Computer Systems Security

1
Computer Systems Security

• Teaching team
• Martin Slade (module leader), room K356, phone
  353554, e-mail M.Slade_at_staffs.ac.uk
• Pirooz Saeidi, room K244, phone 353270, e-mail
  P.Saeidi_at_staffs.ac.uk

2
Classes

• Classes
• 2 lectures 1 tutorial/practical per week -
  tutorial/practicals start next week - NONE this
  week!
• 3 Tutorial/practical classes timetabled - Monday
  1pm K006, Wednesday 9am K006, Thursday 11am K006
• choose an appropriate class, turn up and sign up
  for it

3
Slides and module information

• For now
• PowerPoint versions of lecture slides, the
  tutorial/practical questions and example answers
  will be made available on my web site at
• www.soc.staffs.ac.uk/mss1/css/css-index.htm
• lecture slides will be put on the web page the
  day before the lecture
• Also information about assessments (at an
  appropriate time), announcements, and other
  information about the module will appear on the
  web page from time to time

4

• Later
• module will appear on Blackboard
• Blackboard.staffs.ac.uk/
• Information that appears on Web site will appear
  on Blackboard

5
Learning outcomes

• 1. To be able to explain the fundamental
  principles and concepts involved in the securing
  of computer systems both stand-alone and networks
  based.
• 2. To be able to specify, design and implement an
  exemplar component of a security system.
• 3. To be able to critically evaluate various
  techniques used in exemplar security systems.

6
Assessment

• Assignment (50) - researching about and writing
  a security component - learning outcomes 2 3
• Exam (50) - 2 hours long - learning outcomes 1
  3

7
Study time guide

• Module has a total of 150 hours allocated to it
  as
• 36 hours for lectures/tutorials 2 hours for
  exam
• Giving 112 hours left over to be organised by
  you, example breakdown
• 26 hours revision for exam
• 50 hours work on assignment
• 36 hours for tutorial preparation and weekly
  reading 3 hours of reading and advance review
  of tutorial material per week - less on this will
  require more on the others

8

• Textbook
• Pfleeger, C.P., S. L. Pfleeger, Security in
  Computing, Prentice Hall, 3rd edition, 2002
• Alternative book
• Gollmann, D, Computer Security, Wiley, 2nd
  edition, 2006
• Reference will be made to other books/articles as
  relevant, particularly Stallings book on network
  security

9
Lecture 1

• In this lecture we will be looking at
• 1. What is computer system security.
• 2. How things are insecure.
• Reading to accompany this lecture
• Pfleeger, chapter 1.

10
Definition of Computer System

• System a set of components/elements (hardware,
  software, people, organisations) that interact to
  produce specific input/output behaviour across
  some boundary
• elements of a system may be systems themselves
  (sub-systems)
• Computer a machine that can (in principle)
  execute any algorithm that is specified for it -
  an algorithmic mechanism
• may be stand-alone, embedded or distributed

11

• Computer System any system in which a computer
  executing an algorithm produces the outputs for
  the system
• includes hardware (computer and I/O peripherals),
  software (algorithms) and data (inputs to and
  outputs from algorithms)
• Network of computers that communicate with each
  other would count as a computer system because
  final outputs are determined by end nodes of
  network

12
Security - informal exposition

• Security - notion we are familiar with as
  security of physical objects
• useful analogy - informally something is secure
  if it is safe/protected from loss or harm
• loss - we can no longer use object
• harm - we can no longer use object in ways that
  we want to (because it no longer functions
  properly or as it should)
• THUS - security is really about ensuring that
  something we have is available to us to use in
  ways that we want to use them

13

• 3 fundamental concepts that underpin this
  common-sense notion of security are
• Ownership
• Authority
• Possession

14
Ownership

• Ownership - relationship that is recognised as
  legal between an owner (person or organisation,
  etc) and something owned i.e. property (object,
  resource, information, software, etc.)
• owner can determine (subject to legal
  constraints) who does what with owners property
  - can also determine when, where and how
  something is used - although such things are
  often left as assumptions about reasonable use.
• In other words - the owner determines the nature
  of the usage of the property/object

15

• We can see this as a set of usage permissions (in
  literature on security these are often called
  Access Permissions)
• Owner - has legal right to determine the usage
  permissions of their property, subject to
  constraints on usage specified by the legal
  authorities.

