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KN (WS 2007/08) Network Security General Overview on the fundamentals of Network Security Prof. KN (WS 2007/08) Network Security General Overview on the fundamentals of Network Security Prof. Wael Adi 07. 02. 2008 V-6 Technische Universität Carolo-Wilhelmina zu Braunschweig Institut für Datentechnik und Kommunikationsnetze (IDA) -- INSTITUT FÜR DATENTECHNIK UND KOMMUNIKATIONSNETZE

Chapter 8: Network Security Copyright notice: Most of the slides (those without foot note) Chapter 8: Network Security Copyright notice: Most of the slides (those without foot note) are adapted from the slides of the following text book adopted for this course: Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross, Addison-Wesley, July 2004. Chapter goals: r understand principles of network security: m m cryptography and its many uses beyond “confidentiality” authentication message integrity key distribution r security in practice: m firewalls m security in application, transport, network, link layers 8: Network Security 2

Chapter 8 roadmap Historical overview 8. 1 What is network security? 8. 2 Principles Chapter 8 roadmap Historical overview 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 3

What is network security? Confidentiality: only sender, intended receiver should “understand” message contents m What is network security? Confidentiality: only sender, intended receiver should “understand” message contents m sender encrypts message m 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 8: Network Security 4

Security is dealing with Friends and enemies: Alice, Bob, Trudy r well-known in network Security is dealing with Friends and enemies: Alice, Bob, Trudy r well-known in network security world r Bob, Alice (lovers!) want to communicate “securely” r Trudy (intruder) may intercept, delete, add messages Alice data channel secure sender Bob data, control messages secure receiver data Trudy 8: Network Security 5

Who might Bob, Alice be? … well, real-life Bobs and Alices! r Web browser/server Who might Bob, Alice be? … well, real-life Bobs and Alices! r Web browser/server for electronic transactions (e. g. , on-line purchases) r on-line banking client/server r DNS servers r routers exchanging routing table updates r other examples? 8: Network Security 6

There are bad guys (and girls) out there! Q: What can a “bad guy/Attacker” There are bad guys (and girls) out there! Q: What can a “bad guy/Attacker” do? A: a lot! (there are many types of attacks on the system) m eavesdrop: intercept messages m actively insert messages into connection m impersonation: can fake (spoof) source address in packet (or any field in packet) m hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place m denial of service: prevent service from being used by others (e. g. , by overloading resources) more on this later …… 8: Network Security 7

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 8

Cryptography Historical overview Cryptography: Is it Art ob Science ? The story has three Cryptography Historical overview Cryptography: Is it Art ob Science ? The story has three epochs: I. Conventional Cryptography as Art • Julius Caesar Cipher Arabic origins of cryptography • Alkindi “Istihrag Almuamma” First Algorithms for Cryptoanalysis 850 • Arabic Encyclopedia (ch-8)Al-Qalkashandi “Subh Al-Asha fi Sinaat Alinsha” 1418 • Kaisiski “ The Art of Deciphering” 1863, . . . Gauss • Vernam (AT&T) 1926, first unbreakable system • II world ware 1945, Enigma, Hagelin. . Alan Turing II. As a Modern Science 1949 C= B log 2 (1 + S/N) • C. Shannon (AT&T) 1948 ‚Founded the Communication Theory ‘ • C. Shannon (AT&T) 1949 'Communication Theory of Secrecy Systems‘ III. Breakthrough to Modern Cryptology 1976 • Diffie and Hellmann 1976 Public key Cryptography (Stanford University) • RSA 1978 (public key secrecy system) (MIT) • . . . . Any new Breakthrough expected ? ! 8: Network Security 9

The language of cryptography K Alice’s encryption key A plaintext encryption algorithm K Bob’s The language of cryptography K Alice’s encryption key A plaintext encryption algorithm K Bob’s decryption key B ciphertext decryption plaintext algorithm Symmetric/Secret key cryptography: sender, receiver keys identical Asymmetric/public-key cryptography: encryption key public, decryption key secret (private) 8: Network Security 10

Secret and Public key Cryptography Secret Key Systems Public-Key Security Systems K-public K-secret K-open Secret and Public key Cryptography Secret Key Systems Public-Key Security Systems K-public K-secret K-open = K-close (Symmetric System) -Open and close with the same key which has to be agreed on secretly !! K-open K-close (Asymmetric System) - Open and close with different keys!! - No Secret Key Agreement required 8: Network Security 11

Secret Key Crypto-System : mechanical simulation SENDER RECEIVER Secret key agreement Key = Z Secret Key Crypto-System : mechanical simulation SENDER RECEIVER Secret key agreement Key = Z Z Key = Z Message Lock Z 8: Network Security 12

Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq ……. Example: Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard to break this simple cipher? : q brute force (how hard? Check all possibilities) q other? 8: Network Security 13

