Data validation & authentication routines – GCSE Computer Science exam questions with mark scheme

1Grade 1–33 marks

State the type of validation check being described in each case. [3]

(a) An exam mark entered must be between 0 and 100.
(b) The username field must not be left empty.
(c) A password must contain at least 8 characters.

(a) Range check(1)
(b) Presence check(1)
(c) Length check(1)
Answer: (a) Range (b) Presence (c) Length
Examiner tip: A length check tests the number of characters; a range check tests the size of a value.

2Grade 6–75 marks

A program asks the user how many concert tickets they want to buy. The number must be between 1 and 6 inclusive.

Write an algorithm, using pseudo-code or program code, that keeps asking the user to enter the number of tickets until a valid number is entered, displaying an error message each time an invalid number is entered. You may assume the user always enters an integer. [5]

Gets the number of tickets from the user (e.g. tickets ← STRING_TO_INT(USERINPUT)); ignore missing conversion as integer input is assumed(1)
Uses a loop that repeats (WHILE / REPEAT…UNTIL)(1)
Correct condition to identify invalid input: tickets < 1 OR tickets > 6 (or valid: tickets ≥ 1 AND tickets ≤ 6 for UNTIL)(1)
Outputs an error message only when the input is invalid(1)
Re-input happens inside the loop, so the loop ends only when a valid number is entered(1)

Examiner tip: Using AND in the invalid condition (tickets < 1 AND tickets > 6) can never be true, so it loses the condition mark.

3Grade 6–75 marks

This algorithm is an authentication routine.

attempts ← 0
REPEAT
   OUTPUT 'Enter password'
   pw ← USERINPUT
   attempts ← attempts + 1
UNTIL pw = 'Tr0ub4dor' OR attempts = 3
IF pw = 'Tr0ub4dor' THEN
   OUTPUT 'Access granted'
ELSE
   OUTPUT 'Locked'
ENDIF

(a) The user enters abc then Tr0ub4dor. State the final value of attempts and the final output. [2]

(b) State the final output if the user enters red, blue and then green. [1]

(d) Suggest one other way the authentication could be made more secure. [1]

(a) attempts = 2(1)
(a) Access granted(1)
(b) Locked(1)
(c) Stops an attacker trying many passwords / a brute-force attack / guessing until they find the right one(1)
(d) e.g. two-factor authentication (code sent to phone); don't store the password in the code / store it hashed or encrypted; require a stronger password; lock the account for a set time / alert the owner after failed attempts. Do not credit simply 'limit the number of attempts' (already given)(1)
Answer: (a) 2, Access granted (b) Locked (c) prevents brute-force guessing (d) e.g. two-factor authentication
Examiner tip: Count carefully: attempts increases once every time the loop body runs, including the successful attempt.

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