1. 8-1 Chapter 8 Security Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)  If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved

2. 8-2Network Security Chapter 8: Network Security Chapter goals: To learn how computer networks can be defended  understand principles of network security:  cryptography and its many uses beyond “confidentiality”  (end-point) authentication  message integrity  security in practice:  security in application, transport, network, link layers  firewalls and intrusion detection systems

3. 8-3Network Security Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography (confidentiality) 8.3 Message integrity 8.4 End-point authentication 8.5 Securing e-mail (application) 8.6 Securing TCP connections: SSL (transport) 8.7 Network layer security: IPsec (network) 8.8 Securing wireless LANs (MAC) 8.9 Operational security: firewalls and IDS

4. 8-4Network Security What is network security? Desirable properties of secure communications 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 CIA

5. 8-5Network Security Friends and enemies: Alice, Bob, Trudy  well-known in network security world  Bob and Alice want to communicate “securely”  Trudy (intruder) may eavesdrop (sniff and record), modify, delete, insert messages secure sender s secure receiver channel data, control messages data Alice Bob Trudy

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

7. 8-7Network Security 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)

8. 8-8Network Security Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography (confidentiality) 8.3 Message integrity 8.4 End-point authentication 8.5 Securing e-mail (application) 8.6 Securing TCP connections: SSL (transport) 8.7 Network layer security: IPsec (network) 8.8 Securing wireless LANs (MAC) 8.9 Operational security: firewalls and IDS

9. 8-9 Security: many facets  Alice and Bob would like for the contents of their communication to remain secret from an eavesdropper  They probably would also like to make sure that when they are communicating, they are indeed communicating with each other  If their communication is tampered with by an eavesdropper, that this tampering is detected Cryptography: encrypting communication, authenticating the party with whom one is communicating, and ensuring message integrity Network Security

10. 8-10Network Security The language of cryptography m plaintext message K A (m) ciphertext, encrypted with key K A m = K B (K A (m)) Typically, the encryption algorithm is well known plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B

11. 8-11Network Security Symmetric key cryptography 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 S K S plaintext message, m K (m) S m = K S (K S (m))

12. 8-12Network Security Simple encryption scheme 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.: Encryption key: mapping from set of 26 letters to set of 26 letters

13. 8-13Network Security Breaking an encryption scheme  cipher-text only attack: Trudy has ciphertext she can analyze  two approaches:  brute force: search through all keys  statistical analysis  known-plaintext attack: Trudy has plaintext corresponding to ciphertext  e.g., in monoalphabetic cipher, Trudy determines pairings for a,l,i,c,e,b,o,  chosen-plaintext attack: Trudy can get ciphertext for chosen plaintext

14. 8-14Network Security A more sophisticated encryption approach  n substitution 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 subsitution pattern in cyclic pattern  dog: d from M 1, o from M 3, g from M 4 Encryption key: n substitution ciphers, and cyclic pattern  key need not be just n-bit pattern

15. 8-15Network Security Symmetric key crypto: DES DES: Data Encryption Standard  US encryption standard [NIST 1993]  56-bit symmetric key, 64-bit plaintext input  block cipher with cipher block chaining  how secure is DES?  DES Challenge: 56-bit-key-encrypted phrase decrypted (brute force) in less than a day  no known good analytic attack  making DES more secure:  3DES: encrypt 3 times with 3 different keys

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

17. 8-17Network Security AES: Advanced Encryption Standard  symmetric-key NIST standard, replacied DES (Nov 2001)  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

18. 8-18Network Security Public Key Cryptography symmetric key crypto  requires sender and receiver know shared secret key  Q: how to agree on key in the first place (particularly if they never “met”)? public key crypto  radically different approach [Diffie- Hellman76, RSA78]  sender and receiver do not share secret key  public encryption key known to all  private decryption key known only to receiver

19. 8-19Network Security Public key cryptography 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 - Not only for encryption, but for authentication and digital signature as well There are encryption/decryption algorithms and techniques for choosing public and private keys such that K B - (K B + (m)) = m

20. 8-20Network Security Public key encryption algorithms 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 - + + -

21. 8-21 Symmetric Key vs. Public Key  Symmetric key cryptography: the fact that the sender knows the secret key implicitly identifies the sender to the receiver Network Security

22. 8-22Network Security Prerequisite: modular arithmetic  x mod n = remainder of x when divided 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

23. 8-23Network Security RSA: getting ready  message: just a bit pattern  bit pattern can be uniquely represented by an integer number  thus, encrypting a message is equivalent to encrypting a number example:  m = 10010001. 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)

