**Reading**: *text*, §10 handout (§9 in the book)\\
**Due**: Homework 2, due February 5; Project progress report, due February 8

- Classical Cryptography
- Polyalphabetic: Vigenère,
*f*(_{i}*a*) =*a*+*k*mod_{i}*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:
- Product ciphers: DES, AES
- Public-Key Cryptography
- Basic idea: 2 keys, one private, one public
- Cryptosystem must satisfy:
- Given public key, computationally infeasible to get private key;
- Cipher withstands chosen plaintext attack;
- Encryption, decryption computationally feasible (
*note*: commutativity not required)

- Benefits: can give confidentiality or authentication or both

- Use of public key cryptosystem
- Normally used as key interchange system to exchange secret keys (cheap)
- Then use secret key system (too expensive to use public key cryptosystem for this)

- Diffie-Hellman
- Goal is to share a common key (
*symmetric key exchange protocol*) - Given
*n*,*g*, prime*p*, compute*k*such that*n*=*g*mod^{k}*p* - Choose
*k*as private key, make public key*K*=*g*mod^{k}*p*

- Goal is to share a common key (

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