1. Encryption on the Internet Jeff Cohen

2. Keeping Information Secret What information do we want to be secret? –Credit card number –Social security number –Personal communication Insecure communication (hackers)

3. Shared Secrets Two people, in private, make secret code No one else knows secret No one else can read their messages Great for codes with people you know Not so great on the Internet

4. Requirements No shared secrets Private secrets are OK I’ll keep some information public I’ll publish the rest

5. Factoring You can factor easily: 15 = 3 x 5 But humans can’t factor this: 12413867828429 If the numbers are big enough, neither can a computer We can hide all the information in factoring

6. The RSA Cryptosystem Pick large primes p and q n = pq Pick e and d so that ed / [(p-1)(q-1)] has a remainder of 1 Publish n and e Encryption: E(M) = M e mod n Decryption: D(M) = M d mod n Cannot find d if we only know e and n n = 100761443 = 10037 x 10039 e = 45791531 d = 11 cabbage => 3122175 = M E(3122175) = 14631615 D(14631615) = 3122175