1. CSC 386 – Computer Security Scott Heggen

2. Agenda Cryptography

3. Substitution Cipher (Caesar Cipher) AC BE CF DG EH FI GJ …… k = AliceBob Eve k k “bad egg” E C = E(k,m) = “egc hjj” “bad egg” D “egc hjj”

4. Ciphers How can we improve upon the Caesar Cipher?

5. Other Substitution 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 ……

6. Assignment For Monday: Create a Python script which takes in two files: original_message.txt and key.txt The original_message.txt file can contain any message you chose to send me The key.txt file must contain your B-number, without the letter B The Python script must compute C using the message and the key. Your encryption algorithm E should be a pseudo-random substitution cipher which shifts each letter of message by the integer value in key with the same index (e.g., message[0] should be shifted by key[0] letters) Submit to Moodle: Your Python Script coded_message.txt