Encryption — Symmetric & Asymmetric

Syllabus & Goals 3 min

Cambridge 2.3 · Symmetric and asymmetric encryption Paper 1 · Computer Systems

By the end of this lesson you can:

1. Explain the purpose of encryption, using plaintext and ciphertext.
2. Describe symmetric encryption and its key-distribution problem.
3. Describe asymmetric encryption using a public and private key.

Recap / Warm-Up 5 min

Error checks make sure data arrives intact. Encryption makes sure that, even if data is intercepted, it can't be understood.

Quick starter

You shift every letter of CAT forward by one (C→D, A→B, T→U). What word do you send, and how does your friend read it back?

Key Concept 14 min

1 · The purpose of encryption

On a public network, data can be intercepted by a hacker (an eavesdropper). Encryption can't stop interception, but it makes the data unreadable to anyone without the key.

2 · Symmetric encryption

Symmetric encryption uses the same key to encrypt and to decrypt. It is fast, but both people need that one secret key.

3 · Asymmetric encryption

Asymmetric encryption solves that problem using a matching pair of keys:

• a public key — shared with everyone, used to encrypt;
• a private key — kept secret by the owner, used to decrypt.

Worked Example 12 min

(a) A simple symmetric cipher

Take the plaintext HELLO and a key of 3 (shift each letter three places forward):

Ciphertext = KHOOR. The receiver shifts each letter back by 3 (the same key) to recover HELLO.

The cipher as an algorithm

Cambridge pseudocode

// Symmetric encryption: Caesar shift on letter positions (A = 0 ... Z = 25)
DECLARE Plain : ARRAY[1:5] OF INTEGER
DECLARE Cipher : ARRAY[1:5] OF INTEGER
DECLARE Index, Key : INTEGER
Plain ← [7, 4, 11, 11, 14]
Key ← 3
FOR Index ← 1 TO 5
    Cipher[Index] ← (Plain[Index] + Key) MOD 26
NEXT Index
OUTPUT "Cipher positions: ", Cipher

The same idea in Python (IDLE)

# Symmetric encryption: a Caesar cipher shifts each letter by the key
plain_text = "HELLO"
key = 3
cipher_text = ""
for letter in plain_text:
    shifted = (ord(letter) - ord("A") + key) % 26
    cipher_text = cipher_text + chr(shifted + ord("A"))
print("Cipher text is", cipher_text)

Output

Cipher text is KHOOR

Try It Yourself 12 min

📝 Exam Practice 10 min

Recap & Key Terms 3 min

Encryption turns plaintext into unreadable ciphertext. Symmetric uses one shared key (fast, but the key can be intercepted). Asymmetric uses a public key to encrypt and a secret private key to decrypt. That completes Unit 2 — Data transmission.

Plaintext / ciphertext

The readable original data / the scrambled encrypted output.

Symmetric encryption

Encryption that uses the same key to encrypt and decrypt.

Asymmetric encryption

Encryption using a public key (to encrypt) and a matching private key (to decrypt).

Public / private key

A mathematically linked pair; the public key is shared, the private key is kept secret.

Homework 1 min

Task (≤ 15 min): (a) Encrypt SECURE with a symmetric shift key of 4. (b) State one advantage of asymmetric over symmetric encryption.