CS 54001 -1 Large-Scale Networked Systems Professor Ian

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CS 54001 -1: Large-Scale Networked Systems Professor: Ian Foster TAs: Xuehai Zhang, Yong Zhao Lecture 4 www. classes. cs. uchicago. edu/classes/archive/2003/winter/54001 -1

Week 1: Internet Design Principles & Protocols l An introduction to the mail system l An introduction to the Internet l Internet design principles and layering l Brief history of the Internet l Packet switching and circuit switching l Protocols l Addressing and routing l Performance metrics l A detailed FTP example CS 54001 -1 Winter Quarter 2

Week 2: Routing and Transport l Routing techniques – Flooding – Distributed Bellman Ford Algorithm – Dijkstra’s Shortest Path First Algorithm l Routing in the Internet – Hierarchy and Autonomous Systems – Interior Routing Protocols: RIP, OSPF – Exterior Routing Protocol: BGP l Transport: achieving reliability l Transport: achieving fair sharing of links CS 54001 -1 Winter Quarter 3

Week 3: Measurement & Characterization l What does the Internet look like? l What does Internet traffic look like? l How do I measure such things? l How do such characteristics evolve? l l What Internet characteristics are shared with other networks? Are all those Faloutsos’ related? CS 54001 -1 Winter Quarter 4

Week 4: Security l Context: Attacks, services, & mechanisms l Message encryption l Public key cryptography l Authentication protocols l Message integrity l Public key infrastructure l Firewalls CS 54001 -1 Winter Quarter 5

Attacks, Services and Mechanisms l l l Security Attack: Any action that compromises the security of information. Security Mechanism: A mechanism that is designed to detect, prevent, or recover from a security attack. Security Service: A service that enhances the security of data processing systems and information transfers. A security service makes use of one or more security mechanisms. CS 54001 -1 Winter Quarter 6

Security Attacks CS 54001 -1 Winter Quarter 7

Security Attacks l l Interruption: This is an attack on availability Interception: This is an attack on confidentiality Modification: This is an attack on integrity Fabrication: This is an attack on authenticity CS 54001 -1 Winter Quarter 8

Active and Passive Threats CS 54001 -1 Winter Quarter 9

Methods of Defense l l Encryption Software Controls (access limitations in a data base, in operating system protect each user from other users) l Hardware Controls (smartcard) l Policies (frequent changes of passwords) l Physical Controls CS 54001 -1 Winter Quarter 10

Security Algorithms & Services Security Cryptography algorithms Secret key (e. g. , DES) CS 54001 -1 Public key (e. g. , RSA) Security services Message digest (e. g. , MD 5) Winter Quarter Privacy Chapter 8, Figure 3 Authentication Message integrity 11

Security Services l Confidentiality (privacy) l Authentication (who created or sent the data) l Integrity (has not been altered) l Non-repudiation (the order is final) l Access control (prevent misuse of resources) l Availability (permanence, non-erasure) – Denial of Service Attacks – Virus that deletes files CS 54001 -1 Winter Quarter 12

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Conventional Encryption Principles l An encryption scheme has five ingredients: – Plaintext – Encryption algorithm – Secret Key – Ciphertext – Decryption algorithm l Security depends on the secrecy of the key, not the secrecy of the algorithm CS 54001 -1 Winter Quarter 16

Conventional Encryption Principles CS 54001 -1 Winter Quarter 17

Cryptography l Classified along three independent dimensions: – The type of operations used for transforming plaintext to ciphertext – The number of keys used > symmetric (single key) > asymmetric (two-keys, or public-key encryption) – The way in which the plaintext is processed CS 54001 -1 Winter Quarter 18

Average time required for exhaustive key search Key Size (bits) Number of Alternative Keys Time required at 106 Decryption/µs 32 232 = 4. 3 x 109 2. 15 milliseconds 56 256 = 7. 2 x 1016 10 hours 128 2128 = 3. 4 x 1038 5. 4 x 1018 years 168 2168 = 3. 7 x 1050 5. 9 x 1030 years CS 54001 -1 Winter Quarter 19

Conventional Encryption Algorithms l Data Encryption Standard (DES) – The most widely used encryption scheme – The algorithm is reffered to the Data Encryption Algorithm (DEA) – DES is a block cipher – The plaintext is processed in 64 -bit blocks – The key is 56 -bits in length CS 54001 -1 Winter Quarter 20

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Expansion Phase of DES 4 -bit chunk … … Expanded to 6 bits by stealing a bit from left and right chunks CS 54001 -1 Winter Quarter Chapter 8, Figure 6 23

Cipher Block Chaining Block 1 Block 4 + + DES DES Cipher 1 CS 54001 -1 Block 3 DES IV Block 2 Cipher 3 Cipher 4 Winter Quarter Chapter 8, Figure 7 24

