Encryption on the Internet

Eirik Albrigtsen

Encryption & Decryption

Text Unreadable Text key + encryption method

Unreadable Text Text key + decryption method

Caesar’s cipher

Modular Arithmetic

18 = 6 mod 1215 = 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

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

Caesar’s cipher

the quick brown fox jumps over the lazy dog

WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ

Security of Caesar’s cipher

To decrypt x: D(x) = x-3 mod 26 K=3=shift factor

Max 25 possible values of K.

Security of Caesar’s cipher

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 01+4n: shift by 32+4n: shift by 73+4n: shift by 3

• Example• X = BOB BOB = 1 14 1 1 14 1• E(X) = BRI EOB = 1 17 8 4 14 4

security = published and attacked, but not broken.

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