3.2.8 Robust and Secure Programming

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1 Validation


Learn It: What is validation?

Validation – Is when a program checks if the data being entered into the computer/device is
reasonable, sensible and suitable. This a called data validation. For example, checking
that a person’s DOB is not in the future or that the string is not empty. Validation does
not ensure that data is correct.
  • Most commercial programs or web applications require some form of user input to ensure that the program will not crash or do something unexpectedly if a user enters something incorrectly.
  • Therefore, all data must be checked as soon as it is inputted.
  • Validation can only check if a data input item is reasonable, it cannot check if it's correct.
  • If a web application asks the user for their date of birth, that application can check that the data entered would be appropriate for a particular age group. (i.e. Over 18), but it cannot check if you entered December instead of January by mistake.
  • Although validation can help ensure that any data entered is sensible, verification double checks that the data has been entered correctly. This is an important difference.
  • Data is usually entered twice and the two versions are compared. If they are different, then the user is prompted to try again.
  • This is commonly used with username/email and passwords that are entered on applications.

There are several different types of validation checks that can be used:

  1. Range Check – A number or date should be within a sensible or allowed or specific range.
  2. Type Check – Ensures that the correct data type has been entered such as, an integer, float, string or Boolean.
  3. Length Check – Ensures that a string contains a valid number of characters that is not too short or long, for example, a password is greater than 8 characters or a National Insurance Number is no longer than 9 characters.
  4. Presence Check – This checks that the user input is not left blank/empty. This is essentially a length check that ensures that the length is greater than zero.
  5. Format Check – Checks the format of the input, for example, a postcode or email address is appropriate.

Range Check Example:

  • The following algorithm asks the user to enter an integer between 5 and 10, which is validated by using a while loop that performs a range check. The number is then multiplied by 2 and outputted.

Range_Check.png Type Check Example:

  • The following algorithm asks the user to something, and performs a type check to identify whether it is a string, integer or a float number.

Type_Check.png Length Check Example:

  • The following algorithm asks the user to enter their name, and performs a length check. The name must be between 2 and 20 characters.

Length_Check.png Presence Check Example:

  • The following algorithm asks the user to enter a sentence which uses the .lower() method to return the lowercased string from the given string and performs a presence check.

Presence_Check.png Format Check Example:

  • The following algorithm asks the user to enter their nine-digit National Insurance Number, and performs a format check using string formatting. The number must be 9 characters long and in the format of: LLNNNNNNL.


Learn It: Validation through Menus

Creating menus

  • One easy way to control what a user enters into a system is to use menu commands and options that you want to display for users.
  • They can then enter a letter or a number to select the option they want.
  • This is another way of validating data through the use of menus with set options to choose from, you can use if statements to give the correct response, depending on how the user answers.
  • Below is an example menu algorithm that uses letters for the options and whatever letter is typed in is converted to lowercase before the if statement starts.


Badge It: Coding Challenge

Silver - Validating Input (5 Marks)

  1. Create a Python program that will ask the user to input two numbers between 1 and 100.
  2. Use a subroutine to find out if the data is valid or not. After valid numbers have been inputted the program should show them a menu giving them the option to either add or multiply the numbers together and keep asking for their menu selection until they enter a valid option.
  3. Use subroutines to perform the correct calculation and display the final answer in a user-friendly format (i.e. display a message such as “The answer is…”)

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Learn It: Try and Except

Try and Except

  • Sometimes the user can make a mistake and crash the program which can be very frustrating.
  • Instead of letting the program crash, it is much more user friendly to show them an error message so they can try again.
  • This is where the “Try and Except” can help. It works in a similar way to an if statement, but instead of crashing the program it will run the catch (or except) block.


Learn It: Creating Simple Authentication Routines

Authentication – This is the process of recognising a user’s identity. It is the mechanism
of associating an incoming request with a set of identifying credentials. The credentials
provided are compared to those on a file in a database of the authorised user’s information
on a local operating system or within an authentication server. The software process of
ensuring that the person accessing a system is the person who is supposed to access that
  • Often a computer system will require a user to login to ensure that they are authorised to use the system.

The following authentication methods might be used:

  1. User ID and passwords.
  2. Memorable Information – Prompting the user for something only the real user should know, such as a favourite place or the name of their first pet.
  3. Checking that the user is using their usual computer, by logging their IP address.
  4. Biometric Methods such as, optical, facial or fingerprint recognition.

Authentication techniques are used throughout the Cyber Security world.

