paper 2

• Defensive Design

  Creating a program that is secure and robust
• Defensive Design Considerations

  • Anticipating Misuse
  • Input Sanitisation
  • Validation
  • Verification
  • Authentication
  • Maintainable code
• Anticipating Misuse

  Planning ahead to take steps against potential misuse (e.g Twitter preventing the same tweet sent twice in a row)
• Input Sanitisation

  Cleaning up data that has been input, (e.g. removing special characters to prevent a SQL injection)
• Validation

  Checking whether input data follows criteria and should be accepted (e.g. length check on a password)
• Verification

  Checking whether data has been entered correctly (e.g. double entry)
• Authentication

  Ensuring only authorised users can gain access to a system (e.g. passwords)
• Maintainable code

  Allowing other programmers to understand the code (e.g. comments, indentation, meaningful variable names)
• Input Validation


  A process to check that data is reasonable or sensible before it is accepted
• Validation Checks

  • Range Check
  • Type Check
  • Format Check
  • Presence Check
  • Lookup Table
  • Length Check
• Range Check

  Checks data is within a certain range
• Range Check

  • Age: 34 203
• Type Check

  Checks data is a certain data type
• Type Check

  • Height (in cm): 182 Two metres
• Format Check

  Checks data is entered in a certain way
• Format Check

  • Date of Birth (DD/MM/YYYY) 25/03/2001 25th March 01
• Presence Check

  Checks that data has actually been entered and not left blank
• Lookup Table

  A table of acceptable entries, also known as a list
• Length Check

  Checks the length of the input is within a certain amount
• Length Check

  • Password: fluffythecat123 Telephone Number 08323877319 07383
• Ways to make a program maintainable

  • Using subprograms to reuse code and make them easier to test (modularisation)
  • Appropriate variable names, using a naming convention, ensure the purpose of a variable is immediately understood
  • Using indentation to improve readability and clearly show each 'block' of code
  • Comments enable a programmer to understand the purpose of each line of code
  • Using constants is another method of improving maintainability
• Testing

  The main purpose is to ensure a program works correctly no matter what input has been entered by the user
• Other reasons to test a program

  • Ensuring user requirements have been met
  • Errors have been removed
  • The program doesn't crash while running
• Iterative testing

  Programmer develops a module, tests it, and repeats this process until the module works as expected
• Final testing

  1. Takes place after development and before the program is released to the end user

  2. Ensures the whole program works as originally expected
• Syntax error

  • A mistake in the grammar or spelling of the program
  • Prevents the program from being compiled
• Syntax errors

  • Incorrect spelling: pront("hello")
  • Incorrect punctuation: print("hello"(
• Logical error

  A mistake made by the programmer - the program still works but displays the wrong output
• Logical errors

  • Incorrect calculation: total = num1 - num2 print(total)
  • Incorrect variable printed: age = 16 name = "Steve" print("Nice to meet you", age)
• Types of test data

  • Normal data - Sensible data the program should accept and process
  • Boundary data - Data at the extreme boundary of any data ranges
  • Invalid (erroneous) data - Data the program cannot process and should not accept
• • Define computational thinking.Solving problems that can be implemented by a computer system is known as computational thinking.
  • Define abstraction.Abstraction is the process of removing unnecessary details of a problem to focus on the important features to implement in a solution.
  • Define decomposition.Decomposition is the process of breaking down a large problem into a set of smaller problems to make the problem easier to solve.
• • Low-level languageA low-level language is a programming language that directly translates to machine code understood by the processor.
  • First generation languageMachine code is a first-generation language where instructions are directly executable by the processor and written in binary code.
  • Second generation languageAssembly code is a second-generation language that uses mnemonics that correspond almost exactly to machine code instructions.
  • High-level languageA high-level programming language uses English-like statements to allow users to program with easy to use code.
• • State two advantages of high-level languages?The advantages of high-level languages are that they are easier to:
  • read
  • write
  • debug
  • maintain.
  • State two disadvantages of high-level languages?The disadvantages of high-level languages are:
  • users cannot directly manipulate hardware
  • the program may be less efficient.
• • Give one example of a high-level programming language.Examples of high-level programming languages include:
  • Python
  • Java
  • Basic
  • C++.
• Translator

  A program that translates program source code into machine code so it can be executed by a processor
• Compiler


  A translator that translates high-level languages into machine code all in one go, typically used when a program is finished
• Interpreter

  A translator that translates high-level languages into machine code one line at a time as the program runs
• Assembler

  Used to translate low-level assembly code into machine code
• Compiler

  Translates the entire program code at once
• Interpreter

  Translates line by line as it executes