Symmetric key cryptography KA-B plaintext message, m Same key encryption ciphertext algorithm K (m) Symmetric key cryptography KA-B plaintext message, m Same key encryption ciphertext algorithm K (m) A-B KA-B decryption plaintext algorithm m=K A-B ( KA-B(m) ) symmetric key crypto: Bob and Alice share know same (symmetric) key: K A-B r e. g. , key is knowing substitution pattern in mono alphabetic substitution cipher r Q: how do Bob and Alice agree on key value? 8: Network Security 14

A Perfect Cipher: One time Pad Invented by Vernam (AT&T 1926) proved to be A Perfect Cipher: One time Pad Invented by Vernam (AT&T 1926) proved to be unbreakable by Shannon (AT&T 1949) The only known unbreakable cipher! Cipher Text X+Z Clear Text X+ Z+Z =X Clear Text X + + Z Z Key-tape One Time secret Key Unconditional. Secrecyif : One Time secret Key length = Clear text length (Shannon 1949) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 15

Symmetric key cryptosytem: DES: Data Encryption Standard r US encryption standard [NIST 1993/1976] r Symmetric key cryptosytem: DES: Data Encryption Standard r US encryption standard [NIST 1993/1976] r 56 -bit symmetric key, 64 -bit plaintext input r How secure is DES? m DES Challenge: 56 -bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months m no known “backdoor” decryption approach r making DES more secure: m use three keys sequentially (3 -DES) on each datum m use cipher-block chaining 8: Network Security 16

Symmetric key crypto: DES operation initial permutation 16 identical “rounds” of function application, each Symmetric key crypto: DES operation initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation 8: Network Security 17

DES: Data Encryption Standard First introduced 1976 NIST / IBM Message Key 64 Combinational DES: Data Encryption Standard First introduced 1976 NIST / IBM Message Key 64 Combinational Logic Cryptogram 64 Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering 64 W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 18

The Core of DES Cipher In (64 Bits) 1 R 2 R 3 R The Core of DES Cipher In (64 Bits) 1 R 2 R 3 R 16 R Key Map . . Key (64 Bits) Out (64 Bits) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 19

DES Round Structure R L R (32 Bits each) Key: Ki + f (32 DES Round Structure R L R (32 Bits each) Key: Ki + f (32 Bits each) L´ R´ Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 20

Involution An involution is a function that is self-inverse M 2 M 1 F Involution An involution is a function that is self-inverse M 2 M 1 F F M 1 x F F Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering M 2 W. Adi 2005 = 1 F = F-1 Security Design Fundamentals Lecture number: 11301 Page : 21

L R DES Core Involution Ki + f Key function is : L + L R DES Core Involution Ki + f Key function is : L + f(R, Ki) R L + f(R, Ki) f = highly non-linear function ! Nobody was able since 1976 to break this involution mechanism in DES efficiently ! Ki + f R = L + f(R, Ki) Same as input ! L Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 22

AES: Advanced Encryption Standard r new (Nov. 2001) symmetric-key NIST standard, replacing DES r AES: Advanced Encryption Standard r new (Nov. 2001) symmetric-key NIST standard, replacing DES r processes data in 128 bit blocks r 128, 192, or 256 bit keys r brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES 8: Network Security 23

AES Advanced Encryption Standard (finalized in 2001) Basic concept Key Expansion Round Keys X AES Advanced Encryption Standard (finalized in 2001) Basic concept Key Expansion Round Keys X K 1 K 2 . . . K 9 K 10 R 1 R 2 . . . R 9 R 10 Y 10 Encryption Rounds R 1 … R 10 Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 24

Basic AES: Encryption Round Functions Clear Text (16 bytes) a 16 Byte sub a Basic AES: Encryption Round Functions Clear Text (16 bytes) a 16 Byte sub a . . b 16 a Byte sub b a 1 b = [M] a-1 + C Byte sub b b 1 The Only non-linear mapping ! Transposition A 4 x 32 bits Mix column B = [C] A Linear mapping B 4 x 32 bits + Round-Key Ki (128 bits) Cipher Text (16 byts) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 25

Public-Key Cryptography Scientific Breakthrough in 1976 Technical University of Braunschweig IDA: Institute of Computer Public-Key Cryptography Scientific Breakthrough in 1976 Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 26

Public Key Cryptography symmetric key crypto r requires sender, receiver know shared secret key Public Key Cryptography symmetric key crypto r requires sender, receiver know shared secret key r Q: how to agree on key in first place (particularly if never “met”)? public key cryptography r radically different approach [Diffie. Hellman 76, RSA 78] r sender, receiver do not share secret key r public encryption key known to all r private decryption key known only to receiver 8: Network Security 27

