- Greetings and Felicitations!
- Puzzle of the Day
- Classical Ciphers
- monoalphabetic (simple substitution):
*f*(*a*) =*a*+*k***mod***n* - example: Cæsar with
*k*= 3,`RENAISSANCE`->`UHQDLVVDQFH` - polyalphabetic: Vigenère,
*f*_{i}(*a*) =*a*+*k*_{i}**mod***n* - cryptanalysis: do index of coincidence to see if it's monoalphabetic or polyalphabetic, then Kasiski method.
- problem: eliminate periodicity of key

- monoalphabetic (simple substitution):
- Long key generation
- 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*.) - Enigma/rotor systems; wheels, 3 rotors and a reflecting one. Go
through it; UNIX uses this for
*crypt*(1) command. - 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`?

- Running-key cipher:
- DES
- Go through the algorithm

- Public-Key Cryptography
- Basic idea: 2 keys, one private, one public
- Cryptosystem must satisfy:
- given public key, CI 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 PKC
- Normally used as key interchange system to exchange secret keys (cheap)
- Then use secret key system (too expensive to use PKC for this)

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Send email to cs153@csif.cs.ucdavis.edu.

Department of Computer Science

University of California at Davis

Davis, CA 95616-8562