Lecture 1 Dependability and Security

Question 1

What is meant by Dependability

Answer 1

Dependability is the ability for a system to deliver a service that can be trusted and reliable. This generally reflects upon the extent of the user’s confidence that it will operate as users expect and it will not ‘fail’ in normal circumstances.

Summary: The dependability of a system reflects the user’s degree of trust in that system.

Question 2

Why is system dependability important?

Answer 2

• System failures may have widespread effects with large numbers of people affected by the failure.
• Systems that are not dependable and are unreliable, unsafe or insecure may be rejected by their users.
• The costs of system failure may be very high if the failure leads to economic losses or physical damage.
• Undependable systems may cause information loss with a high consequent recovery cost.

Question 3

How is Dependability subjective

Answer 3

The dependability of a system is not absolute but depends on the judgement of a system’s stakeholders. What seems to be a system failure to one stakeholder is normal behaviour to another.

Note: The dependability of a system reflects the user’s degree of trust in that system.

Question 4

How may a system Specification be incorrect or incomplete in relation to dependability?

(DHFT)

Answer 4

Dependability can only be defined formally with respect to a system specification i.e. a failure is a deviation from a specification.

However, many specifications are incomplete or incorrect - hence, a system that conforms to its specification may ‘fail’ from the perspective of system users.

Furthermore, users don’t read specifications don’t know how the system is supposed to behave.

Therefore perceived dependability is more important in practice.

Question 5

Name the Principal Dependability Properties

(Probabilty that the system)
(A judgement of how likely it is that)

Answer 5

Availability
- The probability that the system will be up and running and able to deliver useful services to users.
Reliability
- The probability that the system will correctly deliver services as expected by users
Safety
- A judgment of how likely it is that the system will cause damage to people or its environment
Security
- A judgment of how likely it is that the system can resist accidental or deliberate intrusions.

Question 6

Name examples of a system not being dependable

Answer 6

• Safe system operation depends on the system being available and operating reliably.
• A system may be unreliable because its data has been corrupted by an external attack (insecurity).
• Denial of service attacks on a system (insecurity) are intended to make it unavailable.
• If a system is infected with a virus (insecurity), you cannot be confident in its reliability or safety.

Question 7

What is Availability and Reliability in terms of Dependability?

Answer 7

Reliability: the probability of failure-free system operation over a specified time in a given environment for a given purpose.

Alternative Defintion of Reliability: The probabilty that a system, will correctly deliver the resquested services, at that point in time.

Availability: the probability that a system, at a point in time, will be
operational and able to deliver the requested services.

Both reliability and availability attributes can be expressed quantitatively, e.g., availability of 0.999 means that the system is up and running for 99.9% of the time.

Question 8

Explain whether Availability and Reliability are connected /related or not?

Answer 8

Availability and reliability are closely related

• Obviously if a system is unavailable it is not delivering the specified system services.
• However, it is possible to have systems with low reliability that must be available.
• So long as system failures can be repaired quickly and does not damage data, some system
  failures may not be a problem.
• Availability is therefore best considered as a separate attribute reflecting whether or not the system can deliver its services.
• Availability takes repair time into account, if the system has to be taken out of service to repair
  faults.

Question 9

How can perceived availability be measured?

Answer 9

Availability is usually expressed as a percentage of the time that a system is available to deliver services e.g. 99.95%.

However, this ignores:
- The number of users affected by the service outage. Loss of service in the middle of the night is less important for many systems than loss of service during peak usage periods.

• The length of the outage. The longer the outage, the more the disruption. Several short outages are less likely to be disruptive than 1 long outage. Long repair times are a particular problem.

Question 10

Checkpoint (summary of above)

Answer 10

• Economic and human activities are increasingly dependent on software intensive systems. These can be thought of as critical systems.
• For critical systems, the costs of failure are likely to significantly exceed the costs of system development and operation.
• Consequently, the dependability and security of the system are the most important development considerations.
• Critical systems are often subject to external regulation.

Question 11

Why is it important to continuously check the dependability of a system?

(3 points, Evolution, New Functionality)

Answer 11

• It is important for us to continuously check the dependability of a system.
• The evolution of a system throughout its lifetime will require us to check that correct service is still delivered (e.g., changes in underlying protocols, etc)
• New functionality added to a system may introduce unforeseen problems and hence requires appropriate checks and evidence of dependability

Question 12

Name and Describe each Reliability Terminology (4)

Answer 12

Human Error or Mistake
- Human behaviour that results in the introduction of faults into a system. For example, in the wilderness weather system, a programmer might decide that the way to compute the time for the next transmission is to add 1 hour to the current time. This works except when the transmission time is between 23.00 and midnight (midnight is 00.00 in the 24-hour clock).

