni

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Certification Authorities Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 60 Certification authority (CA): binds public key to particular entity, E. E (person, router) registers its public key with CA. – E provides “proof of identity” to CA. – CA creates certificate binding E to its public key. – certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key” Bob’s public key K B + Bob’s identifying information digital signature (encrypt) CA private key K CA - K B + certificate for Bob’s public key, signed by CA

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Certification Authorities Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 61 when Alice wants Bob’s public key: – gets Bob’s certificate (Bob or elsewhere). – apply CA’s public key to Bob’s certificate, get Bob’s public key Bob’s public key K B + digital signature (decrypt) CA public key K CA + K B +

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Certificates: summary Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 62 primary standard X.509 (RFC 2459) certificate contains: – issuer name – entity name, address, domain name, etc. – entity’s public key – digital signature (signed with issuer’s private key) Public-Key Infrastructure (PKI) – certificates, certification authorities – often considered “heavy”

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