Encryption converts 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.
Plaintext: HELLO Key: 3 (Caesar cipher — shift each letter by 3) Ciphertext: KHOOR
Encryption protects data in transit and at rest — essential for online banking, passwords, email
Symmetric Encryption
One Key for Both Sides
Symmetric encryption: the same key is used to encrypt and decrypt. Both the sender and receiver must have the same secret key. Fast and efficient.
Advantage: very fast, efficient for large amounts of data (e.g. AES encryption)
Disadvantage: the key must be shared securely — how do you share it safely in the first place?
Example: AES (Advanced Encryption Standard) — used in Wi-Fi (WPA2), disk encryption
Asymmetric Encryption
Public and Private Keys
Asymmetric encryption: uses a mathematically linked key pair. The public key encrypts data (shared freely); the private key decrypts it (kept secret). Used in HTTPS, email signing.
Advantage: no need to share a secret key — public key can be published openly
Disadvantage: much slower than symmetric encryption — computationally expensive
Used to establish a secure channel, then symmetric keys are exchanged within it
Exam Practice
Have a go at this question
Cambridge IGCSE 0478 style
State one advantage and one disadvantage of asymmetric encryption compared to symmetric encryption.
2 marks
Advantage: the public key can be shared freely so there is no need to exchange a secret key beforehand [1]. Disadvantage: asymmetric encryption is much slower/more computationally expensive [1].