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140120107044 ins ala.ppt

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Paneliya Prince
Paneliya Prince

The document discusses the Playfair cipher and polyalphabetic ciphers. It explains that the Playfair cipher encrypts plaintext letters in pairs using a 5x5 keyword matrix. It encrypts letters based on whether they are in the same row, column, or different rows and columns. The document also explains that polyalphabetic ciphers improve security over monoalphabetic ciphers by using multiple cipher alphabets selected by a repeating keyword to encrypt the message letter by letter.

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GANDHINAGAR INSTITUTE OF TECHNOLOGY Department of Computer Engineering Playfair and polyalphabetic cipher Group ID: CE_A3_03 Students Name with Enrollment No: Gabani Pruthvi (140120107044) Name of Faculty: Prof. Sonali Virparia Information and network security(2170709)
Playfair Cipher Not even the large number of keys in a monoalphabetic cipher provides security one approach to improving security was to encrypt multiple letters the Playfair Cipher is an example invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair
Playfair Key Matrix a 5X5 matrix of letters based on a keyword fill in letters of keyword (sans duplicates) fill rest of matrix with other letters eg. using the keyword MONARCHY MONAR CHYBD EFGIK LPQST UVWXZ
Encrypting and Decrypting plaintext encrypted two letters at a time: 1. if a pair is a repeated letter, insert a filler like 'X', eg. "balloon" encrypts as "ba lx lo on" 2. if both letters fall in the same row, replace each with letter to right (wrapping back to start from end), eg. ar" encrypts as "RM" 3. if both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom), eg. mu" encrypts to "CM" 4. otherwise each letter is replaced by the one in its row in the column of the other letter of the pair, eg. hs" encrypts to "BP", and ea" to "IM" or "JM" (as desired)
Polyalphabetic Ciphers another approach to improving security is to use multiple cipher alphabets called polyalphabetic substitution ciphers makes cryptanalysis harder with more alphabets to guess and flatter frequency distribution use a key to select which alphabet is used for each letter of the message use each alphabet in turn repeat from start after end of key is reached
140120107044 ins ala.ppt
Example write the plaintext out write the keyword repeated above it use each key letter as a caesar cipher key encrypt the corresponding plaintext letter eg using keyword deceptive key: deceptivedeceptivedeceptive plaintext: wearediscoveredsaveyourself ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
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140120107044 ins ala.ppt

  • 1. GANDHINAGAR INSTITUTE OF TECHNOLOGY Department of Computer Engineering Playfair and polyalphabetic cipher Group ID: CE_A3_03 Students Name with Enrollment No: Gabani Pruthvi (140120107044) Name of Faculty: Prof. Sonali Virparia Information and network security(2170709)
  • 2. Playfair Cipher Not even the large number of keys in a monoalphabetic cipher provides security one approach to improving security was to encrypt multiple letters the Playfair Cipher is an example invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair
  • 3. Playfair Key Matrix a 5X5 matrix of letters based on a keyword fill in letters of keyword (sans duplicates) fill rest of matrix with other letters eg. using the keyword MONARCHY MONAR CHYBD EFGIK LPQST UVWXZ
  • 4. Encrypting and Decrypting plaintext encrypted two letters at a time: 1. if a pair is a repeated letter, insert a filler like 'X', eg. "balloon" encrypts as "ba lx lo on" 2. if both letters fall in the same row, replace each with letter to right (wrapping back to start from end), eg. ar" encrypts as "RM" 3. if both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom), eg. mu" encrypts to "CM" 4. otherwise each letter is replaced by the one in its row in the column of the other letter of the pair, eg. hs" encrypts to "BP", and ea" to "IM" or "JM" (as desired)
  • 5. Polyalphabetic Ciphers another approach to improving security is to use multiple cipher alphabets called polyalphabetic substitution ciphers makes cryptanalysis harder with more alphabets to guess and flatter frequency distribution use a key to select which alphabet is used for each letter of the message use each alphabet in turn repeat from start after end of key is reached
  • 7. Example write the plaintext out write the keyword repeated above it use each key letter as a caesar cipher key encrypt the corresponding plaintext letter eg using keyword deceptive key: deceptivedeceptivedeceptive plaintext: wearediscoveredsaveyourself ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