Security

1
Chapter 9

• Security

2
Topics

• Introduction
• Threats, mechanisms, cryptography
• Security channel
• Authentication, integrity, confidentiality
• Access control
• Firewall, secure mobile code
• Security management
• Examples
• Kerberos, E-commerce

3
What Do We Need to Protect?

• Data
• Information we keep on computers (product design,
  financial records, personnel data)
• Resources
• Unauthorized use of computer time space
• Reputation
• Misrepresentation, forgery, negative publicity

4
Fundamental Security Objectives

• Confidentiality - Protection from unauthorized
  persons
• Integrity - consistency of data no unauthorized
  creation, alteration or destruction
• Availability - ensuring access to legitimate
  users
• Access control - ensuring appropriate use by
  authorized users

5
Security Threats

• Interception
• Unauthorized access to a service or data
• Eavesdropping
• Interruption
• Unavailable of service or data
• Denial of service attack
• Modification
• Unauthorized changing of data
• Fabrication
• Adding data or activity normally not exist.
• Security policy

6
Examples Threat
Eavesdropping
Denial of service
7
Example Security Policy

• Chinese Wall Model widely used in financial
  world
• Group datasets into conflict of interest
  classes
• Subjects are allowed to access to at most one
  dataset belonging to each such conflict of
  interest class
• Subject s can access company cs data only if
• a) s has already accessed cs data or
• b) s has not yet accessed any of cs competitors
  data
• s can write to cs data only if s can not read
  any other companys sensitive data
• Mandatory security policy for UK Stock Exchange.

8
Security Mechanisms

• Encryption
• Transform data to achieve confidentiality and
  integrity
• Authentication
• Verify the identify of user
• Authorization
• Check the permission
• Auditing
• Trace the accesses, used for analysis

9
Cryptography

• Intruders and eavesdroppers in communication.

10
Classifications

• Symmetric cryptography shared Key
• PDK(EK(P))
• DES
• Asymmetric cryptography a pair of keys
• PDKD(EKE(P))
• RAS
• Hash function one way function, not reversible
• hH(m)
• MD5

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Notations
12
DES
64-bit data block

• The principle of DES
• Outline of one encryption round

13
Key Generation
14
Attacking DES

• Cryptanalysis
• Relies on nature of the encryption algorithm and
  additional knowledge of the general types of
  plain texts (frequencies of letters etc.)
• Some samples of plain- and cipher texts
• Brute-force
• Test every possible key on some cipher text until
  readable result be done in advance if key is not
  changed

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Brute-force Key Search
Dont get impressed easily DES can now be
cracked in hours!
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Triple DES
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Public-Key Cryptosystems
Encryption
Decryption
Plaintext P
C
P
Ciphertext
DK-(.)
E K (.)
Public key K
Private key K-
Encryption
Decryption
P
Plaintext P
Ciphertext
C
DK(.)
E K- (.)
Private key K-
Public key K
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Idea

• Questions
• 314159265358979 314159265358979?
• 3912571506419387090594828508241 ??
• Idea Use easy algorithm for encryption. Use
  difficult algorithm for decryption
• A user picks a public key/private key pair
• publish the public key
• private key not published

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RSA Rivest, Shamir and Adleman

• Foundation no known method that can efficiently
  find the prime factors of large numbers.
• In RSA, private and public keys are constructed
  from very large prime numbers (consisting of
  hundreds of decimal digits)
• Four steps to construct the keys
• Choose two very large prime numbers, p and q
• Compute n p x q and z (p 1) x (q 1)
• Choose a number d that is relatively prime to z
• Compute the number e such that e x d 1 mod z

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How It Works?

• How it works?
• Encryption C Pe mod n
• Decryption P Cd mod n
• K (e, n), K- (d, n)
• The intruder needs to factor n into p and q to
  crack the code.
• Higher cost of computation.
• Problems
• 1) Is the number of primes infinite? Yes!
• 2) Are they scarce? Yes! 4 of the first 25
  billion numbers. And the percentage drops as the
  numbers get bigger.
• Implication it is tricky to propose a new prime
  number. E.g., is 687,532,127 a prime?

