Lecture 10: April 20, 2026

Reading: text, §10.2.3–10.3, 10.3.2
Assignments: Homework 2, due April 24, 2026

1. Greetings and felicitations!

2. Product ciphers
   1. DES
   2. AES

3. Public-Key Cryptography
   1. Basic idea: 2 keys, one private, one public
   2. Cryptosystem must satisfy:
      1. Given public key, computationally infeasible to get private key;
      2. Cipher withstands chosen plaintext attack;
      3. Encryption, decryption computationally feasible (note: commutativity not required)
   3. Benefits: can give confidentiality or authentication or both; simplifies key distribution

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

5. RSA
   1. Provides both authenticity and confidentiality
   2. Based on difficulty of computing totient, φ(n), when n is difficult to factor

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 153A, Computer & Information Security & Privacy I Version of April 20, 2026 at 1:55PM

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