System Fault
- A characteristic of a software system that can lead to a system error. The fault is the inclusion of the code to add 1 hour to the time of the last transmission, without a check if the time is greater than or equal to 23.00.

System Error
- An erroneous system state that can lead to system behaviour that is unexpected by system users. The value of transmission time is set incorrectly (to 24.XX rather than 00.XX) when the faulty code is executed.

System Failure
- An event that occurs at some point in time when the system does not
deliver a service as expected by its users. No weather data is transmitted because the time is invalid.

Question 13

How do failures occur and how can they be prevented?

Answer 13

• Failures are usually a result of system errors that are derived from faults in the system
• However, faults do not necessarily result in system errors
• The erroneous system state resulting from the fault may be transient and ‘corrected’ before an error arises.
• The faulty code may never be executed.
• Errors do not necessarily lead to system failures
• The error can be corrected by built-in error detection/recovery
• The failure can be protected against by built-in protection facilities. These may, for example,
  protect system resources from system errors

Question 14

What are the three key areas to achieving reliability?

(Development Techniques, V & V techniques, Run-time techniques)

Answer 14

Fault Avoidance
- Development Techniques are used that either minimise the possibility of mistakes or trap mistakes before they result in the introduction of system faults.

Fault Detection and Removal
- Verification and validation techniques that increase the probability of detecting and correcting errors before the system goes into service.

Fault Tolerance
- Run-time techniques are used to ensure that system faults do not result in system errors and/or that system errors do not lead to system failures.

Question 15

What is meant by the term “Safety” ? in context to Critical systems?

Answer 15

Safety is a property of a system that reflects the system’s ability to operate, normally or abnormally, without danger of causing human injury or death and without damage to the system’s environment.

It is important to consider software safety as most devices whose failure is critical now incorporate software-based control systems

Question 16

Name and Describe the two levels of safety critical systems?

Answer 16

Primary Safety Critical systems
- Embedded software systems whose failure can cause the associated hardware to fail and directly threaten people. Example is an insulin pump control system.

Secondary Safety-Critical Systems:
- Systems whose failure results in faults in other (socio-technical) systems, which can then have safety consequences. For example, a medical records system is safety critical as failure may lead to inappropriate treatment being prescribed

Question 17

Explain whether safety and reliability are linked/connected or not, and explain why?

Answer 17

• Safety and reliability are related but distinct
• In general, reliability and availability are necessary but not sufficient conditions for system safety
• Reliability is concerned with conformance to a given specification and delivery of service
• Safety is concerned with ensuring the system cannot cause damage
  irrespective of whether or not it conforms to its specification
• A reliable system can be unsafe if the requirements are incorrect or incomplete

Question 18

What is meant by ‘unsafe reliable system’

Answer 18

These are systems which exhibit dormant (hidden) faults that have remained undetected in the system for many years and only rarely arise after a series of events.

Remember Plane example

Question 19

Name an explain different types of faults/errors that could occur in ‘unsafe reliable systems’

SHC

Answer 19

Specification errors
- The system specification is incorrect, and the system behaves (perception) as specified but still causes an accident.

Hardware failures
- Hardware failures generating spurious (false) inputs. These are hard to anticipate in the specification. E.g. gives incorrect input, system thinks there is an issue (temperature)

Context-Sensitive Commands (i.e. issuing right command at the wrong time)
- Usually the result of operator error.

Question 20

Name and Describe each Safety Terminology (6)

Answer 20

Accident (or mishap)
- An unplanned event or sequence of events which results in human death or injury, damage
to property, or to the environment. An overdose of insulin is an example of an accident.

Hazard
- A condition with the potential for causing or contributing to an accident. A failure of the sensor that measures blood glucose is an example of a hazard.

Damage
- A measure of the loss resulting from a mishap. Damage can range from many people being killed as a result of an accident to minor injury or property damage. Damage resulting from an overdose of insulin could be serious injury or the death of the user of the insulin pump.

Hazard Severity
- An assessment of the worst possible damage that could result from a particular hazard. Hazard severity can range from catastrophic, where many people are killed, to minor, where only minor damage results. When an individual death is a possibility, a reasonable assessment of hazard severity is ‘very high’.