Public key cryptography + Bob’s public B key K K plaintext message, m encryption Public key cryptography + Bob’s public B key K K plaintext message, m encryption ciphertext algorithm + K (m) B - Bob’s private B key decryption plaintext algorithm message + m = K B(K (m)) B 8: Network Security 28

Public-Key Secrecy Systems - K-open K A (m) K-close K + (m) A • Public-Key Secrecy Systems - K-open K A (m) K-close K + (m) A • Every key can remove the effect of the other • Knowing one of then does not lead to the other + K (K (m)) = m A A - Open and close with different keys!! - No Secret Key Agreement required Two Major Schemes in Public Key Cryptography: • Diffie-Hellman Public Key exchange scheme • RSA public Key secrecy system Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 or - + K (K (m)) = m A A Security Design Fundamentals Lecture number: 11301 Page : 29

Sharing Secrets without exchange of secrets (Diffie Public-Key Cryptography Breakthrough 1976 & Hellman) “Mechanical Sharing Secrets without exchange of secrets (Diffie Public-Key Cryptography Breakthrough 1976 & Hellman) “Mechanical Scenario” A Open Register Secret key-B Secret key-A inje B ctio n SHIELD t injec ion ! Same thing ! Shared Secret Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 30

How to “publicly” hide (shield) a secre One-Way function: Secret shielded secret 6 9 How to “publicly” hide (shield) a secre One-Way function: Secret shielded secret 6 9 SHIELD = One Way Function How: 2 6 mod 11 = 9 log 2 9 (mod 11) = 6 Discrete logarithm : no formula is known to compute log 2 9 modulo 11 ! Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 31

Example for Diffie-Hellman key exchange scheme 1976 Widely use in internet and banking. . Example for Diffie-Hellman key exchange scheme 1976 Widely use in internet and banking. . . Open Agreement and Register Shielding function is : y = (5 x) mod 7 A 3 Secret key-A= 3 5 = 6 K-open-A= 6 53 Secret key-B= 5 55 3 55 ) 5 5. 3 Shield ! same thing ! Z=6 Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering 55=3 3 6 3 ( K-open-B= B W. Adi 2005 ( ) 5 53 5 3. 5 Security Design Fundamentals Lecture number: 11301 Page : 32

Basic Public Key Secrecy System (RSA system) (Mechanical simulation: user B gets a secured Basic Public Key Secrecy System (RSA system) (Mechanical simulation: user B gets a secured message from A) User A User B Public register Ko= Kc-1 Close Kc Kc M ( )Kc (mod m) MKc. Ko = M open Ko MKc (MKc)Ko Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 33

Mathematical Model of a Public-Key Secrecy-system (using asymmetric keys) Sender Message X Receiver E Mathematical Model of a Public-Key Secrecy-system (using asymmetric keys) Sender Message X Receiver E ( Zp, X ) Y = E (Zp, X) D ( Zs, Y ) X Message Channel Zp Zs Secret-Key Zs Public-Key Zp Public Directory Z. . Zp Z. . . Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 34

RSA Public Key Secrecy System 1978 USER A: Open directory Na = pa. qa RSA Public Key Secrecy System 1978 USER A: Open directory Na = pa. qa open modulus of A pa. qa tow secret large primes (Na) = (pa-1). (qa -1) Ea = open Encryption key of A Da = Ea-1 [mod (Na) ] User A Na Ea USER B: Nb = pb. qb open modulus of B pb. qb tow secret large primes (Nb) = (pb-1). (qb -1) User B Nb Eb Eb = open Encryption key of B Db = Eb-1 [mod (Nb) ] Condition: gcd [Ea , (Na) ] = 1 Condition: gcd [ Eb , (Nb) ] = 1 Number of possible keys = [ (Na)] Number of possible keys = [ (Nb)] A sends Message M to B: Y= MEb mod Nb (Encrypt) Db Y =M mod Nb =M (Decrypt) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering Eb Db W. Adi 2007 Security Design Fundamentals Lecture number: 11301 Page : 35

Example: Construct RSA secrecy system using the two prime pairs 7, 5 and 3, Example: Construct RSA secrecy system using the two prime pairs 7, 5 and 3, 11. Encrypt the message M=2 sent to user B Solution: Open directory USER A: Na = 7 x 5 open modulus of A pa. qa two secret large primes (Na) = (pa-1). (qa -1)=24 5= open Encryption key of A Da = 5 -1 [mod 24) ] =5 User A 35 5 USER B: Nb = 3. 11 open modulus of B pb. qb two secret large primes (Nb) = (pb-1). (qb -1) =20 User B 33 3 3 = open Encryption key of B Db = Eb-1 [mod (Nb) ]=7 gcd [Ea , (Na) ] = 1 gcd [ Eb , (Nb) ] = 1 A sends Message M=2 to B: 3 Y= 2 mod 33=8 Y =8 7 8 mod 33 = 2 = M (Decrypt) (Encrypt) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2007 Security Design Fundamentals Lecture number: 11301 Page : 36

