* Reading*: §9.2

- Greetings and felicitations!
- Quick review for midterm

- Classical Cryptography
- Polyalphabetic: Vigenère,
*f*_{i}(*a*) =*a*+*k*_{i}mod*n* - Cryptanalysis: first do index of coincidence to see if it is monoalphabetic or polyalphabetic, then Kasiski method.
- Problem: eliminate periodicity of key

- Polyalphabetic: Vigenère,
- Long key generation
- Autokey cipher:
*M*= THETREASUREISBURIED;*K*= HELLOTHETREASUREISB;*C*= ALPEFXHWNIIIKVLVQWE - Running-key cipher:
*M*= THETREASUREISBURIED;*K*= THESECONDCIPHERISAN;*C*= MOILVGOFXTMXZFLZAEQ; wedge is that (plaintext, key) letter pairs are not random (T/T, H/H, E/E, T/S, R/E, A/O, S/N, etc.) - Perfect secrecy: when the probability of computing the plaintext message is the same whether or not you have the ciphertext
- Only cipher with perfect secrecy: one-time pads;
*C*= AZPR; is that DOIT or DONT?

- Autokey cipher:
- DES

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