Time to break a code (106 decryptions/µs) CS 54001 -1 Winter Quarter 25

Public-Key Cryptography CS 54001 -1 Winter Quarter 27

Applications for Public-Key Cryptosystems l Three categories: – Encryption/decryption: The sender encrypts a message with the recipient’s public key – Digital signature: The sender ”signs” a message with its private key – Key exchange: Two sides cooperate to exhange a session key CS 54001 -1 Winter Quarter 28

Requirements for Public-Key Cryptography l l l Computationally easy for a party B to generate a pair (public key KUb, private key KRb) Easy for sender to generate ciphertext Easy for the receiver to decrypt ciphertect using private key CS 54001 -1 Winter Quarter 29

Requirements for Public-Key Cryptography l l l Computationally infeasible to determine private key (KRb) knowing public key (KUb) Computationally infeasible to recover message M, knowing KUb and ciphertext C Either of the two keys can be used for encryption, with the other used for decryption CS 54001 -1 Winter Quarter 30

Public-Key Cryptographic Algorithms l l RSA and Diffie-Hellman RSA - Ron Rives, Adi Shamir and Len Adleman at MIT, in 1977. – RSA is a block cipher – The most widely implemented l Diffie-Hellman – Exchange a secret key securely – Compute discrete logarithms CS 54001 -1 Winter Quarter 31

The RSA Algorithm – Key Generation 1. Select p, q 2. Calculate n = p x q 3. Calculate 4. Select integer e 5. Calculate d 6. Public Key KU = {e, n} 7. Private key KR = {d, n} CS 54001 -1 Winter Quarter p and q both prime 32

Example of RSA Algorithm CS 54001 -1 Winter Quarter 33

The RSA Algorithm - Encryption l Plaintext: M

The RSA Algorithm - Decryption l Ciphertext: C l Plaintext: M = Cd (mod n) CS 54001 -1 Winter Quarter 35

Key Management Public-Key Certificate Use CS 54001 -1 Winter Quarter 36

Authentication Protocols l Simple three-way handshake – Assume two parties share a secret key l Trusted third party – E. g. , Kerberos l Public key authentication CS 54001 -1 Winter Quarter 38

Simple Three-Way Handshake E(m, k) denotes encryption of message m with key k CS 54001 -1 Winter Quarter Chapter 8, Figure 9 39

Trusted Third Party S A B A, B E((T , L, K , B), KA ), L, K , A), KB ) E((A , T), E((T K), , L, K , A ), KB ) + 1, K E (T ) Ka and Kb are secret keys shared with server S T=timestamp, L=lifetime, K=session key CS 54001 -1 Winter Quarter Chapter 8, Figure 10 40

Public Key Cryptography CS 54001 -1 Winter Quarter Chapter 8, Figure 11 41

Message Integrity l Requirements - must be able to verify that: – Message came from apparent source or author, – Contents have not been altered, – Sometimes, it was sent at a certain time or sequence. l Protection against active attack (falsification of data and transactions) CS 54001 -1 Winter Quarter 42

Approaches to Message Integrity l Conventional encryption – Only the sender and receiver should share a key l Message integrity without message encryption – An authentication tag is generated and appended to each message l Message Authentication Code – Calculate the MAC as a function of the message and the key. MAC = F(K, M) CS 54001 -1 Winter Quarter 43

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One-way HASH function CS 54001 -1 Winter Quarter 45

One-way HASH function l Secret value is added before the hash and removed before transmission. CS 54001 -1 Winter Quarter 46

Secure HASH Functions l l Purpose of the HASH function is to produce a ”fingerprint. Properties of a HASH function H : 1. 2. 3. 4. 5. 6. CS 54001 -1 H can be applied to a block of data at any size H produces a fixed length output H(x) is easy to compute for any given x. For any given block x, it is computationally infeasible to find x such that H(x) = h For any given block x, it is computationally infeasible to find with H(y) = H(x). It is computationally infeasible to find any pair (x, y) such that H(x) = H(y) Winter Quarter 47

Simple Hash Function l One-bit circular shift on the hash value after each block is processed would improve CS 54001 -1 Winter Quarter 48

Message Digest Generation Using SHA-1 CS 54001 -1 Winter Quarter 49

SHA-1 Processing of single 512 -Bit Block CS 54001 -1 Winter Quarter 50

Sender identity and message integrity confirmed if checksums match Calculate MD 5 checksum over message contents Sign checksum using RSA with sender’ s private key Calculate MD 5 checksum on received message and compare against received value Decrypt signed checksum with sender’ s public key Transmitted message CS 54001 -1 Winter Quarter Chapter 8, Figure 13 51

Create a random secret key k Original message Encrypt message using DES with secret key k Decrypt message using DES with secret key k Encrypt k using RSA with recipient’ s public key Decrypt E(k) using RSA with my private key -> k Encode message + E(k) in ASCII for transmission Convert ASCII message Transmitted message CS 54001 -1 Winter Quarter Chapter 8, Figure 14 52

