Outline for October 10, 2025

Reading: text, §10.2.3–10.5
Assignments: Homework 1, due October 8; Project selection, due Oct 10

1. Class overview

2. Product ciphers
   1. DES
      1. DES modes
   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

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 235A, Computer and Information Security Version of October 10, 2025 at 10:27AM

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