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 37

Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. 0: Alice says “I am Alice” Failure scenario? ? 8: Network Security 38

Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. 0: Alice says “I am Alice” in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice 8: Network Security 39

Authentication: another try Protocol ap 2. 0: Alice says “I am Alice” in an Authentication: another try Protocol ap 2. 0: Alice says “I am Alice” in an IP packet containing her source IP address Alice’s “I am Alice” IP address Failure scenario? ? 8: Network Security 40

Authentication: another try Protocol ap 2. 0: Alice says “I am Alice” in an Authentication: another try Protocol ap 2. 0: Alice says “I am Alice” in an IP packet containing her source IP address Alice’s IP address Trudy can create a packet “spoofing” “I am Alice” Alice’s address 8: Network Security 41

Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends her secret password to “prove” it. Alice’s “I’m Alice” IP addr password Alice’s IP addr OK Failure scenario? ? 8: Network Security 42

Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends her secret password to “prove” it. Alice’s “I’m Alice” IP addr password Alice’s IP addr OK playback attack: Trudy records Alice’s packetand later plays it back to Bob Alice’s “I’m Alice” IP addr password 8: Network Security 43

Authentication: yet another try Protocol ap 3. 1: Alice says “I am Alice” and Authentication: yet another try Protocol ap 3. 1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Alice’s encrypted “I’m Alice” IP addr password Alice’s IP addr OK Failure scenario? ? 8: Network Security 44

Authentication: another try Protocol ap 3. 1: Alice says “I am Alice” and sends Authentication: another try Protocol ap 3. 1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Alice’s encrypted “I’m Alice” IP addr password Alice’s IP addr OK record and playback still works! Alice’s encrypted “I’m Alice” IP addr password 8: Network Security 45

Authentication: yet another try Goal: avoid playback attack Nonce: number (R) used only once Authentication: yet another try Goal: avoid playback attack Nonce: number (R) used only once –in-a-lifetime ap 4. 0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R KA-B(R) Failures, drawbacks? Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice! 8: Network Security 46

Authentication: ap 5. 0 ap 4. 0 requires shared symmetric key r can we Authentication: ap 5. 0 ap 4. 0 requires shared symmetric key r can we authenticate using public key techniques? ap 5. 0: use nonce, public key cryptography “I am Alice” R Bob computes + - - K A (R) “send me your public key” + KA KA(KA (R)) = R and knows only Alice could have the private key, that encrypted R such that + K (K (R)) = R A A 8: Network Security 47

ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) I am Alice R K (R) A Send me your public key - + m = K (K (m)) A A + K (m) A K T K (R) Send me your public key + A Trudy gets - + m = K (K (m)) sends T to Alice m T + K (m) T encrypted with Alice’s public key 8: Network Security 48

ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Difficult to detect: q Bob receives everything that Alice sends, and vice versa. (e. g. , so Bob, Alice can meet one week later and recall conversation) q problem is that Trudy receives all messages as well! 8: Network Security 49

Practical Secured Identity (Authentic Ide International Mobile Equipment Identity IMEI (non-secured) Subscriber Identity Module Practical Secured Identity (Authentic Ide International Mobile Equipment Identity IMEI (non-secured) Subscriber Identity Module SIM (secured) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 50

Challenge-Response Identification Mechanisms Well known Explicit Secret Key Signature Authenticity without Secrecy Prover A Challenge-Response Identification Mechanisms Well known Explicit Secret Key Signature Authenticity without Secrecy Prover A Set up: Agree on a secret key Ki and a One-Way Function F Verifier Ki Ki Who are you ? : Prove by using R that you know Ki Authentication Request Generate a random R (Challenge) I am A, and this is the proof : RES (Response) If RES = F(Ki , R) then accept RES=F(Ki , R) Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering W. Adi 2005 Security Design Fundamentals Lecture number: 11301 ? Page : 51

GSM Authentication: Challenge-Response Subscriber Identification Mechanism Mobil-Station Identity key max. 128 Bit Ki RAND GSM Authentication: Challenge-Response Subscriber Identification Mechanism Mobil-Station Identity key max. 128 Bit Ki RAND Verifier-Station Random Generator 128 bits RAND Authentication request A 3/COMP 128 32 Bit XRES SIM Card Ki A 3/COMP 128 XRES Authentication response XRES = Authentication Result Technical University of Braunschweig IDA: Institute of Computer and Communication Network Engineering RAND W. Adi 2005 Security Design Fundamentals Lecture number: 11301 Page : 52

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Message integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 53

