Plaintext · Ciphertext · Caesar Cipher · Symmetric & Asymmetric
CSZoneAQA GCSE Computer Science 8525
What is Encryption?
Scrambling Data to Keep it Safe
Encryption is the process of converting readable data (plaintext) into an unreadable form (ciphertext) using an algorithm and a key. Only someone with the correct key can decrypt and read the data.
Each letter is shifted a fixed number of places along the alphabet. The shift value is the key. This is a substitution cipher — each letter is replaced by another.
Key = 3 (shift right by 3)
Plaintext: H E L L O Ciphertext: K H O O R
A→D, B→E, C→F, D→G, E→H ... Z→C
To decrypt: shift LEFT by 3 K→H, H→E, O→L, O→L, R→O = HELLO
Symmetric vs Asymmetric Encryption
Types of Encryption Used Today
SYMMETRIC
Same key is used to encrypt AND decrypt. Faster, but the key must be shared securely.
Examples: AES
ASYMMETRIC
Two keys: a public key (shared) encrypts; a private key (secret) decrypts. Slower but more secure.
Examples: RSA, HTTPS
HTTPS:Uses asymmetric encryption to share a symmetric key, then symmetric for the actual data.
Exam Practice
Have a go at this question
AQA-style question
Using a Caesar cipher with a key of 4, encrypt the word 'GCSE'. Show your working. Give one limitation of the Caesar cipher compared to modern encryption.
3 marks
G→K, C→G, S→W, E→I → KGWI [1] Limitation: only 25 possible keys so it can be cracked by brute force / trial and error very easily [1]. Modern ciphers have vastly more possible keys making brute force infeasible [1].
Key Takeaways
What to Remember
Encryption converts plaintext → ciphertext using a key and algorithm
Caesar cipher: shift each letter by a fixed amount (the key)
Symmetric: one shared key · Asymmetric: public key encrypts, private key decrypts
Used in HTTPS, banking, messaging — protects data in transit and storage