Lecture 22 Chapter 15: Security

1. Lecture 22Chapter 15: Security

2. Aspects of Security • Security Services • Enhance the security of data processing systems and information transfers of an organization. • Counter security attacks. • Security Attack • Action that compromises the security of information owned by an organization. • Security Mechanisms • Designed to prevent, detect or recover from a security attack.

3. Security Services • Enhance security of data processing systems and information transfers • Authentication • Assurance that the communicating entity is the one claimed • Authorization • Prevention of the unauthorized use of a resource • Availability • Data is available in a timely manner when needed

4. Security Services • Confidentiality • Protection of data from unauthorized disclosure • Integrity • Assurance that data received is as sent by an authorized entity • Non-Repudiation • Protection against denial by one of the parties in a communication

5. Security Attacks Information source Information destination Normal Flow

6. Security Attacks Information source Information destination Interruption Attack on availability (ability to use desired information or resources)

7. Denial of Service Smurf Attack ICMP = Internet Control Message Protocol ICMP echo (spoofed source address of victim) Sent to IP broadcast address ICMP echo reply Internet 1 SYN Perpetrator Victim 10,000 SYN/ACKs – Victim is dead Innocentreflector sites

8. Security Attacks Information source Information destination Interception Attack on confidentiality (concealment of information)

9. Packet Sniffing Every network interface card has a unique 48-bit Media Access Control (MAC) address, e.g. 00:0D:84:F6:3A:1024 bits assigned by IEEE; 24 by card vendor Packet Sniffer Server Client Network Interface Card allows only packets for this MAC address Packet sniffer sets his card to promiscuous mode to allow all packets

10. Security Attacks Information source Information destination Fabrication Attack on authenticity (identification and assurance of origin of information)

11. IP Address Spoofing • IP addresses are filled in by the originating host • Using source address for authentication • r-utilities (rlogin, rsh, rhosts etc..) C 2.1.1.1 • Can A claim it is B to the server S? • ARP Spoofing • Can C claim it is B to the server S? • Source Routing Internet S 1.1.1.3 A 1.1.1.1 1.1.1.2 B

12. Security Attacks Information source Information destination Modification Attack on integrity (prevention of unauthorized changes)

13. TCP Session Hijack • When is a TCP packet valid? • Address / Port / Sequence Number in window • How to get sequence number? • Sniff traffic • Guess it • Many earlier systems had predictable Initial Sequence Number • Inject arbitrary data to the connection

14. Security Attacks Passive attacks Traffic analysis Message interception eavesdropping, monitoring transmissions Active attacks Masquerade Replay Modification of message contents Denial of service some modification of the data stream

15. Security Mechanism • Feature designed to • Prevent attackers from violating security policy • Detect attackers’ violation of security policy • Recover, continue to function correctly even if attack succeeds. • No single mechanism that will support all services • Authentication, authorization, availability, confidentiality, integrity, non-repudiation

17. Cryptography as a Security Tool • Broadest security tool available • Source and destination of messages cannot be trusted without cryptography • Means to constrain potential senders (sources) and / or receivers (destinations) of messages • Based on secrets (keys)

18. Secure Communication over Insecure Medium

19. Encryption • Encryption algorithm consists of • Set of K keys • Set of M Messages • Set of C ciphertexts (encrypted messages) • A function E : K → (M→C). That is, for each k K, E(k) is a function for generating ciphertexts from messages. • Both E and E(k) for any k should be efficiently computable functions. • A function D : K → (C → M). That is, for each k K, D(k) is a function for generating messages from ciphertexts. • Both D and D(k) for any k should be efficiently computable functions. • An encryption algorithm must provide this essential property: Given a ciphertext c  C, a computer can compute m such that E(k)(m) = c only if it possesses D(k). • Thus, a computer holding D(k) can decrypt ciphertexts to the plaintexts used to produce them, but a computer not holding D(k) cannot decrypt ciphertexts. • Since ciphertexts are generally exposed (for example, sent on the network), it is important that it be infeasible to derive D(k) from the ciphertexts