Digital Signatures Cryptographic technique analogous to handwritten signatures. r sender (Bob) digitally signs document, Digital Signatures Cryptographic technique analogous to handwritten signatures. r sender (Bob) digitally signs document, establishing he is document owner/creator. r verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document 8: Network Security 54

Digital Signatures (public key) Simple digital signature for message m: r Bob signs m Digital Signatures (public key) Simple digital signature for message m: r Bob signs m by encrypting with his private key -, creating “signed” message, K- (m) K B Bob’s private Bob’s message, m key Dear Alice Public key encryption algorithm Oh, how I have missed you. I think of you all the time! …(blah) Bob (m) If equal Then accept - K B(m) + K (K (m)) = m B B Public key decryption algorithm Bob’s message, m, signed (encrypted) signe with his private key - K B(m) verify + K B Bob’s public key 8: Network Security 55

Digital Signatures (more) - r Suppose Alice receives msg m, digital signature K B(m) Digital Signatures (more) - r Suppose Alice receives msg m, digital signature K B(m) r Alice verifies m signed by Bob by applying Bob’s + - public key KB to KB(m) then checks KB(KB(m) ) = m. + - r If KB(KB(m) ) = m, whoever signed m must have used Bob’s private key. Alice thus verifies that: ü Bob signed m. ü No one else signed m. ü Bob signed m and not m’. Non-repudiation: ü Alice can take m, and signature KB(m) to court and prove that Bob signed m. 8: Network Security 56

Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easyto-compute digital “fingerprint” r Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easyto-compute digital “fingerprint” r apply hash function H to m, get fixed size message digest, H(m). large message m H: Hash Function H(m) Hash function properties: r many-to-1 r produces fixed-size msg digest (fingerprint) r given message digest x, computationally infeasible to find m such that x = H(m) 8: Network Security 57

Internet checksum: poor crypto hash function Internet checksum has some properties of hash function: Internet checksum: poor crypto hash function Internet checksum has some properties of hash function: ü produces fixed length digest (16 -bit sum) of message ü is many-to-one But given message with given hash value, it is easy to find another message with same hash value: message IOU 1 00. 9 9 BOB ASCII format 49 4 F 55 31 30 30 2 E 39 39 42 4 F 42 B 2 C 1 D 2 AC message IOU 9 00. 1 9 BOB different messages but identical checksums! ASCII format 49 4 F 55 39 30 30 2 E 31 39 42 4 F 42 B 2 C 1 D 2 AC 8: Network Security 58

Digital signature = signed message digest Alice verifies signature and integrity of digitally signed Digital signature = signed message digest Alice verifies signature and integrity of digitally signed message: Bob sends digitally signed message: large message m H: Hash function Bob’s private key + - KB encrypted msg digest H(m) digital signature (encrypt) encrypted msg digest KB(H(m)) large message m H: Hash function KB(H(m)) Bob’s public key + KB digital signature (decrypt) H(m) equal ? 8: Network Security 59

Hash Function Algorithms r MD 5 hash function widely used (RFC 1321) m computes Hash Function Algorithms r MD 5 hash function widely used (RFC 1321) m computes 128 -bit message digest in 4 -step process. m arbitrary 128 -bit string x, appears difficult to construct msg m whose MD 5 hash is equal to x. r SHA-1 is also used. m US standard [NIST, FIPS PUB 180 -1] m 160 -bit message digest 8: Network Security 60

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 61

Trusted Intermediaries Symmetric key problem: Public key problem: r How do two entities establish Trusted Intermediaries Symmetric key problem: Public key problem: r How do two entities establish shared secret key over network? Solution: r trusted key distribution center (KDC) acting as intermediary between entities r When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: r trusted certification authority (CA) 8: Network Security 62

Key Distribution Center (KDC) r Alice, Bob need shared symmetric key. r KDC: server Key Distribution Center (KDC) r Alice, Bob need shared symmetric key. r KDC: server shares different secret key with each registered user (many users) r Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC KA-KDC KP-KDC KB-KDC KA-KDC KX-KDC KY-KDC KB-KDC KZ-KDC 8: Network Security 63

Key Distribution Center (KDC) (Secret key) Q: How does KDC allow Bob, Alice to Key Distribution Center (KDC) (Secret key) Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R 1 KA-KDC(A, B) Alice knows R 1 KA-KDC(R 1, KB-KDC(A, R 1) ) KB-KDC(A, R 1) Bob knows to use R 1 to communicate with Alice and Bob communicate: using R 1 as session key for shared symmetric encryption 8: Network Security 64

Certification Authorities (Public key) r Certification authority (CA): binds public key to particular entity, Certification Authorities (Public key) r Certification authority (CA): binds public key to particular entity, E. r E (person, router) registers its public key with CA. m m m 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 Bob’s identifying information + KB digital signature (encrypt) CA private key K- CA + KB certificate for Bob’s public key, signed by CA 8: Network Security 65

