1. Introduction to Cryptography Lecture 4

2. Caesar Cipher Gaius Julius Caesar (100 B.C.- 44 B.C.) General Politician Dictator of Rome Creator of Caesar Cipher

3. Caesar Cipher The cipher key: This cipher shift alphabet three letters to the left. ABCDEFGHIJKLMNOPQRSTUVWXYZ DEFGHIJKLMNOPQRSTUVWXYZABC

4. Shift Cipher Definition: The shift cipher is the cipher which shifts alphabet k letter to the left/right. Caesar cipher is example of shift cipher. Question: Is shift cipher a secure cipher?

5. ASCII codes for printable characters

6. Modular Arithmetic & Shift Cipher Assume the message written using capital letters only Then we can represent every letter using ASCII using number 65 – 90  Then apply mod 65 on each number  As the result: A become 0, B become 1, …

7. Modular Arithmetic & Shift Cipher Do the shift of k letters by taking. To get letter representation, do: c + 65, and then use ASCII table. The book gives a nice example of how Excel does it.

8. Modular Arithmetic & Shift Cipher Example: SKYISCLEAR 83758973836776696582 1810248182114017 2719331727112013926 119717111201390 66847282667685787465 BTHRBLUNJA

9. Modular Arithmetic & Shift Cipher For English alphabet shift cipher takes plaintext x as input and gives ciphertext y as output, where.

10. Homework Read pg.57-66. Exercises: 9, 11, 14 on pg.67-68. Those questions will be a part of your collected homework.