How is data kept secure, accurate and recoverable in a software system, and how do backup, validation and encryption protect it?
Describe data security and integrity measures: backup and recovery, validation and verification, error detection, and encryption in depth.
A focused answer to WJEC A-Level Computer Science Unit 3 data security and integrity, covering backup and recovery, validation and verification, error detection methods such as parity and checksums, and encryption in depth.
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What this dot point is asking
WJEC wants you to describe how a system keeps data secure, accurate and recoverable: backup and recovery strategies, validation and verification, error-detection methods such as parity, checksums and check digits, and encryption in depth. This rounds off the Unit 3 systems content and links to the transmission and analysis topics. Expect questions on error detection (especially parity), on backup types, and on validation versus verification.
The answer
Backup and recovery
The trade-off is speed versus restore simplicity: full backups restore in one step but are large and slow to make; incremental backups are quick to make but need the last full backup plus every increment to restore.
Validation and verification
Both run together in practice because they catch different faults: validation rejects nonsense, while verification catches mistyped but otherwise plausible values.
Error detection in transmission
Transmission can corrupt data, so error-detection methods are added. A parity bit makes the total number of 1s even or odd, as agreed, and the receiver re-checks it. A checksum is a value calculated from the data and sent with it for the receiver to recalculate and compare. A check digit is an extra digit calculated from a number (such as a barcode or ISBN) to catch entry errors.
Encryption in depth
Examples in context
- Example 1. The 3-2-1 backup rule in practice
- A business keeps three copies of its data, on two different media, with one off-site. When a fire destroys the office, the off-site copy survives and the business recovers. This concretely shows why a recovery plan specifies not just that backups exist but where they are kept, so a single disaster cannot take them all.
- Example 2. A check digit on a barcode
- The last digit of a barcode is calculated from the others, so a scanner that misreads one digit produces a mismatch and rejects the read. This everyday error detection catches the most common entry mistakes at the point of capture, illustrating why check digits are built into identification numbers.
- Example 3. Encryption plus error detection together
- Sending a file over the internet, the data is encrypted for confidentiality and carries a checksum for integrity. Encryption stops an interceptor reading it; the checksum lets the receiver confirm it arrived uncorrupted. The two address different risks, which is why a robust system uses both rather than choosing one.
Try this
Q1. State the type of backup that copies only the data changed since the last backup. [1 mark]
- Cue. An incremental backup.
Q2. State one limitation of using a single parity bit for error detection. [1 mark]
- Cue. It detects only an odd number of bit errors; an even number of flipped bits goes undetected, and it cannot correct errors.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20204 marksExplain how a parity check detects an error in transmitted data, and state one limitation of it.Show worked answer →
Describe how the parity bit is set and checked, then give a limitation.
A parity bit is added to each unit of data so that the total number of 1s is even (even parity) or odd (odd parity), as agreed. The sender sets the parity bit accordingly; the receiver counts the 1s and checks the parity matches. If it does not, an error has occurred during transmission.
Limitation: a single parity bit detects only an odd number of bit errors. If two bits (or any even number) are flipped, the parity is unchanged and the error goes undetected. It also cannot correct errors, only detect them.
Markers reward setting the parity bit for even or odd total, the receiver re-checking, and the limitation that an even number of errors is missed.
WJEC 20224 marksExplain why regular backups are essential and describe one difference between a full backup and an incremental backup.Show worked answer →
Explain the purpose of backups, then contrast the two backup types.
Regular backups are essential so that data can be recovered after loss or corruption from hardware failure, human error, malware (such as ransomware) or disaster. Without recent backups, lost data may be unrecoverable.
A full backup copies all the selected data every time, so recovery is simple but each backup is large and slow. An incremental backup copies only the data changed since the last backup, so each backup is small and fast, but recovery needs the last full backup plus all subsequent increments in order.
Markers reward the recovery-after-loss purpose, and a correct contrast such as full copying everything versus incremental copying only changes.
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