Cryptography and Computer Security - Lecture 1 - Introduction, Cryptography

Objectives

Preliminary Reading

Lecture Content

Introduction

1. Introduction - Computer Security - Why?
   • information is a strategic resource
   • security was by physical & administrative means
   • spread of IT requires automated/technical means also
   • a significant portion of budget spent on IT
   • concerned both with information
     • stored and processed within a computer
     • transferred between computers
   • need to define requirements for security and identify how to meet them
2. Security Services
   • confidentiality - protect info content/access
   • authentication - protect info origin (sender)
   • integrity - protect info accuracy
   • nonrepudiation - protect from deniability
   • user identification - ensure identity of users
   • access control - control access to info/resources
   • availability - ensure info delivery

     Useful to start by looking at what security services may be required. Consider both similarities and differences with traditional paper documents. Note that there is a degree of ambiguity as to the meaning of various terms.

3. Security Mechanisms

   Personnel

   Access Tokens, Biometrics

   Physical

   Integrated Access control

   Managerial

   Security Education

   Data Networking

   Encryption, Config control

   S/W & O/S

   Testing, Evaluation, Certification

   H/W

   TCB, Tamper-proof, Encryption

   No single mechanism can provide all the security services wanted. Usually have a range to choose from. But encryption is a key enabling technology.

4. Security Threats
   • threats can come from a range of sources
   • various surveys, with results of order:
   • 55% human error
   • 10% disgruntled employees
   • 10% dishonest employees
   • 10% outsider access
   • also have "acts of god" (fire, flood etc)

     Note that in the end, it always comes back to PEOPLE. Technology can only assist so much, always need to be concerned about the role of people in the threat equation - who and why.

5. Security Attacks
   • interception - of info-traffic flow, attacks confidentiality (DoS)
   • interruption - of service, attacks availability
   • modification - of info, attacks integrity
   • fabrication - of info, attacks authentication

     Can consider attacks from the perspective of how the flow of information from sender to receiver is influenced.

6. Passive vs Active Attacks
   

   Useful to catagorise these as passive vs active attacks. Passive attacks only involve monitoring (interception) of the information, leading to loss of confidentiality or traffic analysis (monitoring exchange of information without knowing precise contents), and are hard to prevent. Active attacks involve intervention in the information flow (interception, modification, fabrication), and can be easier to detect.

7. Response to Threats
   • identify key assets
   • evaluate threat posed to assets
   • implement suitable countermeasures
   • manage implementation
   • cryptography a key technology

     Knowing that threats exist to assets, its necessary to choose appropriate countermeasures, commensurate with the risk involved, and manage their implementation. The details for this are outside the scope of this course (cf postgrad Computer Security course), but note that encryption is a key enabling technology.

8. Security Models - Communications
   

   In considering the place of encryption, its useful to use the following two models. The first models information flowing over an insecure communications channel, in the presence of possible opponents. Hence an appropriate security transform (encryption alg) can be used, with suitable keys, possibly negotiated using the presence of a trusted third party.

9. Security Models - Communications
   • using this model requires us to:
   • design an algorithm for the security transformation
   • generate the secret information used by the algorithm
   • develop methods to distribute the secret information
   • specify a protocol enabling the principals to use the transformation & secret info for a security service
10. Security Models - Computer
    

    The second model is concerned with controlled access to information or resources on a computer system, in the presence of possible opponents. Here appropriate controls are needed on the access and within the system, to provide suitable security. Some cryptographic techniques are useful here also.

11. Security Models - Computer
    • using this model requires us to:
    • select appropriate gatekeeper functions to identify users
    • implement security controls to ensure only authorised users access designated information or resources
    • trusted computer systems can be used to implement this model

Introduction to Cryptography

1. Introduction to Cryptography
   • cryptography is the study of secret (crypto-) writing (-graphy)
   • concerned with developing algorithms which may be used to:
     • conceal the context of some message from all except the sender and recipient (privacy or secrecy), and/or
     • verify the correctness of a message to the recipient (authentication or integrity)
   • basis of many technological solutions to computer and communications security problems
2. Basic Terminology

   cryptography

   the art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back to its original form

   plaintext

   the original intelligible message

   ciphertext

   the transformed message

   cipher

   an algorithm for transforming an intelligible message into one that is unintelligible by transposition and/or substitution methods

   key

   some critical information used by the cipher, known only to the sender & receiver

   encipher (encode)

   the process of converting plaintext to ciphertext using a cipher and a key

   decipher (decode)

   the process of converting ciphertext back into plaintext using a cipher and a key

   cryptanalysis (codebreaking)

   the study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key.

   cryptology

   the field encompassing both cryptography and cryptanalysis

   code

   an algorithm for transforming an intelligible message into an unintelligible one using a code-book

   Briefly review some terminology we'll be using throughout the course.