Certification Authorities r When Alice wants Bob’s public key: m gets Bob’s certificate (Bob Certification Authorities r When Alice wants Bob’s public key: m gets Bob’s certificate (Bob or elsewhere). m apply CA’s public key to Bob’s certificate, get Bob’s public key + KB digital signature (decrypt) CA public key + KB Bob’s public key + K CA 8: Network Security 66

A certificate contains: r Serial number (unique to issuer) r info about certificate owner, A certificate contains: r Serial number (unique to issuer) r info about certificate owner, including algorithm and key value itself (not shown) r info about certificate issuer r valid dates r digital signature by issuer 8: Network Security 67

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 68

Firewalls firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, Firewalls firewall isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others. public Internet administered network firewall 8: Network Security 69

Firewalls: Why prevent denial of service attacks: m SYN flooding: attacker establishes many bogus Firewalls: Why prevent denial of service attacks: m SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections. prevent illegal modification/access of internal data. m e. g. , attacker replaces CIA’s homepage with something else allow only authorized access to inside network (set of authenticated users/hosts) two types of firewalls: m application-level m packet-filtering 8: Network Security 70

Packet Filtering Should arriving packet be allowed in? Departing packet let out? r internal Packet Filtering Should arriving packet be allowed in? Departing packet let out? r internal network connected to Internet via router firewall r router filters packet-by-packet, decision to forward/drop packet based on: m m source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits 8: Network Security 71

Packet Filtering r Example 1: block incoming and outgoing datagrams with IP protocol field Packet Filtering r Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. m All incoming and outgoing UDP flows and telnet connections are blocked. r Example 2: Block inbound TCP segments with ACK=0. m Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside. 8: Network Security 72

Application gateways r Filters packets on application data as well as on IP/TCP/UDP fields. Application gateways r Filters packets on application data as well as on IP/TCP/UDP fields. r Example: allow select internal users to telnet outside. host-to-gateway telnet session application gateway-to-remote host telnet session router and filter 1. Require all telnet users to telnet through gateway. 2. For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections 3. Router filter blocks all telnet connections not originating from gateway. 8: Network Security 73

Limitations of firewalls and gateways r IP spoofing: router can’t know if data “really” Limitations of firewalls and gateways r IP spoofing: router can’t know if data “really” comes from claimed source r if multiple app’s. need special treatment, each has own app. gateway. r client software must know how to contact gateway. m r filters often use all or nothing policy for UDP. r tradeoff: degree of communication with outside world, level of security r many highly protected sites still suffer from attacks. e. g. , must set IP address of proxy in Web browser 8: Network Security 74

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8: Network Security 75

Internet security threats Mapping: m before attacking: “case the joint” – find out what Internet security threats Mapping: m before attacking: “case the joint” – find out what services are implemented on network m Use ping to determine what hosts have addresses on network m Port-scanning: try to establish TCP connection to each port in sequence (see what happens) m nmap (http: //www. insecure. org/nmap/) mapper: “network exploration and security auditing” Countermeasures? 8: Network Security 76

Internet security threats Mapping: countermeasures m record traffic entering network m look for suspicious Internet security threats Mapping: countermeasures m record traffic entering network m look for suspicious activity (IP addresses, ports being scanned sequentially) 8: Network Security 77

Internet security threats Packet sniffing: m broadcast media m promiscuous NIC reads all packets Internet security threats Packet sniffing: m broadcast media m promiscuous NIC reads all packets passing by m can read all unencrypted data (e. g. passwords) m e. g. : C sniffs B’s packets C A src: B dest: A payload B Countermeasures? 8: Network Security 78

Internet security threats Packet sniffing: countermeasures m all hosts in organization run software that Internet security threats Packet sniffing: countermeasures m all hosts in organization run software that checks periodically if host interface in promiscuous mode. m one host per segment of broadcast media (switched Ethernet at hub) C A src: B dest: A payload B 8: Network Security 79

Internet security threats IP Spoofing: m can generate “raw” IP packets directly from application, Internet security threats IP Spoofing: m can generate “raw” IP packets directly from application, putting any value into IP source address field m receiver can’t tell if source is spoofed m e. g. : C pretends to be B C A src: B dest: A Countermeasures? payload B 8: Network Security 80

Internet security threats IP Spoofing: ingress filtering m routers should not forward outgoing packets Internet security threats IP Spoofing: ingress filtering m routers should not forward outgoing packets with invalid source addresses (e. g. , datagram source address not in router’s network) m great, but ingress filtering can not be mandated for all networks C A src: B dest: A payload B 8: Network Security 81