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Example (1)

• To find a key pair e, d
• 1. Choose two large prime numbers, P and Q (each
  greater than 10100), and form
• n P x Q
• Z (P1) x (Q1)
• 2. For d choose any number that is relatively
  prime with Z (that is, such that d has no common
  factors with Z).
• We illustrate the computations involved using
  small integer values for P and Q
• P 13, Q 17 gt n 221, Z 192
• d 5
• 3. To find e solve the equation
• e x d 1 mod Z
• That is, e x d is the smallest element divisible
  by d in the series Z1, 2Z1, 3Z1, ... .
• e x d 1 mod 192 1, 193, 385, ...
• 385 is divisible by d
• e 385/5 77

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Example (2)

• To encrypt text using the RSA method, the
  plaintext is divided into equal blocks of length
  k bits where 2k lt n (that is, such that the
  numerical value of a block is always less than n
  in practical applications, k is usually in the
  range 512 to 1024).
• k 7, since 27 128
• The function for encrypting a single block of
  plaintext M is
• E'(e, n, M) Me mod n
• for a message M, the ciphertext is M77 mod 221
• The function for decrypting a block of encrypted
  text c to produce the original plaintext block
  is
• D'(d, n, c) cd mod n
• Rivest, Shamir and Adelman proved that E' and D'
  are mutual inverses (that is, E'(D'(x))
  D'(E'(x)) x) for all values of P in the range 0
  P n.

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Secret Message
24
Signature
Remark Goal of a signature is to guarantee, that
the receiver is sure that the received message is
from the sender. However, anyone with Gerds
public key of Gerd can also read.
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Message Digest

• Cryptographic checksum
• Just as a regular checksum protects the receiver
  from accidental changes to the message , a
  cryptographic checksum protects the receiver from
  malicious changes.
• One-way function
• Given a cryptographic checksum for a msg, it is
  virtually impossible to figure out what msg
  produced that checksum it is not computationally
  feasible to find two msg that hash to the same
  cryptographic checksum.
• Relevance
• If you are given a checksum for a message you
  are able to compute exactly the same checksum for
  that message, then it is highly likely this
  message produced the checksum you were given.

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Hash Function MD5

• For each round, four functions are applied. And
  each function has 16 iterations.

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MD5 Iterations
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Requirements
m MD5(m)
Received msg
Compare
MD5(m)
Weak collision resistance given m and h,
difficult to find m such that hH(m) Strong
collision resistance given h, difficult to find
m and m such that H(m)H(m).
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Tamper Proof
Using K and K-
m K- MD5(m)
Received msg
K K- MD5(m)
Compare
MD5(m)
30
Secure Channels

• Main model of DS client-server
• Servers may be distributed and replicated
• How to secure a DS?
• Establish secure communication between
  client/server
• Authentication of communicating partners
• Ensuring message integrity and confidentiality
• Establish authorization
• How to be sure on the server side, that a client
  is allowed to get the requested service?
• Access control
• Two principles
• Set-up phase precedes message exchange
• Session keys to ensure message integrity

31
Setup Phase

• Suppose Alice and Bob want to communicate with
  each other, Alice at machine M1 and Bob at
  machine M2
• 1. Alice is setting up a communication channel,
• a) Either by sending a message directly to Bob or
• b) by sending a corresponding message to a
  trusted third party, helping to set up this
  channel
• 2. Once the channel has been set up, both sides
  know for sure, that they can exchange messages

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Authentication on Shared Key
33
Optimization?
34
Reflection Attack
Consequence use different challenges for
initiator and responder
35
Scalability of Session Keys

• Suppose we have N hosts each sharing a secret key
  with each of the other N-1 hosts
• DS has (N-1)N/2 secret session keys and each
  host has manage (N-1) session keys
• For large N session keys will be a problem
• Instead you can install a trusted key
  distribution center KDC on one of the nodes of
  the DS

36
Authentication Key Distribution Center
37
Improvement
Ticket

• Using a ticket and letting Alice set up a
  connection to Bob.

