What Makes Encryption Strong?
Two properties. Claude Shannon identified them in 1945.
Diffusion: A small change in input causes a big change in output.
Confusion: The relationship between the key and output is complex.
Together, they make a cipher nearly impossible to break.
Diffusion
Imagine you encrypt a message. Then you change one letter and encrypt again.
What happens to the output?
Weak cipher:
- One letter changes in the output
- The two ciphertexts look almost identical
- Patterns survive
Strong cipher:
- The entire output changes
- The two ciphertexts look completely different
- Patterns are destroyed
This is called the avalanche effect.
One small change at the input cascades through the entire output.
Why Diffusion Matters
Languages have patterns.
- In English, ‘e’ is the most common letter
- Words like “the” appear constantly
- Certain letter pairs are predictable
Without diffusion, these patterns might survive encryption.
An attacker could use frequency analysis to spot them, then work backwards to crack the cipher.
With strong diffusion?
Even nearly identical messages produce completely different ciphertexts.
No patterns. Nothing to analyze.
Confusion
Now think about the key.
If an attacker has your ciphertext, can they figure out the key?
Weak cipher:
- The key affects the output in a simple way
- Change one part of the key → one part of the output changes
- Easy to trace backwards
Strong cipher:
- The key affects the output in a complex way
- Every part of the key influences every part of the output
- Impossible to untangle
Why Confusion Matters
Think about the Caesar cipher.
Key = 3 means shift every letter by 3:
- A → D
- B → E
- C → F
An attacker sees ‘D’ in the ciphertext.
They know it probably came from ‘A’. The relationship is obvious.
One guess and they have the key.
A strong cipher mixes the key into the output so thoroughly that there’s no pattern to exploit.
Knowing the output tells you nothing about the key.
Together
- Diffusion hides patterns in your message
- Confusion hides the key
Modern ciphers apply both, over and over. That’s what makes them strong.