24. 8-24Network Security RSA: Creating public/private key pair 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<n) that has no common factors with z (e, z are “relatively prime”). 4. choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5. public key is (n,e); private key is (n,d). K B + K B -

25. 8-25Network Security RSA: encryption, decryption 0. given (n,e) and (n,d) as computed above 1. to encrypt message m (<n), compute c = m mod n e 2. to decrypt received bit pattern, c, compute m = c mod n d m = (m mod n) e mod n d magic happens! c

26. 8-26Network Security 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). bit pattern m m e c = m mod n e 0000l000 12 24832 17 encrypt: encrypting 8-bit messages. c m = c mod n d 17 481968572106750915091411825223071697 12 c d decrypt:

27. 8-27 p=5, q=7 (e = 5, n = 35) Network Security

28. 8-28Network Security RSA: another important property 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!

29. 8-29Network Security 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 ?

30. 8-30Network Security Why does RSA work?  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

31. 8-31Network Security 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

32. 8-32Network Security RSA in practice: session keys  exponentiation in RSA is computationally intensive  DES is at least 100 times faster than RSA  use public key cryto to establish secure connection, then establish second key – symmetric session key – for encrypting data 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

33. 8-33Network Security Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography (confidentiality) 8.3 Message Integrity and Digital Signatures 8.4 End-point authentication 8.5 Securing e-mail (application) 8.6 Securing TCP connections: SSL (transport) 8.7 Network layer security: IPsec (network) 8.8 Securing wireless LANs (MAC) 8.9 Operational security: firewalls and IDS

34. 8-34 Message Integrity  Also known as message authentication  To authenticate a message sent by Alice, Bob needs to verify  the message indeed originated from Alice  the message was not tampered with on its way to him  Message integrity is a critical concern in just about all secure networking protocols  e.g., OSPF (link-state routing protocol) Network Security

35. 8-35Network Security Cryptography Hash Function Hash function properties:  produces fixed-size message digest  computationally infeasible to find any two different messages x and y such that H(x) = H(y)  i.e., computationally infeasible for an intruder to substitute one message for another message that is protected by the hash function  i.e., given message digest x, computationally infeasible to find m such that x = H(m) large message m H: Hash Function H(m) message digest

36. 8-36Network Security Hash function algorithms  MD5 hash function widely used (RFC 1321)  computes 128-bit message digest in 4-step process  Secure Hash Algorithm (SHA-1)  US standard [NIST, FIPS PUB 180-1]  160-bit message digest

37. 8-37Network Security 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: I O U 1 0 0. 9 9 B O B 49 4F 55 31 30 30 2E 39 39 42 D2 42 message ASCII format B2 C1 D2 AC I O U 9 0 0. 1 9 B O B 49 4F 55 39 30 30 2E 31 39 42 D2 42 message ASCII format B2 C1 D2 AC different messages but identical checksums!

38. 8-38 Message Integrity: attempt one 1. Alice creates message m and calculates the hash H(m) (for example with SHA-1) 2. Alice then appends H(m) to the message m, creating an extended message [m, H(m)], and sends the extended message to Bob 3. Bob receives an extended message [m, h] and calculates H(m). If H(m) == h, Bob concludes that everything is fine  Any issue?  Trudy can create a bogus message m’ in which she says she is Alice, calculate H(m’), and send Bob (m’, H(m’)). When Bob receives the message, everything checks out in Step 3, so Bob doesn’t suspect any funny business Network Security

39. 8-39 Message Authentication Code  In addition to using cryptographic hash functions, Alice and Bob will need a shared secret s (authentication key) 1. Alice creates message m, concatenates s with m to create m + s, and calculates the hash H(m + s) (for example with SHA-1). H(m + s) is called the message authentication code (MAC) 2. Alice then appends the MAC to the message m, creating an extended message [m, H(m + s)], and sends the extended message to Bob 3. Bob receives an extended message (m, h) and knowing s, calculates the MAC H(m + s). If H(m + s) == h, Bob concludes that everything is fine Network Security

40. 8-40 Message Authentication Code Network Security No encryption algorithm used e.g., routing concerns message integrity, not message confidentiality How to distribute shared authentication key s ???