38
Needham-Schroeder Authentication Protocol

• In early distributed systems (1974-84) it was
  difficult to protect the servers
• E.g. against masquerading attacks on a file
  server because there was no mechanism for
  authenticating the origins of requests
• public-key cryptography was not yet available or
  practical
• computers too slow for trap-door calculations
• RSA algorithm not available until 1978
• Needham and Schroeder therefore developed an
  authentication and key-distribution protocol for
  use in a local network
• An early example of the care required to design a
  safe security protocol
• Introduced several design ideas including the use
  of nonces.

39
Illustration
nonce
Nonce a random number used only once. The
purpose is to uniquely relate two messages to
each other.
Q1 Why include B in message 2?
Q2 How about if a chuck knows an old key KA,B?
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Enhancement

• Protection against malicious reuse of a
  previously generated session key in the
  Needham-Schroeder protocol.

41
Authentication Using Public-Key Cryptography

• Mutual authentication in a public-key
  cryptosystem.

Q how to exchange public keys?
42

• Message Integrity Confidentiality

43
Digital Signature

• Goals
• To authenticate stored document files as well as
  messages
• To protect against forgery
• To prevent the signer from repudiating a signed
  document (denying their responsibility)
• Encryption of a document in a secret key
  constitutes a signature
• impossible for others to perform without
  knowledge of the key
• strong authentication of document
• strong protection against forgery
• weak against repudiation (signer could claim key
  was compromised)

44
Illustration

• Digital signing a message using public-key
  cryptography.

45
Digital Signature (2)

• Digitally signing a message using a message
  digest.

46
Certificate Authority (CA)

• Verify the owner of a public key
• Maintain the (owner, public_key) by a certificate
  authority
• CA are organized in a hierarchy.
• For each merchant, it issues a certificate.
• The names of CA are widely known, e.g. Verisign.
• Chain of trust
• Certified by a higher-level CA the central
  authority IPRA

47
CA Hierarchy
IPRA Internet Policy Registration
Authority (root)
PCA policy certification authority
CA certification authority
48
Certificate Authorities in X.509
49
X.509 Certificate Format
50
SSL Handshake
(PK_alg, encr_alg, MD)
Optional
K-C R
51
SSL Record Protocol
Message digest
52
Confidential Group Communication

• Goal secure channels between each pair of nodes
• Share one key?
• Share a key between each pair of nodes?
• Each node has its own private key but all the
  nodes share a public key.

53

• Access Control

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General Issues in Access Control

• General model of controlling access to objects.

55
Access Control

• Access control Matrix
• Access Control List
• Capabilities.

56
Protection Domains

• The hierarchical organization of protection
  domains as groups of users.

57
Firewalls
Common implementations of a firewall, e.g. a
packet-filtering router or an application gateway
58
Firewall Solutions

• Definition - hardware /or software components
  that restrict access between a restricted network
  the Internet or between networks
• Logically - a separator, restricter, analyzer
• Rarely a single object
• Restricts people to entering at a controlled
  point
• Prevents attackers from getting close to other
  defenses (host controls)
• Restricts people to leaving at a controlled point

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Firewall Capabilities

• Focus security decisions - single point to
  leverage control
• Enforce security policy -minimize exceptions
• Log Internet activity - analysis
• Limit exposure - separate sensitive areas of one
  network from another or outside world

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Firewall Limitations

• Cant protect against
• malicious insiders
• connections that dont go through it
• new threats
• viruses
• scans for source destination addresses port
  numbers, not details of data

61
Types of Firewalls

• Simple traffic logging systems
• audit log file of files accessed (HTTPD)
• site usage/demand hours/links/browsers used
• IP Packet Screening Routers (packet filtering
  gateway)
• not only looks at can it route, but should it
• selectively routes or blocks packets based on
  rules
• based on protocols, destination (port 80), known
  source IP addresses

62
Types of Firewalls (cont.)

• Hardened Firewall Host (hardware)
• Halts unauthorized users
• Concentrates security, hides internal system
  names, centralizes simplifies net management
• Proxy Server (software)
• Deals with external server requests on behalf of
  internal clients
• May limit certain HTTP methods (CGI or Java
  applets)

63
Filtering Router
Check the source and destination address. Make
decisions based on security policies.
64
Filtering Router and Bastion Host

• Firewall Architectures
• Dual-homed host (two network interfaces)
• One communicates externally, one internally
• No direct communication internal to external hosts

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Advantages

• All accesses can be logged
• Reduce the number of Internet connections by
  making it a caching proxy
• Does not reveal the names and addresses of actual
  clients inside
• But slow down page downloading by an order of
  magnitude.