Internet security threats Denial of service (DOS): m flood of maliciously generated packets “swamp” Internet security threats Denial of service (DOS): m flood of maliciously generated packets “swamp” receiver m Distributed DOS (DDOS): multiple coordinated sources swamp receiver m e. g. , C and remote host SYN-attack A C A SYN SYN SYN B Countermeasures? SYN 8: Network Security 82

Internet security threats Denial of service (DOS): countermeasures m filter out flooded packets (e. Internet security threats Denial of service (DOS): countermeasures m filter out flooded packets (e. g. , SYN) before reaching host: throw out good with bad m traceback to source of floods (most likely an innocent, compromised machine) C A SYN SYN SYN B SYN 8: Network Security 83

Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography Chapter 8 roadmap 8. 1 What is network security? 8. 2 Principles of cryptography 8. 3 Authentication 8. 4 Integrity 8. 5 Key Distribution and certification 8. 6 Access control: firewalls 8. 7 Attacks and counter measures 8. 8 Security in many layers 8. 8. 1. Secure email 8. 8. 2. Secure sockets 8. 8. 3. IPsec 8. 8. 4. Security in 802. 11 8: Network Security 84

Secure e-mail q Alice wants to send confidential e-mail, m, to Bob. KS m Secure e-mail q Alice wants to send confidential e-mail, m, to Bob. KS m KS Alice: K (. ) S + . K B( ) K+ B KS(m ) Encrypted Message + Internet + KS(m ) + KB(KS ) Encrypted Key q generates random symmetric private key, KS. q encrypts message with KS (for efficiency) q also encrypts KS with Bob’s public key. q sends both KS(m) and KB(KS) to Bob. . K S( ) m KS - . K B( ) KB Bob: q uses his private key to decrypt and recover KS q uses KS to decrypt KS(m) to recover m 8: Network Security 85

Secure e-mail (continued) • Alice wants to provide sender authentication message integrity. m . Secure e-mail (continued) • Alice wants to provide sender authentication message integrity. m . H( ) KA - . K A( ) - - KA(H(m)) + + KA Internet m m + . K A( ) H(m ) compare . H( ) H(m ) • Alice digitally signs message. • sends both message (in the clear) and digital signature. 8: Network Security 86

Secure e-mail (continued) • Alice wants to provide secrecy, sender authentication, message integrity. m Secure e-mail (continued) • Alice wants to provide secrecy, sender authentication, message integrity. m . H( ) KA - Signature . K A( ) - KA(H(m)) + Message Encryption . K S( ) m KS KS + . K B( ) K+ B + Internet + KB(KS ) Encrypted key Alice uses three keys: her private key, Bob’s public key, newly created symmetric key 8: Network Security 87

Pretty good privacy (PGP) PGP • • Internet e-mail encryption scheme, defacto standard. uses Pretty good privacy (PGP) PGP • • Internet e-mail encryption scheme, defacto standard. uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. • provides secrecy, sender authentication, integrity. • inventor, Phil Zimmerman, was target of 3 -year federal investigation. A PGP signed message: ---BEGIN PGP SIGNED MESSAGE--Hash: SHA 1 Bob: My husband is out of town tonight. Passionately yours, Alice ---BEGIN PGP SIGNATURE--Version: PGP 5. 0 Charset: noconv yh. HJRHh. GJGhgg/12 Ep. J+lo 8 g. E 4 v. B 3 mq. Jh FEv. ZP 9 t 6 n 7 G 6 m 5 Gw 2 ---END PGP SIGNATURE--- 8: Network Security 88

Secure sockets layer (SSL) SSL r transport layer security to any TCPbased app using Secure sockets layer (SSL) SSL r transport layer security to any TCPbased app using SSL services. r used between Web browsers, servers for e-commerce (shttp). r security services: m m m server authentication data encryption client authentication (optional) TCP: Transmission Control Protocol r server authentication: m SSL-enabled browser includes public keys for trusted CAs. m Browser requests server certificate, issued by trusted CA. m Browser uses CA’s public key to extract server’s public key from certificate. r check your browser’s security menu to see its trusted CAs. 8: Network Security 89

SSL (continued) Encrypted SSL session: r Browser generates symmetric session key, encrypts it with SSL (continued) Encrypted SSL session: r Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. r Using private key, server decrypts session key. r Browser, server know session key m r SSL: basis of IETF Transport Layer Security (TLS). r SSL can be used for non-Web applications, e. g. , IMAP. r Client authentication can be done with client certificates. All data sent into TCP socket (by client or server) encrypted with session key. IMAP: Internet Message Access Protocol 8: Network Security 90

