What Is Encryption? KS3 Computing Explained

Encryption scrambles data into an unreadable form so that only someone with the correct key can reverse the process. Every time you see the padlock in your browser's address bar, encryption is protecting your password and payment details from eavesdroppers. At KS3 it sits at the heart of cyber security and network safety.

What problem does encryption solve?

When you send data across a network it passes through many devices — routers, switches, servers — before reaching its destination. Without encryption, anyone who intercepts a packet along that route could simply read its contents. A username, a bank card number, a private message: all visible as plain text.

Encryption transforms the original readable data — called plaintext — into a scrambled form called ciphertext. Converting plaintext to ciphertext is encryption; reversing it is decryption. Even if an attacker captures every packet, they see only meaningless ciphertext without the key.

What is a cipher and what is a key?

A cipher is the algorithm — the set of mathematical rules — used to perform encryption and decryption. A key is a piece of data (typically represented as a very long number) that the cipher uses to perform the scrambling. Without the correct key, reversing a modern cipher would take longer than the age of the universe, even with powerful computers.

A simple historical example is the Caesar cipher, which shifts every letter in the alphabet by a fixed number of places:

Plaintext | Key (shift) | Ciphertext
HELLO | 3 | KHOOR
ATTACK | 3 | DWWDFN
MEET ME | 3 | PHHW PH

To decrypt, the receiver shifts each letter back by 3. The Caesar cipher is trivially broken today — there are only 25 possible keys — but it illustrates the principle that underpins all modern encryption.

What is symmetric encryption?

In symmetric encryption the same key is used to both encrypt and decrypt the data. Both the sender and the receiver must possess a copy of the shared secret key before they communicate.

Plaintext ──[encrypt with key K]──► Ciphertext ──[decrypt with key K]──► Plaintext

Advantage: symmetric encryption is computationally fast, making it suitable for encrypting large volumes of data — entire hard drives, video streams, or large files.

Disadvantage: both parties need the same key, which raises a problem: how do you share the key securely in the first place? If you send the key over the network and an attacker intercepts it, your encrypted data is immediately compromised.

What is asymmetric encryption?

Asymmetric encryption (also called public-key encryption) solves the key-sharing problem by using two mathematically linked keys: a public key and a private key.

The public key can be shared freely with anyone. It is used to encrypt data.

The private key is kept secret by the owner. It is the only key that can decrypt data encrypted with the corresponding public key.

Think of it as a letterbox with a slot on the outside. Anyone can push a letter through the slot (encrypt using the public key), but only the person with the key to the letterbox (the private key) can open it and read the letters inside.

Feature | Symmetric encryption | Asymmetric encryption
Number of keys | 1 shared secret key | 2 keys: public + private
Speed | Fast | Slower
Key distribution | Difficult — must be shared secretly | Easy — public key can be published
Typical use | Bulk data encryption | Key exchange, digital signatures, HTTPS handshake

How does HTTPS use encryption?

HTTPS (HyperText Transfer Protocol Secure) uses a clever combination of both types. When your browser connects to a secure website, it performs a TLS handshake:

The server sends its public key (embedded in an SSL/TLS certificate).

Your browser uses the public key to securely share a random session key.

All subsequent data in that session is encrypted using symmetric encryption with that session key, because symmetric encryption is fast enough for real-time communication.

This gives the best of both worlds: asymmetric encryption solves the key-sharing problem; symmetric encryption handles the bulk data efficiently. The padlock icon in your browser's address bar confirms this handshake completed successfully.

Why does encryption appear in the national curriculum?

The DfE computing programme of study requires KS3 students to understand how to evaluate and apply measures to make systems safe and secure, and to appreciate how data is represented and protected (gov.uk/government/publications/national-curriculum-in-england-computing-programmes-of-study). Encryption is the primary technical mechanism for protecting data both in transit (over a network) and at rest (on a storage device).

At GCSE, encryption is an explicit topic on all major specifications including OCR J277 and AQA 8525. Understanding the principles now — plaintext, ciphertext, symmetric and asymmetric encryption — means the GCSE material builds on solid ground rather than starting from scratch.

Frequently asked questions

What is encryption in simple terms for KS3?

Encryption is the process of scrambling data so that it can only be read by someone with the correct key. The original readable data is called plaintext; the scrambled version is ciphertext. Decryption is the process of turning ciphertext back into plaintext using the key.

What is the difference between symmetric and asymmetric encryption?

Symmetric encryption uses one shared key for both encrypting and decrypting — it is fast but requires both parties to already have the same key. Asymmetric encryption uses two keys: a public key (which anyone can use to encrypt data) and a private key (which only the owner holds and uses to decrypt). Asymmetric encryption is slower but solves the key-distribution problem.

What is a Caesar cipher and how does it relate to modern encryption?

A Caesar cipher shifts each letter in a message by a fixed number of places — for example, a shift of 3 turns A into D, B into E, and so on. It demonstrates the concepts of encryption (transforming plaintext with a key) and decryption (reversing it), but it is trivially easy to crack since there are only 25 possible shift values. Modern ciphers apply the same principles to binary data using mathematically complex algorithms, making them computationally infeasible to crack without the key.

What does the padlock icon in a browser mean?

The padlock means the connection between your browser and the website is encrypted using HTTPS. The website has a valid SSL/TLS certificate, which both confirms the website's identity and provides the public key needed to initiate the encrypted session. Any data you send — passwords, form entries, payment details — is encrypted and cannot be read by an eavesdropper.

Can encrypted data ever be broken?

Modern encryption algorithms like AES-256 are considered computationally secure, meaning that breaking them by trying every possible key would take far longer than the age of the universe even with the most powerful computers available. However, encryption can be broken indirectly — for example, by stealing the key itself, by exploiting implementation flaws in software, or through social engineering. The weakest link in encryption is almost always human behaviour, not the mathematics.

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