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. A=0, B=1, C=2, ... , Y=24, Z=25. To encrypt x: E(x) = x+3 mod 26
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 =
E(X) = BRI EOB =

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

12. Trust This