Lecture on SWEC and Computer Security

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Summer 2011

Lesson ² 5

Software Engineering Concepts

and Computer Security

CSE 101



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What is the difference between software

engineering and computer science?

Computer Science Software Engineering

is concerned with

System Engineering is concerned with all aspects of computer-based

systems development including hardware, sof tware and process engineering.

theory

fundamentals

the practicalities of developing

delivering useful software



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What is a software process?

� A set of activities whose goal is the development or evolution of software.

� Generic activities in all software processes are:

± Specification - what the system should do and its development constraints

± Development - production of the software system

± Validation - checking that the software is what the customer wants

± Evolution - changing the software in response to changing demands.

� Upper-CASE

± Tools to support the early process activities of requirements and design� Lower-CASE

± Tools to support later activities such as programming, debugging and

testing

CASE (Computer-Aided Sof tware Engineering) :Software systems which areintended to provide automated support for software process activities, such asrequirements analysis, system modelling, debugging and testing



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What are the attributes of good software?

� The sof tware should deliver the required f unctionality andperformance to the user and should be maintainable, dependableand acceptable.

� Maintainability± Sof tware must evolve to meet changing needs (scalable);

� Dependability± Sof tware must be trustworthy (reliable, secured and safe);

� Eff iciency± Sof tware should not make wastef ul use of system resources;

� Acceptability± Sof tware must accepted by the users for which it was designed. This

means it must be understandable, usable and compatible with othersystems.



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What are the key challenges facing

Software Engineering?

� Heterogeneity

± Developing techniques for building sof tware that can cope with heterogeneous platforms and execution environments;

� Delivery

± Developing techniques that lead to faster delivery of sof tware;

� Trust

± Developing techniques that demonstrate that sof tware can betrusted by its users.

± Reliable, Secured and Safe.



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Generic Software Process Models

� A simplified representation of a software process,presented from a specif ic perspective

� Examples of process perspectives:

± Workflow perspective represents inputs, outputs and

dependencies ± Data-flow perspective represents data transformation activities

± Role/action perspective represents the roles/activities of thepeople involved in sof tware process

� Generic process models

± Waterfall

± Evolutionary development

± Formal transformation

± Integration from reusable components



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Engineering Example

Building a house:

�Land and finances

�garden, garage, you are used to age wine,

enjoy to sit by the fireplace, lots of storage,

don¶t like Bauhaus

�Architect will define number of floors and

rooms, orientation of the driveway, size of the

garage «

�type of bricks, color of the walls,«

�Construction

�Entering

�Living in the house

�Fixing minor problems, leaking in the roof «



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The Waterfall Model

System Feasibility Validation

Plans +

RequirementsValidation

Product Design Verification

Detailed Design Verification

Code Unit Test

Integration ProductVerification

Integration System Test

Operation +

MaintenanceRevalidation

Waterfall Weakness�High risk for new systems because of specif icationand design problems.

�Low risk for well-understood developments usingfamiliar technology.

�Usually requirements change, are incomplete or

even not known ( Result: µThat¶s not what I meant !¶ (

go back to last step )

�WF-Model reacts very statically: Each stage must

be completed before next one starts

�Too expensive

�Doesn¶t force to discipline



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ValidationFinalversion

Development Intermediateversions

SpecificationInitialversion

Outlinedescription

Concurrentactivities

Evolutionary Process Model

Process Model Weakness:

PrototypingLow risk for new applications because specif ication and program stay in step.High risk because of lack of process visibility.

TransformationalHigh risk because of need for advanced technology and staff skills.



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Risk analysis

Risk

analysis

Risk analysis

Risk

analysis Proto-type 1

Prototype 2

Prototype 3Opera-tionalprotoype

Concept of Operation

Simulations, models, benchmarks

S/Wrequirements

Requirement

validation

DesignV&V

Productdesign Detailed

design

Code

Unit test

Integration

testAcceptance

testService Develop, verify

next-level product

Evaluate alternativesidentify, resolve risks

Determine objectivesalternatives and

constraints

Plan next phase

Integrationand test plan

Development

plan

Requirements planLife-cycle plan

REVIEW

Spiral Process Model

Focuses attention on reuse options.Focuses attention on early error elimination.Puts quality objectives up front.Integrates development and maintenance.Provides a framework for hardware /sof twaredevelopment.Contractual development of ten specif ies process model and deliverables in advance.

Requires risk assessment expertise.



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Professional and ethical responsibility

Sof tware engineering involves wider responsibilities than simply the application of technical skills. Sof tware engineers must behave in an honest and ethicallyresponsible way if they are to be respected as professionals. Ethical behaviour is morethan simply upholding the law.

Conf identiality

± Engineers should normally respect the conf identiality of their employers or clients

irrespective of whether or not a formal conf identiality agreement has beensigned.

Competence

± Engineers should not misrepresent their level of competence.

± They should not knowingly accept work which is outside their competence.

Intellectual property rights

± Engineers should be aware of local laws governing the use of intellectual property

such as patents, copyright, etc.± They should be caref ul to ensure that the intellectual property of employers and

clients is protected.

Computer misuse

± Sof tware engineers should not use their technical skills to misuse other people¶ scomputers.

± Computer misuse ranges from relatively trivial (game playing on an employer¶ smachine, say) to extremely serious (dissemination of viruses).



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Code of ethics - principles

� PUBLIC± Sof tware engineers shall act consistently with the public interest.

� CLIENT AND EMPLOYER

± Sof tware engineers shall act in a manner that is in the best interests of their client and employerconsistent with the public interest.

� PRODUCT

± Sof tware engineers shall ensure that their products and related modif ications meet the highest professional standards possible.