Public-Key Cryptography Principles l l The use of two keys has consequences in: key distribution, confidentiality and authentication. The scheme has six ingredients – Plaintext – Encryption algorithm – Public and private key – Ciphertext – Decryption algorithm CS 54001 -1 Winter Quarter 54

Encryption using Public-Key system CS 54001 -1 Winter Quarter 55

X. 509 Authentication Service l l Distributed set of servers that maintains a database about users Each certificate contains the public key of a user and is signed with the private key of a CA Is used in S/MIME, IP Security, SSL/TLS and SET RSA is recommended CS 54001 -1 Winter Quarter 56

X. 509 Formats CS 54001 -1 Winter Quarter 57

Typical Digital Signature Approach CS 54001 -1 Winter Quarter 58

Obtaining a User’s Certificate l Characteristics of certificates generated by CA: – Any user with access to the public key of the CA can recover the user public key that was certified. – No part other than the CA can modify the certificate without this being detected. CS 54001 -1 Winter Quarter 59

IPRA = Internet Policy Registration Authority (root) PCA n= policy certification authority CA = certification authority IPRA PCA 1 CA User PCA 2 CA User CS 54001 -1 CA CA User PCA 3 CA CA CA User CA User Winter Quarter Chapter 8, Figure 12 60

X. 509 CA Hierarchy CS 54001 -1 Winter Quarter 61

Revocation of Certificates l Reasons for revocation: – The users secret key is assumed to be compromised. – The user is no longer certified by this CA. – The CA’s certificate is assumed to be compromised. CS 54001 -1 Winter Quarter 62

Firewall Design Principles l l The firewall is inserted between the premises network and the Internet Aims: – Establish a controlled link – Protect the premises network from Internetbased attacks – Provide a single choke point CS 54001 -1 Winter Quarter 64

Firewall Characteristics l Design goals: – All traffic from inside to outside must pass through the firewall (physically blocking all access to the local network except via the firewall) – Only authorized traffic (defined by the local security police) will be allowed to pass – The firewall itself is immune to penetration (use of trusted system with a secure operating system) CS 54001 -1 Winter Quarter 65

Four General Techniques l Service control – Determines the types of Internet services that can be accessed, inbound or outbound l Direction control – Determines the direction in which particular service requests are allowed to flow l User control – Controls access to a service according to which user is attempting to access it l Behavior control – Controls how particular services are used (e. g. filter e-mail) CS 54001 -1 Winter Quarter 66

Types of Firewalls l Three common types of Firewalls: – Packet-filtering routers – Application-level gateways – Circuit-level gateways – (Bastion host) CS 54001 -1 Winter Quarter 67

Types of Firewalls l Packet-filtering Router CS 54001 -1 Winter Quarter 68

Types of Firewalls l Packet-filtering Router – Applies a set of rules to each incoming IP packet and then forwards or discards the packet – Filter packets going in both directions – The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header – Two default policies (discard or forward) CS 54001 -1 Winter Quarter 69

Types of Firewalls l Advantages: – Simplicity – Transparency to users – High speed l Disadvantages: – Difficulty of setting up packet filter rules – Lack of Authentication CS 54001 -1 Winter Quarter 70

Types of Firewalls l Possible attacks and appropriate countermeasures – IP address spoofing – Source routing attacks – Tiny fragment attacks CS 54001 -1 Winter Quarter 71

Types of Firewalls l Application-level Gateway CS 54001 -1 Winter Quarter 72

Types of Firewalls l Application-level Gateway – Also called proxy server – Acts as a relay of application-level traffic l Advantages: – Higher security than packet filters – Only need to scrutinize a few allowable applications – Easy to log and audit all incoming traffic l Disadvantages: – Additional processing overhead on each connection (gateway as splice point) CS 54001 -1 Winter Quarter 73

Types of Firewalls l Circuit-level Gateway CS 54001 -1 Winter Quarter 74

Types of Firewalls l Circuit-level Gateway – Stand-alone system or – Specialized function performed by an Application-level Gateway – Sets up two TCP connections – The gateway typically relays TCP segments from one connection to the other without examining the contents CS 54001 -1 Winter Quarter 75

Types of Firewalls l Circuit-level Gateway – The security function consists of determining which connections will be allowed – Typically use is a situation in which the system administrator trusts the internal users – An example is the SOCKS package CS 54001 -1 Winter Quarter 76

Course Outline (Subject to Change) 1. (January 9 th) Internet design principles and protocols 2. (January 16 th) Internetworking, transport, routing 3. (January 23 rd) Mapping the Internet and other networks 4. (January 30 th) Security 5. (February 6 th) P 2 P technologies & applications (Matei Ripeanu) (plus midterm) 6. (February 13 th) Optical networks (Charlie Catlett) 7. *(February 20 th) Web and Grid Services (Steve Tuecke) 8. (February 27 th) Network operations (Greg Jackson) 9. 10. *(March 6 th) Advanced applications (with guest lecturers: Terry Disz, Mike Wilde) (March 13 th) Final exam * Ian Foster is out of town. CS 54001 -1 Winter Quarter 77