  • A simple identification routine is used when you log into the school network, or an online shopping site.
  • Usually you’ll be assigned a user ID and you choose a password when you first login.
  • The password is encrypted and saved in a file.
  • When you enter your user ID and encrypted password it is compared to the one stored for that user ID.

Example Verification login: Verification_ID.png

  • User ID verification is used when a new user is logging on to a website for the first time, they are prompted to choose a password, and then have to re-enter it to verify that they have typed it in correctly.

The following algorithm demonstrates a simple verification process: Verification_Process.png Example Password Authentication algorithm: Password_Auth_Algor.png

Badge It: Coding Challenge

Gold - User Authentication (8 Marks)

  1. Create a simple authentication routine that uses a username and password.
  2. The username and password does not require encrypting and can be in plain text.
  3. The username must have at least 4 characters and the password must have at least 8 characters.
  4. Users should only be given 3 attempts to login, before being given a ‘locked out’ message and the program should terminate.
  5. A welcome message should be given for successful logins.

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Learn It: Testing your Programs

Testing – When developing a program, tests should be performed throughout the process
to ensure that the program functions correctly, is error free and meets the
specification requirements.
  • How do you know that your programs work correctly?
  • They may run and produce some output, but you also need to test that it has worked as expected.
  • We need to test our programs using sensible values to put into our programs to ensure that the outputs are what we expect.

Error Handling Error_Handling_Info.png Types of Data to Test

  • There are three types of data that we should consider.:
    • Normal (Typical) Data.
    • Boundary (Extreme) Data.
    • Erroneous Data.
Normal(Typical) Data - Data that is valid and that represents how the program would
be used. It is data you are expecting, for instance if you are asking for an integer
between 3 and 20 you should test some data that is an integer between 3 and 20,
for instance 7.
Boundary(Extreme) Data – Data that is just barely valid, to check that the extreme
ranges of normal input work correctly. It is data that is on the boundaries. If you
are asking for a number between 3 and 20 inclusive, then test 3 and 20 as well as 0
and 21 to check that the boundaries of the range are working as you expect.
Erroneous Data – Data that should not be accepted by the system. It is data that is
just wrong. If you are asking for an integer test what happens when you enter a number
with a decimal point or a string?

Learn It: Test Planning

Test Plan

  • The best way to approach testing is to devise a test plan, where we can write down the sample data we will use to ensure that the program works correctly.
  • The main aim of testing is to identify errors such as, syntax, logical and run-time errors as-well as dealing with invalid user inputs that stops the program from working correctly.
  • The first four columns are part of the design and planning stage and the final column is completed when you test the finished program.
  • If numbers or dates are being added by the user, then you should use test data that checks; Normal (typical), Boundary (extreme) and Erroneous data.
  • Boundary data includes both ends of the allowed range, as well as data that should not be allowed, just outside this range. (i.e. 2 or 21).


Learn It

  • We can also perform some tests without the use of a computer or any of the real code.
  • Often, when we write an algorithm, we'll want to test that it works correctly.
  • We can use trace tables to achieve this.
X <-- 1
Y <-- 2
WHILE X < 20
    X <-- X + Y
    Y <-- Y + 1
  • The variable in the above pseudocode algorithm can traced using a trace table.
  1 2
1 3 3
3 6 4
6 10 5
10 15 6
15 21 7
  • The trace table allows us to understand what is happening to the values of variables within the algorithm.

Try It

  • Try drawing trace tables for the following algorithms.
Y <-- 3
FOR X <-- 1 TO 5
    Y <-- Y + X
List <-- [10,8,3,5,6,1,2]
Total <-- 0
FOR i <-- 1 TO LEN(List)
    TOTAL <-- TOTAL + List[i]
num <-- 78
answer <-- ''
WHILE num > 0
    r <-- num MOD 2
    num <-- num / 2
    answer <-- STR(r) + answer 
OUTPUT answer
nums = [6,2,8,1,9,2]
n = 0
FOR i <-- 1 TO LEN(nums)
    IF nums[i] > n
        n = nums[i]

Badge It: Exam Questions

Platinum - Exam questions

  1. A program is only as good as its data. Explain how validation and verification can help ensure that the data is as 'good' as possible? (6 Marks)
  2. Describe one method of validation used by online surveys to help them manage the quality of their data? (2 Marks)
  3. Validation is required for the data in the parameters for the function below. Set a test, and show an example of invalid data (2 Marks)
  4. Give two reasons why programmers test their code? (2 Marks)
  5. A program requires a date of birth from the 20th century to be entered. Provide normal, extreme and erroneous test data examples in the format DD/MM/YYYY. (3 Marks)
Function WorkingTime (TimeIn,TimeOut)

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