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Playfair Cipher andShift CipherKriptografi – 3rd Week

Aisyatul Karima, 2012

Standar kompetensi Pada akhir semester, mahasiswa menguasai

pengetahuan, pengertian, & pemahaman tentangteknik-teknik kriptografi. Selain itu mahasiswadiharapkan mampu mengimplementasikan salah satuteknik kriptografi untuk mengamankan informasi yang akan dikirimkan melalui jaringan.

Kompetensi dasar Mahasiswa menguasai teknik playfair cipher Mahasiswa menguasai teknik shift cipher

Contents

1 Play fair Cipher Method

2 Shift Cipher Method

Playfair Cipher Method

Playfair cipher or Playfair square is symetricencryption technique that member of digraphsubstitution technique. This technique encrypt digraph or pair of

alphabet Based on the reason, this technique hard to

encode compare with the simple substitutiontechnique.

Playfair Cipher Method

This technique foundby CharlesWheatstone onphysics he is founderof wheatstone bridgeon 1854. But, popularized by

Lord Playfair.

Charles Wheatstone

Lord Playfair

Playfair Cipher Method

The process of playfair cipher : The key composed by 25 letters that arranged

in a square 5x5 by removing the letter J from alphabet.

S T A N DE R C H BK F G I LM O P Q UV W X Y Z

?????

contoh kunci yang digunakan Jumlah kemungkinan kunci dari sistem ini

adalah :25!=15.511.210.043.330.985.984.000.000

Playfair Cipher Method

The keys on square expanded by adding the6th column and the 6th row.

S T A N D SE R C H B EK F G I L KM O P Q U MV W X Y Z VS T A N D 6th row = 1st row

6th column = 1st column

Playfair Cipher Method

The plaintext that hidden must be arrangedbefore encryption process :

• Change letter J with the letter I• Write the plaintext on pair of letters• There must be no same pair of letters, if there is

the same pair of letter, insert Z in the middle.• If the amount of letter is odd, add the letter Z at the

end

Playfair Cipher Method

Example : plaintext

GOOD BROOMS SWEEP CLEAN

There is no letter J, so write the plaintext on letter pairs.GO OD BR OZ OM SZ SW EZ EP CL EA NZ

Ciphertext = … ???

Playfair Cipher Method The encryption algorithm :

If there are two letters on the same key rows, then eachletter changed by the letter on the right side.

If there are two letters on the same key column, then eachletter changed by the letter on the bottom side.

If there are no two letter on the same row or the samecolumn, then the 1st letter changed by the letter on rowintersected by the 1st letter with the 2nd letter. The 2nd letterchanged by the letter on the 4th vertex of rectangle thatbuilt from 3 letters that used so far.

plainteksGOOD BROOMS SWEEP CLEAN

Cipherteks:FP UT EC UW PO DV TV BV CM BG CS DY

Playfair Cipher Method plaintext

UNIVERSITAS DIAN NUSWANTORO

FAKULTAS ILMU KOMPUTER

DUMUNING INGSUN ANGRAKSO NEGORO

SEMARANG JAWA TENGAH

TEKNIK INFORMATIKA

LAWANG SEWU KERAMAT

Shift Cipher Method

Based on history, the shift cipher was ever beenused at the time of ancient roman on JuliusCaesar period. The method was very simple by moving each

letters of plaintext.

Shift Cipher Method

For the example, if X is the plaintext on the form of number : K is Key with 0 ≤ K ≤ 25 and Y is ciphertext on the form of number

The encryption process given by function : eK(x) = (x + K) mod 26 The decryption process given by function :

dK(y) = (y – K) mod 26.

Shift Cipher Method

For the key K = 3, shift cipher often calledby Caesar Cipher, For the Key K = 13 often called by Rot-13

cipher. Example : Define the ciphertext of plaintext “saya” using

K = 3 “…..” ???

Shift Cipher Method

Contoh: Shift Cipher with 13 movement :

Plaintext :

MERWAN MINTA MAAF PADA MARWAN

Ciphertext = …. ???

A B C D E F G H I J K L M N O P Q R S T U V W X Y ZM N O P Q R S T U V W X Y Z A B C D E F G H I J K L

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