Post on 11-May-2019
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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
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
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