66
Other Variations

• Multiple Bastion Hosts
• Performance, redundancy, need to separate data
  servers
• Usenet, SMNP/DNS, FTP/WWW
• Merge Interior Exterior Routers
• Sufficient capability to specify inbound
  outbound filters
• Usually on the perimeter network
• Merge Bastion Host Exterior Router
• Use Multiple Exterior Routers
• Multiple connections to Internet or Internet
  other sites
• Multiple Perimeter Nets
• Redundancy, privacy

67
Futures

• Third-generation Firewalls
• combined features of packet filtering proxy
  systems
• Client server apps with native support for
  proxied environments
• Dynamic packet filtering
• Packet rules modified on the fly in response to
  triggers
• Underlying Internet protocol undergoing revisions
  - IPv6

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Not Recommended

• Merging Bastion Host Interior Router
• Breach of host leaves access to internal net
• Using Multiple Interior Routers
• Routing software could decide fastest way to
  another internal system is via the perimeter net
• Difficult to keep multiple interior routers
  configured correctly
• Most important complex set of packet filters
• May need to use multiples to resolve performance
  bottlenecks or separate internal networks

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Private Network
70
Virtual Private Network
Internet
Intranet B
Intranet A
Tunneling
Router RA
Router RB
200 Data
RB
Station 200
Station 100
encrypted
71
Tunneling
72

• Virus

73
Virus
74
Memory-Resident Virus
Runs whenever certain interrupts occur.
75
Encrypted virus
To conceal signature.
76
Worms Illustration
Low address
Program
UNIX Address Space
Statically allocated data
Stack
High address
77
Procedure Call
E.g., finger aabbcc
aa bb cc

Buffer area allocated by called fingerd (512
bytes)
Return address
PC?
ret
para2
para1
Stack
High address
78
Buffer Overflow
E.g., finger aabbzz
aa bb cc
0100

Malicious program (binary)
Return address
PC?
0100
para2
para1
Stack
79

• Security Management

80
Key Establishment

• The principle of Diffie-Hellman key exchange.

81
Key Distribution (1)

• Secret-key distribution

82
Key Distribution (2)

• Public-key distribution Certificate

83
Secure Group Management

• Securely admitting a new group member P.

84

• Authorization Management

85
Capabilities

• A capability in Amoeba.

86
Capabilities Generation

• Generation of a restricted capability from an
  owner capability.

87
Delegation

• Transfer the access rights on files, resources,
  etc.
• Suppose Alice wants to delegate rights to Bob
• If Alice knows everyone, broadcast the
  certificate
• Otherwise, construct a certificate saying The
  bearer of this certificate has rights R.
• Problems?
• Using proxy, a token that allows its owner to
  operate with the rights granted in the token.

88
The General Structure of A Proxy
89
Delegating And Exercising Rights
90
Example Kerberos (1)

• Authentication in Kerberos.

91
Example Kerberos (2)

• Setting up a secure channel in Kerberos.

92
Electronic Payment Systems (1)

• Payment systems based on direct payment between
  customer and merchant.
• Paying in cash.
• Using a check.
• Using a credit card.

93
Electronic Payment Systems (2)

• Payment systems based on money transfer between
  banks.
• Payment by money order.
• Payment through debit order.

94
Privacy Issue

• Using cash
• Using credit card
• Online

95
Digital Money

• Suppose Alice wants to pay 12 to Bob
• Contact her bank and request withdrawal 12
• Bank hands out digital money (each note is
  signed)
• Each note carries a unique serial number
• Hand over the notes to Bob
• Bob contact the bank if the money has been used.
• Problem privacy issue.
• Solution blind signature

96
E-cash

• The principle of anonymous electronic cash using
  blind signatures.