IPsec: Network Layer Security IPsec r Network-layer secrecy: secrecy m sending host encrypts the IPsec: Network Layer Security IPsec r Network-layer secrecy: secrecy m sending host encrypts the data in IP datagram m TCP and UDP segments; ICMP and SNMP messages. r Network-layer authentication m destination host can authenticate source IP address r Two principle protocols: m authentication header (AH) protocol m encapsulation security payload (ESP) protocol r For both AH and ESP, source, destination handshake: m create network-layer logical channel called a security association (SA) r Each SA unidirectional. r Uniquely determined by: m security protocol (AH or ESP) m source IP address m 32 -bit connection ID 8: Network Security 91

Authentication Header (AH) Protocol r provides source authentication, data integrity, no confidentiality r AH Authentication Header (AH) Protocol r provides source authentication, data integrity, no confidentiality r AH header inserted between IP header, data field. r protocol field: 51 r intermediate routers process datagrams as usual IP header AH header includes: r connection identifier r authentication data: source- signed message digest calculated over original IP datagram. r next header field: specifies type of data (e. g. , TCP, UDP, ICMP) data (e. g. , TCP, UDP segment) 8: Network Security 92

ESP Protocol r provides secrecy, host r ESP authentication, data field is similar to ESP Protocol r provides secrecy, host r ESP authentication, data field is similar to AH integrity. authentication field. r data, ESP trailer encrypted. r Protocol = 50. r next header field is in ESP trailer. authenticated encrypted IP header ESP TCP/UDP segment header trailer ESP authent. 8: Network Security 93

IEEE 802. 11 security r War-driving: drive around Bay area, see what 802. 11 IEEE 802. 11 security r War-driving: drive around Bay area, see what 802. 11 networks available? m More than 9000 accessible from public roadways m 85% use no encryption/authentication m packet-sniffing and various attacks easy! r Securing 802. 11 m encryption, authentication m first attempt at 802. 11 security: Wired Equivalent Privacy (WEP): a failure m current attempt: 802. 11 i 8: Network Security 94

Wired Equivalent Privacy (WEP): WEP r authentication as in protocol ap 4. 0 m Wired Equivalent Privacy (WEP): WEP r authentication as in protocol ap 4. 0 m host requests authentication from access point sends 128 bit nonce m host encrypts nonce using shared symmetric key m access point decrypts nonce, authenticates host r no key distribution mechanism r authentication: knowing the shared key is enough 8: Network Security 95

WEP data encryption r Host/AP share 40 bit symmetric key (semi- permanent) r Host WEP data encryption r Host/AP share 40 bit symmetric key (semi- permanent) r Host appends 24 -bit initialization vector (IV) to create 64 -bit key r 64 bit key used to generate stream of keys, ki. IV IV r ki used to encrypt ith byte, di, in frame: ci = di XOR ki. IV r IV and encrypted bytes, ci sent in frame 8: Network Security 96

802. 11 WEP encryption Sender-side WEP encryption 8: Network Security 97 802. 11 WEP encryption Sender-side WEP encryption 8: Network Security 97

Breaking 802. 11 WEP encryption Security hole: r 24 -bit IV, one IV per Breaking 802. 11 WEP encryption Security hole: r 24 -bit IV, one IV per frame, -> IV’s eventually reused r IV transmitted in plaintext -> IV reuse detected r Attack: m Trudy causes Alice to encrypt known plaintext d 1 d 2 d 3 d 4 … IV m Trudy sees: ci = di XOR ki IV m Trudy knows ci di, so can compute ki IV IV IV m Trudy knows encrypting key sequence k 1 k 2 k 3 … m Next time IV is used, Trudy can decrypt! 8: Network Security 98

802. 11 i: improved security r numerous (stronger) forms of encryption possible r provides 802. 11 i: improved security r numerous (stronger) forms of encryption possible r provides key distribution r uses authentication server separate from access point 8: Network Security 99

802. 11 i: four phases of operation STA: client station AP: access point AS: 802. 11 i: four phases of operation STA: client station AP: access point AS: Authentication server wired network 1 Discovery of security capabilities 2 STA and AS mutually authenticate, together generate Master Key (MK). AP servers as “pass through” 3 STA derives Pairwise Master Key (PMK) 4 STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity 3 AS derives same PMK, sends to AP 8: Network Security 100

EAP: extensible authentication protocol r EAP: end-end client (mobile) to authentication server protocol r EAP: extensible authentication protocol r EAP: end-end client (mobile) to authentication server protocol r EAP sent over separate “links” m mobile-to-AP (EAP over LAN) m AP to authentication server (RADIUS over UDP) wired network EAP TLS EAP over LAN (EAPo. L) IEEE 802. 11 RADIUS UDP/IP 8: Network Security 101

Network Security (summary) Basic techniques…. . . m cryptography (symmetric and public) m authentication Network Security (summary) Basic techniques…. . . m cryptography (symmetric and public) m authentication m message integrity m key distribution …. used in many different security scenarios m secure email m secure transport (SSL) m IP sec m 802. 11 8: Network Security 102