1. CSC 386 – Computer Security Scott Heggen

2. Agenda Cryptography (i.e., Confusion and Diffusion)

3. The Fundamental Flaw of Ciphers Original Alphabet Caesar Offset (shift +2) Random Substitution … ACL BEK CFM DGY EHA FIJ GJF ……… k = Message index Pseudo- Random Offset 0Shift +4 1Shift -4 2Shift +6 3Shift +22 4Shift +1 5Shift -14 6Shift -8 ……

4. Frequency Analysis The English language has trends: “e” appears in English about 12% of the time “t” = ~9% “a” = ~8%  Common digrams: “he” “an” “in” “th”

5. Data Encryption Standard (DES) Developed by IBM in the 70s as the standard for encryption by the U.S. government Based on Feistel networks Remained the standard for 20+ years Has since been broken by exhaustive search and replaced with a new standard

6. Feistel Network Reversible function! (meaning the decryption method is the same as encryption)

7. The Key A single key is permutated into 16 subkeys A series of left-shifts on the original key Each subkey is then permuted (diffused)

8. The F-function First, the block is expanded Second, the expanded block is X- OR’ed with the subkey Next, fed into “S boxes” which map 6 bits to 4 bits Based on a table Finally, output is permuted bits are diffused

9. DES

10. Assignment For Wednesday: Read up on DES (Chapter 14, Page 265 – 266). I highly suggest reading up on additional sources on the web, such as: http://en.wikipedia.org/wiki/Data_Encryption_Standard http://www.youtube.com/watch?v=qHZKze24kVo http://people.eku.edu/styere/Encrypt/JS-DES.html Quiz on DES: 1 question on The Feistel Structure 1 question on the f-function 1 question on Sub-key Generation