Cryptography / Classic ciphers
Lesson 4 of 25
Substitution and frequency analysis
A substitution cipher swaps each letter for another. It has huge numbers of keys, but letters keep their frequency: E is the most common letter in English, so patterns give it away.
Key points
- E, T, A, O are common in English
- Count letters in the ciphertext
- Shows why keys alone aren't enough
Quiz · +10 XP
What technique breaks simple substitution ciphers?
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