3. Some Cryptographic Concepts

   Encryption

   the mathematical function mapping plaintext to ciphertext using the specified key:
   C = EK(P)

   Decryption

   the mathematical function mapping ciphertext to plaintext using the specified key:
   P = EK-1(C)

   cryptographic system

   the family of transformations from which the cipher function EK is chosen

   key

   is the parameter which selects which individual transformation is used, and is selected from a keyspace K

   More formally we can define the cryptographic system as a single parameter family of invertible transformations

   EK; K in K : P -> C

   with unique inverse P = EK-1; K in K : C -> P

   usually assume the cryptographic system is public, and only the key is secret information

   It can be useful to think about cryptographic functions in a mathematical context (but don't let the terminology scare you :-) For any particular cipher, rather than thinking of it as just one function, its actually a whole family of related functions, and in any given case we use the key to pick one of them out. Obviously these functions must have a single unique inverse, or we'd never be able to recover the original message. When talking about the security of our encryption, it is usual to assume that the opponent knows the family of functions, but doesn't know the key and hence which particular instance is being used.

4. Classifying Cryptographic Algorithms
   • in this course will broadly classify cryptographic algorithms as:
   • private-key encryption algorithms
   • public-key encryption algorithms
   • digital signature algorithms
   • hash functions
   • will be discussing each of these
   • within encryption algorithms, also distinguish between
   • block ciphers
   • stream ciphers

     The first classification is based critically on which security service is provided, and the number of keys used (0, 1, 2). The second, on the basis of how the plaintext is processed (in blocks/units or as individual bits/bytes/characters).

5. Private-Key Encryption Algorithms
   

   All traditional schemes are private-key encryption algorithms, with a single key, used for both encryption and decryption. They are also known as symmetric, since both sender and receiver are equivalent, either can encrypt or decrypt messages.

6. Private-Key Encryption Algorithms
   • a private-key (or secret-key, or single-key) encryption algorithm is one where the sender and the recipient share a common, or closely related, key
   • all traditional encryption algorithms are private-key
   • will cover the other categories later
7. Cryptanalytic Attacks
   • cryptanalysis is the process of breaking an encrypted message without knowledge of the key
   • can identify several different types of attacks to do this:
   • ciphertext only
     • only know algorithm and some ciphertext
     • use statistical attacks only
     • must be able to identify when have plaintext
   • known plaintext
     • know (or strongly suspect) some plaintext-ciphertext pairs
     • use this knowledge in attacking cipher
   • chosen plaintext
     • can select plaintext and obtain corresponding ciphertext
     • use knowledge of algorithm structure in attack
   • chosen ciphertext
     • can select ciphertext and obtain corresponding plaintext
     • use knowledge of algorithm structure in attack
   • chosen plaintext-ciphertext
     • can select plaintext and obtain corresponding ciphertext, or select ciphertext and obtain plaintext
     • allows further knowledge of algorithm structure to be used

     Attacks use increasing amounts of information about, and ability to manipulate the cipher, short of knowing the key. In all cases assume the attacker knows the details of the algorithm. In practise this is often true, and it can be surprisingly easy to arrange for known or even chosen text to be processed (sending a document that will be relayed, automated response systems etc). In general, if analyse a ciphers resistance to attack in these circumstances gives greater confidence of its security in practise.

8. Exhaustive Key Search
   • always theoretically possible to simply try every key
   • most basic attack, directly proportional to key size
   • assume either know or can recognise when plaintext is found
   • tabulate for reasonable assumptions about number of operations possible:

     Key Size (bits)	Time (1us/test)	Time (1us/106test)
     32	35.8 mins	2.15 msec
     40	6.4 days	550 msec
     56	1140 years	10.0 hours
     64	~500000 years	107 days
     128	5 x 1024years	5 x 1018 years

     Comment here that its pretty obvious you can just try every key, but it is necessary to be able to recognise when you've cracked the message, either by knowing explicitly the message you want, or being able to recognise it (ie English text for example). Some forms of data (graphics, machine code) can be much much harder to recognise. Also have the circumstance where there is simply insufficient ciphertext to ever be able to recognise the corresponding message (see Shannon entropy theory and unicity distance later).

9. Unconditional and Computational Security

   unconditional security

   no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext

   computational security

   given limited computing resources (eg time needed for calculations is greater than age of universe), the cipher cannot be broken

   Unconditional security would be nice, but the only known such cipher is the one-time pad (later). For all reasonable encryption algorithms, have to assume computational security where it either takes too long, or is too expensive, to bother breaking the cipher.

Summary

1. Summary
   • have discussed:
   • some reasons why we need security, for which encryption is a key technology
   • security services, threats, attacks, and models
   • introduced some cryptographic concepts, terminology, and types of attacks

Exercises

1. Exercises
   1. Which security service(s) would be required in each of these applications and why (in a sentence or two each) when:
      1. providing a system to control a chemical processing plant
      2. providing a system to manage sensitive organisational research records
      3. sending a command to alter the frequency and power used by a remote radio repeater station over the radio link to it
      4. sending a web order for some books to an online merchant
      5. sending email with details of your latest strategy to discredit your political/ business opponents to a colleague
      6. sending the current status of a door alarm sensor back to the central control panel
   2. For each of the above, identify which Security Model is most applicable, identify the parties involved, and discuss your choices.
   3. What type of Cryptanalytic Attack is used in each of the following scenarios and why (in a sentence or two each) when an attacker analyses:
      1. a network dump of a secure web credit card order
      2. a photocopy of a scrambled message
      3. the interaction with a smartcard in his custom reader

Additional References