41. 8-41Network Security Digital Signatures (DS) Your signature attests to the fact that you (as opposed to someone else) have acknowledged and/or agreed with the document’s contents DS is a cryptographic technique analogous to hand-written signatures:  sender (Bob) digitally signs document, establishing he is document owner/creator.  must be verifiable and nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document  to prove that a document signed by an individual was indeed signed by that individual (the signature must be verifiable) and that only that individual could have signed the document (the signature cannot be forged)

42. 8-42  How we might design a digital signature scheme?  When Bob signs a message, Bob must put something on the message that is unique to him  Bob could consider attaching a MAC for the signature, where the MAC is created by appending his key (unique to him) to the message, and then taking the hash  But for Alice to verify the signature, she must also have a copy of the key, in which case the key would not be unique to Bob Network Security Digital Signatures (DS)

43. 8-43Network Security 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 m,K - B (m) Digital Signatures Does meet our requirements of being verifiable and nonforgeable??? K - B (m)

44. 8-44Network Security - Alice thus verifies that: üBob signed m üno one else signed m üBob signed m and not m’ (DS also provides MI) non-repudiation: Alice can take m and signature K B (m) to court and prove that Bob signed m (Bob cannot deny!) - Digital signatures  suppose Alice receives msg m with signature: m, 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 ( knowing the public key is of no help in learning the private key) - - - + + +

45. 8-45Network Security 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 m,K - B (m) Digital Signatures What is the drawback of this solution?

46. 8-46Network Security 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, 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 ? Digital signature = signed message digest Sign the hash of a message rather than the message itself

47. 8-47Network Security Public-key certification  Motivation: Trudy plays pizza prank on Bob  Trudy creates an e-mail 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 pepperoni pizzas to Bob  Bob doesn’t even like pepperoni  What can we do???

48. 8-48Network Security Certification Authorities  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

49. 8-49Network Security  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 + Certification authorities

50. 8-50Network Security Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography (confidentiality) 8.3 Message integrity 8.4 End-point authentication 8.5 Securing e-mail (application) 8.6 Securing TCP connections: SSL (transport) 8.7 Network layer security: IPsec (network) 8.8 Securing wireless LANs (MAC) 8.9 Operational security: firewalls and IDS

51. 8-51 End-Point Authentication  The process of one entity proving its identity to another entity over a computer network  People authenticate each other in many way s  We recognize each other’s faces when we meet  We are authenticated by the customs official who checks us against the picture on our passport  Client/server processes that authenticate each other or user authenticates him/herself to an e-mail server  A subtly different problem from proving that a message received at some point in the past did indeed come from that claimed sender Network Security

52. 8-52 End-Point Authentication  When performing authentication over the network, the communicating parties cannot rely on biometric information (e.g., visual or voice)  authentication must be done solely on the basis of messages and data exchanged as part of an authentication protocol  Typically, an authentication protocol would run before the two communicating parties run some other protocol  The authentication protocol first establishes the identities of the parties to each other’s satisfaction  only after authentication do the parties get down to the work at hand Network Security

53. 8-53Network Security Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” Failure scenario??? “I am Alice”

54. 8-54Network Security in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice “I am Alice” Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice”

55. 8-55Network Security Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario??? “I am Alice” Alice’s IP address Alice has a well know IP address

56. 8-56Network Security Trudy can create a packet “spoofing” Alice’s address “I am Alice” Alice’s IP address Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Alice has a well know IP address IP spoofing: IP spoofing can be avoided if Trudy’s first-hop router is configured to forward only datagrams containing Trudy’s IP source address, which, however, is not universally deployed or enforced

57. 8-57Network Security Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario??? “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr Authentication: another try

58. 8-58Network Security playback attack: Trudy eavesdrops and records Alice’s packet and later plays it back to Bob “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I’m Alice” Alice’s IP addr Alice’s password Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Authentication: another try

59. 8-59Network Security Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Failure scenario??? “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr Assume that Alice and Bob share a symmetric secret key

60. 8-60Network Security Eavesdrops, record and playback still works! “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Assume that Alice and Bob share a symmetric secret key

61. 8-61 Why both ap3.0 and ap3.1 failed? Network Security “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I’m Alice” Alice’s IP addr Alice’s password “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password A: Bob could not distinguish between the original authentication of Alice and the later playback of Alice’s original authentication Any solution idea? Hint: How can you differentiate two instances (over time)?

62. 8-62Network Security Goal: avoid playback attack Failures? nonce: number (R) used only once-in-a-lifetime ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R K (R) A-B Alice is live, and only Alice knows (the secret) key to encrypt nonce, so it must be Alice! Authentication: yet another try Drawbacks? R is to ensure Alice is both who she says she is and live! (Assume that Alice and Bob share a symmetric secret key)