Software of Safety Important I&C Systems - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 158

Requirements for
Development of Software

software or hardware versions. Software diversity
should not create a danger of non-fulfillment of
functional requirements.

Requirements for methods of software development
are divided into two main groups: 3. Establishment of access categories, applica-

tion of different password systems, digital
signature procedures, use of special encod-
ing algorithms and others. These methods
assure protection from errors of personnel
and unauthorized actions.
4. Monitoring reliability and protection of
incoming information from distortions. In
this case one should check: incoming signals
being present in zones of access, established
in accordance with the specification; logical
non-contradictory nature of values of input
variables and so forth.

1. Requirements to use of formal methods.
2. Requirements to programming methods and
approaches.

The first group of requirements is to a certain
extent recommendatory in nature and indicates
the need (expediency) of using at all stages of
development and verification formal methods that
are based on rigorous mathematical description
of formulations of problems related to different
stages of software improvement and verification
with use of a theoretical apparatus of algorithms,
mathematical logic, graph theory and so forth, and
also on proof of the correctness of solving these
problems by means of standard procedures. Such
methods are used in particular for:

Note that during the use of all protection
methods software is an object of protection and
a means of assuring reliability and safety. In
this case, there must be the introduction of ad-
ditional software components, which in turn can
be sources of failures and therefore they must be
carefully analyzed. Functional characteristics of
the software and I&C system as a whole should
not be degraded to an unacceptable value (just as
during monitoring and diagnostics).
Protection against cyber threats: The software
of I&C system shall be protected from undesirable
and unsafe interference to work and unauthorized
changes via external computer networks and the
use of non-resident storage media.
To achieve such protection connection with
Internet shall be excluded and any changed can
be possible only after appropriate authorization.
Also special methods of protection from viruses
and other malware should be used.
At the same time, measures against cyber
threats should not affect the execution of appli-
cations software and deteriorate performance of
the functions that are implemented by software.

1. Transition from verbal to formal descrip-
tion of general and functional requirements
for software and development of its formal
specifications.
2. Mathematical proof of the conformity of
software to specifications or requirements
of previous stages of development.
3. Development of application programs using
formal procedures of synthesis.
4. Analysis of syntactic and semantic correct-
ness and carrying out test verifications of
execution of functional requirements for
software.

5. Improvement of the verifiability of soft-

ware and formalization of the evaluation of
results during independent verification and
validation.