� JUDGMENT

± Sof tware engineers shall maintain integrity and independence in their professional judgment.

� MANAGEMENT

± Sof tware engineering managers and leaders shall subscribe to and promote an ethical approach tothe management of sof tware development and maintenance.

� PROFESSION

± Sof tware engineers shall advance the integrity and reputation of the profession consistent with the

public interest.

� COLLEAGUES

± Sof tware engineers shall be fair to and supportive of their colleagues.

� SELF

± Sof tware engineers shall participate in lifelong learning regarding the practice of their professionand shall promote an ethical approach to the practice of the profession.



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Computer Crime ² Software Piracy

� Sof tware piracy ± the illegal copying of sof tware programs ± is the biggest legalissue affecting the computer industry.

� Piracy is of greatest concern to developers of commercial sof tware, orprograms that must be purchased before using.

� Piracy is less of a concern for shareware makers, whose programs must be

registered but not always purchased.

� Piracy is not a concern for freeware, which is sof tware that can be freely distributed by anyone.

Various forms of copy protection have been used to discourage piracy,including:

� Installation diskettes that record the number of times the sof tware is installed.

� Hardware locks, without which the program cannot f unction.

� Passwords, serial numbers, or other codes required for installation.



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Computer Viruses ² Categories of Viruses

Boot sector viruses Self-encrypting viruses

Cluster viruses Self-changing viruses

File-infecting viruses Stealth viruses

Worms Macro viruses

Bombs Joke programs

Trojan Horses Bimodal virusesPolymorphic viruses Bipartite viruses

Self-garbling viruses Multipartite viruses

E-mail viruses Macro viruses

A virus is a parasitic program that infects another program (the host). Most viruses fall into thefollowing categories:

Viruses are spread in several ways. The most common are:

�R eceiving an infected disk.

�Downloading an infected executable file from a network or the Internet.�Copying a document file that is infected with a macro virus.

To avoid viruses, you should:

�Treat all disks as though they are infected.

�Install an antivirus program and keep its virus definitions (database of virus information) up to date.

�R un your antivirus program regularly.



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Network Security

� Classic properties of secure systems:± Conf identiality

± Encrypt message so only sender and receiver can understand it.

± Authentication± Both sender and receiver need to verif y the identity of the other party in a

communication: are you really who you claim to be?± Authorization

± Does a party with a verif ied identity have permission to access (r /w /x /«)information? Gets into access control policies.

± Integrity± During a communication, can both sender and receiver detect whether a

message has been altered?± Non-Repudiation

± Originator of a communication can¶ t deny later that the communicationnever took place

± Availability± Guaranteeing access to legitimate users. Prevention of Denial-of-Service(DOS) attacks.



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Cryptography

� Encryption algorithm also called a cipher

� Cryptography has evolved so that modernencryption and decryption use secret keys� Only have to protect the keys! => Key distribution

problem� Cryptographic algorithms can be openly published

Encryption Decryption plaintext ciphertext plaintext


Key K A

Key K B



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Cryptography

� Cryptanalysis ± Type of attacks:± Brute force: try every key

± Ciphertext-only attack:

± Attacker knows ciphertext of several messages encrypted with same key (but doesn¶ t know plaintext).

± Possible to recover plaintext (also possible to deduce key) by looking at frequency of ciphertext letters

± Known-plaintext attack:

± Attacker observes pairs of plaintext/ciphertext encrypted with same key.

± Possible to deduce key and /or devise algorithm to decrypt ciphertext.

± Chosen-plaintext attack:

± Attacker can choose the plaintext and look at the paired ciphertext.

± Attacker has more control than known-plaintext attack and may be able to gain more infoabout key

± Adaptive Chosen-Plaintext attack:

± Attacker chooses a series of plaintexts, basing the next plaintext on the result of previous encryption

± Differential cryptanalysis ± very powerf ul attacking tool But DES is resistant to it

� Cryptanalysis attacks of ten exploit the redundancy of natural language

± Lossless compression before encryption removes redundancy



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Principle of Confusion and Diffusion

� Terms courtesy of Claude Shannon, father ofInformation Theory

� ´Confusionµ = Substitution� a -> b� Caesar cipher

� ´Diffusionµ = Transposition or Permutation� abcd -> dacb� DES


Key K A Key K B



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� Modern substitution ciphers take in N bits andsubstitute N bits using lookup table: called S-

Boxes

� ´Confusionµ : a classical Substitution Cipher

� Cryptographers often think in terms of theplaintext alphabet as being the alphabet used towrite the original message, and the cipher text

alphabet as being the letters that are substituted inplace of the plain letters. A cipher is the name givento any form of cryptographic substitution, in whicheach letter is replaced by another letter or symbol.



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Caesar Cipher

According to Suetonius, Caesar simply replaced each letter in a message withthe letter that is three places further down the alphabet.

As shown below, it is clear to see that the cipher text alphabet has beenshifted by three places. Hence this form of substitution is often called theCaesar Shift Cipher.

Courtesy:

Andreas

Steffen



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Pigpen Cipher

Plain

Text : I Love

Computer Science

Cipher text :

The Pigpen Cipher was used by Freemasons in the 18th Century to keep theirrecords private. The cipher does not substitute one letter for another; ratherit substitutes each letter for a symbol. The alphabet is written in the grids shown, and then each letter is enciphered by replacing it with a symbol that corresponds to the portion of the pigpen grid that contains the letter. Forexample:



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� ´Diffusionµ : a classical Transposition cipher

� modern Transposition ciphers take in N bits andpermute using lookup table : called P-Boxes

Courtesy:

AndreasSteffen



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Lecture on SWEC and Computer Security

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