16
Authority

• The legal right means that interference with an
  owners determination of usage permissions is
  subject to legal remedies (remedy action taken
  by legal authorities to punish wrongdoer or
  obtain appropriate restitution to wronged party)
• Having legal right to do something means having
  the authority to do something
• owner is authorised to determine the usage
  permissions of their property subject to legal
  constraints

17

• someone is authorised to use some property in a
  given way (possibly, if specified, only at a
  given time or for a given length of time, at a
  certain location, etc) if owner of the property
  has given them usage permissions on the property
  to use property in a given way, etc.

18
Possession

• Possession - possession gives possessor the
  capacity to determine/control the usage of the
  thing possessed, independently of whether or not
  they are legally entitled to determine usage of
  the thing
• normally owner of property also possesses it, but
  owner can also not have possession e.g. if object
  is rented (owner has granted for a fee certain
  usage permissions on the property) or it is
  stolen (thief can determine how property is used
  i.e. determine usage, but does not have any legal
  right to determine such usage)

19
Security - definition

• Property is secure if and only if the owners
  authority to determine usage permissions is not
  interfered with and if the property is used by
  all who use it, only according to the usage
  permissions of the owner.
• Security is the process of attempting to ensure
  that property is secure by
• mechanisms that seek to prevent breaches in
  security from happening
• mechanisms that seek to detect attempts
  (successful and unsuccessful) to breach security
• mechanisms that seek to recover from breaches in
  security (to minimise loss or harm)

20
Computer Systems Security

• Computer system is secure if and only if
• 1.owners of the various components of the system
  from hardware, software or data items can
  determine how those items are used without
  illegal interference
• and
• 2.users of those items only use them if they have
  been granted permission to use them and only use
  them in the ways specified by the owner of the
  item.

21

• Computer systems security is process of
  attempting to ensure that computer system
  components (hardware, software, data) are secure
  by using mechanisms that prevent, detect and
  recover from breaches in security

22
Modes of loss or harm

• Items in an insecure system can suffer loss or
  harm
• Loss - of 2 types
• 1. unauthorised change in possession of an item
• unauthorised loss of possession of an item is a
  special case of this
• 2. unauthorised destruction of an item (this may
  be classified as harm - it does not really
  matter)
• Harm unauthorised change to an item

23
Loss 1. - unauthorised change in possession of an
item

• not easy to see that information or software is
  property that may be stolen
• theft of physical objects is something we
  understand, but different from theft of
  information
• theft of physical objects leaves legal right
  unchanged but removes owners possession of the
  object stolen
• theft of information involves copying information
  normally without deleting or damaging the
  original (thieves want to be covert)

24

• theft of software and data leaves legal right
  unchanged but also usually leaves owner still in
  possession of software/data - possession
  (capacity to control how an item is used) however
  is now shared between owner and thief
• hardware, software or information theft all
  involve unauthorised change in possession of an
  item (for hardware owner has nil possession for
  software/data owner now has shared possession)

25

• Where software and data item is copied but the
  owners copy is deleted or destroyed, then the
  change in possession is similar to hardware theft
  - loss of possession of item rather than sharing
  possession - however such loss of possession may
  be temporary (item may be re-acquired)

26
Loss 2. - unauthorised destruction (deletion) of
item

• physical item or software or data no longer
  exists - the item cannot be identified and even
  if components of item can be identified they are
  so damaged that the item itself cannot even be
  partially re-constructed from those components

27
Harm unauthorised change (modification) to item

• item is modified in some way (data changed,
  software modified so that it behaves differently,
  hardware damaged so that it malfunctions, etc)
• change is not authorised by owner or by user who
  has permission of owner to modify item in a given
  way
• As a result the integrity of the item is
  compromised i.e. it may not hold the information
  that you expect it to (data corruption), or
  behave as specified (with software or hardware)

28

• the modified data item may be accepted as valid,
  thus leading to the possibility of using modified
  data to present a false reality (deception and
  forgery)
• Unauthorised change (modification) damage -
  original does not exist (in original
  form/matching its specification) - but unlike
  destruction can be partially (or even fully)
  re-constructed from its components

29
Person, Owner, User, Usage permissions, Resource
relationship