20. K K A-B A-B K (m) m = K ( ) A-B A-B Symmetric key cryptography symmetric key crypto: Bob and Alice share know 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? encryption algorithm decryption algorithm ciphertext plaintext plaintext message, m K (m) A-B A-B

21. DES operation Symmetric Encryption • Data Encryption Standard • most commonly used symmetric block-encryption algorithm • created by US Govt • Encrypts a block of data at a time initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation

22. Symmetric Encryption (cont) • DES is breakable using brute force • making DES more secure • use three keys sequentially (3-DES) on each datum • use cipher-block chaining • Advanced Encryption Standard (AES) • symmetric-key NIST standard • replacing 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

23. Asymmetric Encryption • Public-key encryption based on each user having two keys: • public key – published key used to encrypt data • private key – key known only to individual user used to decrypt data • Must be an encryption scheme that can be made public without making it easy to figure out the decryption scheme • Most common is RSA block cipher • Efficient algorithm for testing whether or not a number is prime • No efficient algorithm is know for finding the prime factors of a number

24. + K (m) B - + m = K (K (m)) B B Public key cryptography + Bob’s public key K B - Bob’s private key K B encryption algorithm decryption algorithm plaintext message plaintext message, m ciphertext

25. Asymmetric Encryption (Cont.) • Formally, it is computationally infeasible to derive D(kd , N) from E(ke , N), and so E(ke , N) need not be kept secret and can be widely disseminated • E(ke , N) (or just ke) is the public key • D(kd , N) (or just kd) is the private key • N is the product of two large, randomly chosen prime numbers p and q (for example, p and q are 512 bits each) • Encryption algorithm is E(ke , N)(m) = mkemod N, where kesatisfies kekd mod (p−1)(q −1) = 1 • The decryption algorithm is then D(kd , N)(c) = ckdmod N

26. + - K K B B RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<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).

27. 1. To encrypt bit pattern, m, compute e d m = c mod n c = m mod n e (i.e., remainder when m is divided by n) d e m = (m mod n) mod n RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 2. To decrypt received bit pattern, c, compute d (i.e., remainder when c is divided by n) Magic happens! c

28. Asymmetric Encryption Example • For example. make p = 7and q = 13 • We then calculate N = 7∗13 = 91 and (p−1)(q−1) = 72 • We next select kerelatively prime to 72 and< 72, yielding 5 • Finally,we calculate kdsuch that kekdmod 72 = 1, yielding 29 • We how have our keys • Public key, ke, N = 5, 91 • Private key, kd , N = 29, 91 • Encrypting the message 69 with the public key results in the cyphertext 62 • Cyphertext can be decoded with the private key • Public key can be distributed in cleartext to anyone who wants to communicate with holder of public key

29. Encryption and Decryption using RSA Asymmetric Cryptography

30. Cryptography (Cont.) • Note symmetric cryptography based on transformations, asymmetric based on mathematical functions • Asymmetric much more compute intensive • Typically not used for bulk data encryption • Many times a combination is used: • use public key cryptography to share a secret key. • use the secret key to encrypt the bulk of the communication.

32. Authentication • Constraining set of potential senders of a message • Complementary and sometimes redundant to encryption • Also can prove message unmodified • Algorithm components • A set K of keys • A set M of messages • A set A of authenticators • A function S : K → (M→ A) • That is, for each k K, S(k) is a function for generating authenticators from messages • Both S and S(k) for any k should be efficiently computable functions • A function V : K → (M× A→ {true, false}). • That is, for each k K, V(k) is a function for verifying authenticators on messages • Both V and V(k) for any k should be efficiently computable functions

33. Authentication (Cont.) • For a message m, a computer can generate an authenticator a A such that V(k)(m, a) = true only if it possesses S(k) • Thus, computer holding S(k) can generate authenticators on messages so that any other computer possessing V(k) can verify them • Computer not holding S(k) cannot generate authenticators on messages that can be verified using V(k) • Since authenticators are generally exposed (for example, they are sent on the network with the messages themselves), it must not be feasible to derive S(k) from the authenticators

