Lesson 1.7: Classical ciphers and Kerckhoffs's principle
How encryption began: substitution ciphers like Caesar's, and how they fall to frequency analysis because languages use some letters far more than others. Why a secret method is no protection (Kerckhoffs's principle), why the key space must be huge, and what modern ciphers do differently. In the lab you break an intercepted message from an old in-house 'encryption' tool by counting letters, decrypt it with tr, and meet ROT13, the 'encryption' still used to hide spoilers.
16 min
You will learn to
- Explain substitution ciphers and the Caesar cipher
- Break a Caesar cipher with frequency analysis
- State Kerckhoffs's principle
- Explain key space and brute force
- Explain why classical ciphers are only of historical interest
Key terms
- Substitution cipher
- A cipher that replaces each letter with another by a fixed rule.
- Caesar cipher
- A substitution cipher that shifts every letter by the same number of places; the number is the key.
- Frequency analysis
- Breaking a substitution cipher by comparing how often each symbol appears with the known letter frequencies of the language.
- Kerckhoffs's principle
- A cryptosystem should stay secure even if everything about it except the key is public.
- Key space
- The number of possible keys. Brute force tries them all.
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