Alphabet Substitution Technique -...

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LOGO Alphabet Substitution Technique Kriptografi - Week 2 Aisyatul Karima, 2012

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Alphabet Substitution TechniqueKriptografi - Week 2

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 caesar cipher Mahasiswa menguasai substitusi abjad tunggal Mahasiswa menguasai substitusi abjad majemuk

Contents

Caesar Cipher1

Monoalphabetic Cipher2

Polyalphabetic Cipher3

Caesar Cipher

Caesar Cipher is a symmetric cryptography.

Symmetric Cryptography is the cryptography that used the same key for encryption and decryption process.

Caesar Cipher

Decryption process must use the same key with the encryption process to encode the data encrypted. So, the public key is the rule for both of

encryption and decryption process.

Caesar Cipher

Example of Caesar Cipher : Caesar cipher is the encryption method that used on

Julius Caesar period. Only used for alphabet, including the capital and small

alphabet. If the plaintext is a number, so the encryption process is

can’t work perfectly. The encryption method is rotate three steps Example :

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

Monoalphabetic Cipher

Monoalphabetic cipher is substitution encryptionmethod that map out every alphabet with the anotheralphabet randomly not the moving alphabet processlike the caesar cipher.

Example : A -> D, B -> I, C -> Q etc.

It is used for make cryptanalyst difficult to analyze theplaintext pattern.

But, this method easy to encode using frequency analysis. As we know, every language has the alphabet that often

appear and that is called vocal alphabet.

Monoalphabetic Cipher

Mixed Monoalphabetic Cipher

U D I N U S ……. ???

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

Easier Monoalphabetic Cipher

B E R D A S I ………

The weakness : Alphabet appear frequency is not lost.

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

Monoalphabetic Cipher

Polyalphabetic Cipher

Leon Battista Alberti on 1467 is the author ofcipher polyalphabetic on Renaissance period. Alberti uses the mix alphabet to encrypt the

plaintext. Alberti uses decoder device that implement the

substitution polyalphabetic with the mixalphabet.

Polyalphabetic Cipher

Polyalphabetic cipher use some themonoalphabetic cipher. Some of polyalphabetic cipher are reply the key

of mono alphabetic at n period. n period = length of plain text / key.

Polyalphabetic Cipher

P : BOBOLJAMSATUK : KEYKEYKEYKEYC : LS.......

For the example, A=0, B=1 .... Z=25, define the key = KEY, So the key enlarge become KEYKEYKEY until the

key has the same length with the plain text

Polyalphabetic Cipher

(B + K) mod 26 = (1 + 10) mod 26 = 11 = L(O + E) mod 26 = (14 + 4) mod 26 = 18 = S

Polyalphabetic method also produce a random encryption pattern because every the same aplhabetproduce the different encryption.

Notice encryption for the “O” that appear twice.

Polyalphabetic Cipher

Vigenere Cipher The process of encryption is simple:

• Given a key letter x and a plaintext letter y,• the ciphertext letter is at the intersection of the row

labeled x and the column labeled y;• in this case the ciphertext is V.

Polyalphabetic Cipher

Polyalphabetic Cipher

Example Vigenere :

Plaintext M E E T M E A T M I D N I G H TKeyword F U L L M O O N F U L L M O O N

Cipher …. ???

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 ZF F G H I J K L M N O P Q R S T U V W X Y Z A B C D EU U V W X Y Z A B C D E F G H I J K L M N O P Q R S TL L M N O P Q R S T U V W X Y Z A B C D E F G H I J KM M N O P Q R S T U V W X Y Z A B C D E F G H I J K LO O P Q R S T U V W X Y Z A B C D E F G H I J K L M NN 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 M

Plaintext

Keyw

ord

Beuford Cipher

Polyalphabetic Cipher

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 ZF F E D C B A Z Y X W V U T S R Q P O N M L K J I H GU U T S R Q P O N M L K J I H G F E D C B A Z Y X W VL L K J I H G F E D C B A Z Y X W V U T S R Q P O N MM M L K J I H G F E D C B A Z Y X W V U T S R Q P O NO O N M L K J I H G F E D C B A Z Y X W V U T S R Q PN N M L K J I H G F E D C B A Z Y X W V U T S R Q P O

Plaintext

Keyw

ord

Plaintext M E E T M E A T M I D N I G H TKeyword F U L L M O O N F U L L M O O N

Cipher …. ???

Varian Beuford Cipher

Polyalphabetic Cipher

Plaintext

Keyw

ord

Plaintext M E E T M E A T M I D N I G H TKeyword F U L L M O O N F U L L M O O N

Cipher …. ???

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 ZF E D C B A Z Y X W V U T S R Q P O N M L K J I H G FU T S R Q P O N M L K J I H G F E D C B A Z Y X W V UL K J I H G F E D C B A Z Y X W V U T S R Q P O N M LM L K J I H G F E D C B A Z Y X W V U T S R Q P O N MO N M L K J I H G F E D C B A Z Y X W V U T S R Q P ON M L K J I H G F E D C B A Z Y X W V U T S R Q P O N

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