Symmetric encryption

Locking with a key

• Encryption scrambles a message so only someone with the key can read it.
• In symmetric encryption, the same key locks and unlocks. Sender and receiver must both know it.

The Caesar cipher

• The oldest example: shift every letter forward by a fixed number. With shift 3, a → d, b → e, and so on.
• The key is the shift. Decrypting just shifts back.

text = "attack"
shift = 3
out = "".join(chr((ord(c) - 97 + shift) % 26 + 97) for c in text)
print(out)        # dwwdfn

Why Caesar is weak

• There are only 25 possible shifts, so a computer can try them all in a blink.
• This is a brute-force attack on the key — exactly what real encryption must resist.

What real ciphers do

• Modern symmetric ciphers like AES use keys with billions of billions of possibilities.
• The big problem they leave: how do two people share the secret key safely? The next lesson solves it.

Covers: IGCSE 2.3 (symmetric encryption), A-Level 17.1.