Hazard Probability
- The probability of the events occurring which create a hazard. Probability values tend to be arbitrary but range from ‘probable’ (say 1/100 chance of a hazard occurring) to ‘implausible’ (no conceivable situations are likely in which the hazard could occur). The probability of a sensor failure in the insulin pump that results in an overdose is probably low.

Risk
- This is a measure of the probability that the system will cause an accident. The risk is assessed by considering the hazard probability, the hazard severity, and the probability that the hazard will lead to an accident. The risk of an insulin overdose is probably medium to low.

Question 21

What are the three key areas to achieving safety?

Answer 21

Hazard Avoidance
- The system is designed so that some classes of hazard simply cannot arise.

Hazard Detection and Removal
- The system is designed so that hazards are detected and removed before they result in an accident

Damage Limitation
- The system includes protection features that minimise the damage that may result from an accident

Question 22

Why do Accidents occur in Safety Critical systems?

Answer 22

Accidents in complex systems rarely have a single cause as these systems are designed to be resilient to a single point of failure.

Designing systems so that a single point of failure does not cause an accident is a fundamental principle of safe systems design.

Almost all accidents are a result of combinations of malfunctions rather than single failures.

It is probably the case that anticipating all problem combinations, especially, in software controlled systems is impossible so achieving complete safety is impossible. Accidents are inevitable.

Question 23

What is meant by the term Security in relation to Dependability?

Answer 23

The security of a system is a system property that reflects the system’s ability to protect itself from accidental or deliberate external attack.

Security is essential as most systems are networked so that external access to the system through the Internet is possible.

Security is essential for availability, reliability and safety.

Question 24

Name and Describe each Security Terminology (6)

Answer 24

Asset
- Something of value which has to be protected. The asset may be the software system itself or data used by that system.

Exposure
- Possible loss or harm to a computing system. This can be loss or damage to data,
or can be a loss of time and effort if recovery is necessary after a security breach.

Vulnerability
- A weakness in a computer-based system that may be exploited to cause loss or harm.

Attack
- An exploitation of a system’s vulnerability. Generally, this is from outside the system and is a deliberate attempt to cause some damage.

Threats
- Circumstances that have potential to cause loss or harm. You can think of these as a system vulnerability that is subjected to an attack.

Control
- A protective measure that reduces a system’s vulnerability. Encryption is an example of a control that reduces a vulnerability of a weak access control system.

Question 25

An example of Security in action (Summary)

Answer 25

Asset
- The records of each patient that is receiving or has received treatment.

Exposure
- Potential financial loss from future patients who do not seek treatment because they do not trust the clinic to maintain their data. Financial loss from legal action by the sports star. Loss of reputation.

Vulnerability
- A weak password system which makes it easy for users to set guessable passwords. User IDS that are the same as names.

Attack
- An impersonation of an authorised user

Threat
- An unauthorised user will gain access to the system by guessing the credentials (login name and password) of an authorised user.

Control
- A password checking system that disallows user passwords that are proper names or words that are normally included in a dictionary.

Question 26

Name the types of damages that could occur from the result of an attack?

Answer 26

Denial of Service
- The system is forced into a state where normal services are unavailable or where
service provision is significantly degraded

Corruption of programs or data
- The programs or data in the system may be modified in an unauthorised way

Disclosure of confidential information
- Information that is managed by the system may be exposed to people who are not authorised to read or use that information

Question 27

Name the three key areas that ensure/contribute to Security Assurance

Answer 27

Vulnerability Avoidance
- The system is designed so that vulnerabilities do not occur. For example, if there is no external
network connection then external attack is impossible

Attack Detection and Elimination
- The system is designed so that attacks on vulnerabilities are detected and neutralised before they result in an exposure. For example, virus checkers and and remove viruses before they infect a system.

Exposure limitation and recovery
- The system is designed so that the adverse consequences of a successful attack are minimised. For example, a backup policy allows damaged information to be restored.

Question 28

Summary of Topic (Dependability and Security)

Answer 28

Reliability is related to the probability of an error occurring in operational use. A system with known faults may be reliable.

• Safety is a system attribute that reflects the system’s ability to operate without threatening people or the environment.
• Security is a system attribute that reflects the system’s ability to protect itself from external attacks.
• Dependability is compromised if a system is insecure as the code or data may be corrupted.