34. Authentication – Hash Functions • Basis of authentication • Creates small, fixed-size block of data (message digest, hash value) from m • Hash Function H must be collision resistant on m • Must be infeasible to find an m’ ≠ m such that H(m) = H(m’) • If H(m) = H(m’), then m = m’ • The message has not been modified • Common message-digest functions include • MD5, which produces a 128-bit hash • SHA-1, which outputs a 160-bit hash

35. Authentication - MAC • Symmetric encryption used in message-authentication code (MAC) authentication algorithm • Simple example: • MAC defines S(k)(m) = f (k, H(m)) • Where f is a function that is one-way on its first argument • k cannot be derived from f (k, H(m)) • Because of the collision resistance in the hash function, reasonably assured no other message could create the same MAC • A suitable verification algorithm is V(k)(m, a) ≡ ( f (k,m) = a) • Note that k is needed to compute both S(k) and V(k), so anyone able to compute one can compute the other

36. H(.) public Internet m m m append H(.) H(m+s) H(m+s) H(m+s) H(m+s) compare Message Authentication Code (shared secret) s (message) s (shared secret)

37. Authentication – Digital Signature • Based on asymmetric keys and digital signature algorithm • Authenticators produced are digital signatures • In a digital-signature algorithm, computationally infeasible to derive S(ks) from V(kv) • V is a one-way function • Thus, kvis the public key and ksis the private key • Consider the RSA digital-signature algorithm • Similar to the RSA encryption algorithm, but the key use is reversed • Digital signature of message S(ks)(m) = H(m)ks mod N • The key ksagain is a pair d, N, where N is the product of two large, randomly chosen prime numbers p and q • Verification algorithm is V(kv)(m, a) ≡ (akvmod N = H(m)) • Where kvsatisfies kvksmod (p − 1)(q − 1) = 1

38. 8: Network Security simple digital signature for message m: Bob “signs” m by encrypting with his private key KB, creating “signed” message, KB(m) - - K K B B Digital Signatures - - Bob’s private key Bob’s message, m (m) Dear Alice Oh, how I have missed you. I think of you all the time! …(blah blah blah) Bob Bob’s message, m, signed (encrypted) with his private key public key encryption algorithm

39. suppose Alice receives msg m, digital signature KB(m) Alice verifies m signed by Bob by applying Bob’s public key KB to KB(m) then checks KB(KB(m) ) = m. if KB(KB(m) ) = m, whoever signed m must have used Bob’s private key. Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m’. non-repudiation: Alice can take m, and signature KB(m) to court and prove that Bob signed m. Digital Signatures - - - + + - + -

40. Digital signature = signed MAC H: hash function H: hash function large message m large message m + - digital signature (decrypt) digital signature (encrypt) K K B B encrypted msg digest encrypted msg digest + - - KB(H(m)) KB(H(m)) H(m) H(m) Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: H(m) Bob’s private key Bob’s public key equal ?

41. Authentication (Cont.) • Why authentication if a subset of encryption? • Fewer computations (except for RSA digital signatures) • Authenticator usually shorter than message • Sometimes want authentication but not confidentiality • Signed patches et al • Can be basis for non-repudiation

42. Key Distribution • Delivery of symmetric key is huge challenge • Sometimes done out-of-band • Asymmetric keys can proliferate • stored on key ring • Even asymmetric key distribution needs care • man-in-the-middle attack

43. Man-in-the-middle Attack on Asymmetric Cryptography

44. Digital Certificates • Proof of who or what owns a public key • Public key digitally signed a trusted party • Trusted party receives proof of identification from entity and certifies that public key belongs to entity • Certificate authority are trusted party – their public keys included with web browser distributions • They vouch for other authorities via digitally signing their keys, and so on

45. + + digital signature (encrypt) K K B B - K K CA CA + (K ) B Certification Authorities • Certification Authority (CA): binds public key to particular entity, E. • E 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 CA private key certificate for Bob’s public key, signed by CA - Bob’s identifying information

46. + + digital signature (decrypt) K K B B - K K CA CA + (K ) B Certification Authorities • 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 CA public key +