1. Encryption on the Internet Eirik Albrigtsen

2. Encryption & Decryption Text Unreadable Text key + encryption method Unreadable Text Text key + decryption method

3. Caesar’s cipher

4. Modular Arithmetic 18 = 6 mod 12 15 = 3 mod 12 26 = 2 mod 12, as 26 and 2 are 2*12 apart General mathematical statement: a = b mod N  a and b are a multiple of N apart

5. Caesar’s cipher A=0, B=1, C=2,..., Y=24, Z=25. To encrypt x: E(x) = x+3 mod 26 Example: C = 2, E(2) = 2+3 mod 26 = 5 = F Y=24, E(24) = 24+3 mod 26 = 27 mod 26 = 1 = B

6. Caesar’s cipher the quick brown fox jumps over the lazy dog WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ

7. Security of Caesar’s cipher To decrypt x: D(x) = x-3 mod 26 K=3=shift factor Max 25 possible values of K.

8. Security of Caesar’s cipher

9. Vigenere Cipher Keyword K = ADHD = 0 3 7 3 (length 4) Encrypt separately each letter depending on its position in the message. If the position is of the form: 0+4n: shift by 0 1+4n: shift by 3 2+4n: shift by 7 3+4n: shift by 3 Example X = BOB BOB = 1 14 1 1 14 1 E(X) = BRI EOB = 1 17 8 4 14 4

11. security = secrecy security = published and attacked, but not broken.

12. Trust This