3.2.11b Robust & Secure Programming — AQA 8525

Robust Programs

A robust program works correctly across a wide range of inputs, including unexpected or invalid ones, and handles errors gracefully without crashing. Robustness is achieved through input validation, error handling (exception handling), and thorough testing.

Input Validation

Validation checks that data entered by a user is acceptable — within the right range, type, and format — before the program uses it. It does not check whether data is true (that is verification).

Types of validation

Validation type	What it checks	Example
Range check	Value is within permitted limits	Age between 0 and 120
Type check	Data is the correct data type	Score must be an integer
Presence check	A required field is not empty	Name field cannot be blank
Length check	String is the right length	PIN must be exactly 4 digits
Format check	Data matches a required pattern	Date is DD/MM/YYYY

Validation in AQA pseudo-code

SUBROUTINE getValidAge()
    REPEAT
        age ← int(INPUT("Enter age (0–120): "))
        IF age < 0 OR age > 120 THEN
            OUTPUT "Invalid! Age must be between 0 and 120."
        ENDIF
    UNTIL age >= 0 AND age <= 120
    RETURN age
ENDSUBROUTINE

Authentication

Authentication is the process of verifying the identity of a user. It ensures only authorised users can access a system.

Method	Description	Example
Username & password	User knows a secret	Login to a website
Biometrics	User's physical feature	Fingerprint, Face ID
Two-factor (2FA)	Two separate checks combined	Password + SMS code

SUBROUTINE login()
    attempts ← 0
    REPEAT
        user ← INPUT("Username: ")
        pass ← INPUT("Password: ")
        attempts ← attempts + 1
        IF user == "admin" AND pass == "s3cr3t" THEN
            OUTPUT "Access granted"
            RETURN True
        ELSE
            OUTPUT "Incorrect credentials. Try " + str(3 - attempts) + " more time(s)."
        ENDIF
    UNTIL attempts >= 3
    OUTPUT "Account locked."
    RETURN False
ENDSUBROUTINE

Testing

Testing is checking that a program behaves correctly. Good testing uses a range of test data covering all categories of input.

Test data type	Description	Example (age 0–120)
Normal	Typical, expected input within range	25, 40, 70
Boundary	Values at the edge of the valid range	0, 1, 119, 120
Erroneous	Invalid data the program should reject	-1, 121, "hello", 200

Why test at boundaries?

Most programming errors occur at the boundary — e.g., using < instead of <= means the boundary value (120) is incorrectly rejected. Always test both sides of a boundary.

Types of Testing

Type	When?	What is tested?
Iterative testing	During development	Each module/subroutine as it is written
Final (terminal) testing	After all modules are complete	Whole program, end-to-end

Exam tip: For a "give examples of test data" question, always include normal, boundary, and erroneous data. State the expected result for each. AQA exams regularly give you a specific input range and ask for all three types.

⚠️ Common Mistakes

Confusing validation (checking data is acceptable) with verification (checking data is correct/true)
Forgetting boundary testing — testing 0 and 120 as well as -1 and 121 for a 0–120 range
Confusing authentication (proving who you are) with authorisation (what you're allowed to do)
Testing only normal data — erroneous data is where bugs most often hide

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Key points

Robust programs handle invalid input gracefully using validation loops
Five validation types: range, type, presence, length, format
Authentication = who you are; use REPEAT loops with attempt limits
Test data must include normal, boundary, and erroneous examples

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Worksheet — 3.2.11b Robust & Secure Programming

8 questions · 22 marks

Q1State what is meant by validation and give one example.[2]

✅ Mark scheme

Mark scheme

Validation is checking that data entered is acceptable / within allowed limits / in the correct format [1]; any suitable example e.g. a range check to ensure age is between 0 and 120 [1].

Q2State three types of validation check and describe what each checks.[3]

✅ Mark scheme

Mark scheme

Any three from (one mark each): range check — value is within permitted limits [1]; type check — data is the correct data type [1]; presence check — field is not left empty [1]; length check — string is the required length [1]; format check — data matches a required pattern [1].

Q3Write validation code in AQA pseudo-code for a score that must be between 0 and 100 inclusive.[3]

✅ Mark scheme

Mark scheme

REPEAT [1]; score ← int(INPUT(...)); IF score < 0 OR score > 100 THEN OUTPUT error message ENDIF [1]; UNTIL score >= 0 AND score <= 100 [1].

Q4Explain the difference between validation and verification.[2]

✅ Mark scheme

Mark scheme

Validation checks that data is acceptable/in the correct format/within range [1]; verification checks that data entered is the same as the original (e.g. re-entering a password to confirm it was typed correctly) [1].

Q5Give three categories of test data for a program that accepts a PIN of exactly 4 digits (1000–9999). State one example value for each.[3]

✅ Mark scheme

Mark scheme

Normal — e.g. 1234 or 5678 (expected to be accepted) [1]; boundary — e.g. 1000 or 9999 (at the edge, expected to be accepted) [1]; erroneous — e.g. 999, 10000, or "abcd" (expected to be rejected) [1].

Q6What is authentication? Give two methods of authentication.[3]

✅ Mark scheme

Mark scheme

Authentication is the process of verifying/proving the identity of a user before allowing access [1]; any two methods: username and password [1]; biometrics (e.g. fingerprint / face recognition) [1]; two-factor authentication [1] (max 2 from methods).

Q7State the difference between iterative testing and final testing.[2]

✅ Mark scheme

Mark scheme

Iterative testing happens during development — each module or subroutine is tested as it is written [1]; final (terminal) testing happens after all development is complete — the whole program is tested end-to-end [1].

Q8Why is boundary testing important? Give an example of a common boundary-related bug.[4]

✅ Mark scheme

Mark scheme

Most bugs occur at the boundary of valid/invalid ranges [1]; the boundary values themselves (e.g. 0 and 120) may be handled incorrectly by an off-by-one error [1]; example: using IF age < 120 THEN instead of IF age <= 120 THEN means 120 is incorrectly rejected [1]; testing boundary values (0, 1, 119, 120) would reveal this bug [1].

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Robust Programs

Input Validation

Types of validation

Validation in AQA pseudo-code

Authentication

Testing

Why test at boundaries?

Types of Testing

Key points

Worksheet — 3.2.11b Robust & Secure Programming

Mini Test — 3.2.11b Robust Programming