Cryptanalysis

Frequency analysis · attack models · modern resistance

1:00

How classical ciphers are broken through frequency analysis and the index of coincidence, why attack models matter, and how modern ciphers are designed to resist all of them.

By the end, you can

Explain why cryptanalysis is more powerful than brute-force search.
Describe how frequency analysis breaks Caesar and simple substitution ciphers.
Trace the algorithm that scores all 26 Caesar shifts to find the key.
Explain why Vigenère was thought unbreakable ("le chiffre indéchiffrable") and how the index of coincidence reveals the key length, reducing it to several independent Caesar ciphers.
Define and rank the four standard attack models (COA, KPA, CPA, CCA).
Explain how confusion, diffusion, and the avalanche effect make modern ciphers resistant to statistical and frequency analysis.
Distinguish ECB mode's failure from a semantically secure (IND-CPA) mode.

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