Polyalphabetic Substitution Cipher

Polyalphabetic Substitution Cipher

• 1.
  Prof. Neeraj Bhargava Mrs.Shubha Chaturvedi Department of Computer Science, School of Engineering & System Sciences MDS University Ajmer, Rajasthan
• 2.
   A polyalphabeticcipher is any cipher based on substitution, using multiple substitution alphabets .  A letter in the plain text is not always replaced by the same letter in the cipher text.  We can use more then 1 substitution for the same letter.  The selection of the next alphabetic cloud be selected in any of the three ways :- (i) Sequential (ii)Key word coded (iii) Auto encoded
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   The vigenerecipher is an algorithm that is used to encrypting and decrypting the text.  The vigenere cipher is an algorithm of encrypting an alphabetic text that uses a series of interwoven caesar ciphers.  It is based on a keyword's letters. It is an example of a polyalphabetic substitution cipher.  This algorithm is easy to understand and implement. This algorithm was first described in 1553 by Giovan Battista Bellaso.  It uses a Vigenere table or Vigenere square for encryption and decryption of the text. The vigenere table is also called the tabula recta.
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  EXAMPLE: The plaintextis "JAVATPOINT", and the key is "BEST” Encryption The first letter of the plaintext is combined with the first letter of the key. The column of plain text "J" and row of key "B" intersects the alphabet of "K" in the vigenere table, so the first letter of ciphertext is "K". Similarly, the second letter of the plaintext is combined with the second letter of the key. The column of plain text "A" and row of key "E" intersects the alphabet of "E" in the vigenere table, so the second letter of ciphertext is "E". This process continues continuously until the plaintext is finished. Ciphertext = KENTUTGBOX
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  Decryption Decryption is doneby the row of keys in the vigenere table. First, select the row of the key letter, find the ciphertext letter's position in that row, and then select the column label of the corresponding cipher text as the plaintext. Example : In the row of the key is "B" and the ciphertext is "K" and this cipher text letter appears in the column "J", that means the first plaintext letter is "J". Next, in the row of the key is "E" and the ciphertext is "E" and this ciphertext letter appears in the column "A", that means the second plaintext letter is "A". This process continues continuously until the ciphertext is finished. Plaintext = JAVATPOINT
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  Method -2  Whenthe vigenere table is not given, the encryption and decryption are done by Vigenar algebraically formula in this method (convert the letters (A-Z) into the numbers (0-25)).  Formula of encryption is,  Ei = (Pi + Ki) mod 26  Formula of decryption is,  Di = (Ei - Ki) mod 26
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  Example: The plaintextis "JAVATPOINT", and the key is "BEST". Encryption: Ei = (Pi + Ki) mod 26 Plain text : J A V A T P O I N T Plaintext value: 09 00 21 00 19 15 14 08 13 19 Key : B E S T B E S T B E Key value: 01 04 18 19 01 04 18 19 01 04 Ciphertext value (E) : 10 04 13 19 20 19 06 01 14 23 Cipher text: K E N T U T G B O X
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  Decryption: Di = (Ei- Ki) mod 26  If any case (Di) value becomes negative (-ve), in this case, we will add 26 in the negative value. Like, the third letter of the ciphertext;  N = 13 and S = 18  Di = (Ei - Ki) mod 26  Di = (13 - 18) mod 26  Di = -5 mod 26  Di = (-5 + 26) mod 26  Di = 21 Ciphertext : K E N T U T G B O X Ciphertext value (E): 10 04 13 19 20 19 06 01 14 23 Key : B E S T B E S T B E Key value (K) : 01 04 18 19 01 04 18 19 01 04 Plaintext: J A V A T P O I N T Plaintext value (P): 09 00 21 00 19 15 14 08 13 19
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   The HomophonicSubstitution cipher is a substitution cipher in which single plaintext letters can be replaced by any of several different ciphertext letters. They are generally much more difficult to break than standard substitution ciphers. The number of characters each letter is replaced by is part of the key, e.g. the letter 'E' might be replaced by any of 5 different symbols, while the letter 'Q' may only be substituted by 1 symbol.
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  The Polybius square,also known as the Polybius checkerboard. It is 5*5 grid that allows to encrypt letters into numbers . is a device invented by the ancient Greeks Cleoxenus and Democleitus, and made famous by the historian and scholar Polybius.
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  Each letter isthen represented by its coordinates in the grid. For example, "BAT" becomes "12 11 44". The 26 letters of the Latin/English alphabet do not fit in a 5 × 5 square, two letters must be combined (usually I and J as above, though C and K is an alternative). Alternatively, a 6 × 6 grid may be used to allow numerals or special characters to be included as well as letters. EXAMPLE : Find out the polybius square cipher for the plain text ? STEP-1 First we will create a matrix 5*5 ,in order to fit 26 letters of the English Alphabet into the 25 spots letter I and J usually combined. 1 2 3 4 5 1 2 3 4 5 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 Z
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  STEP-2 Assign the numberto letter in the plain text from the about to get cipher text. PLAINTEXT : U N I V E R S I T Y CIPHER TEXT : 45 33 24 51 15 42 43 24 44 54
• 14.
  Q. Find outthe Vigenere Cipher for the plain text message ”MaharashiUniversity” by using the Keyword “Orange”.