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Computer Security

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Playfair Cipher

• Multiple-letter encryption cipher
• Treats the set of plain text as a single unit
• Invented by sir Charles Wheatstone in 1854
• Known by the his friend Baron Playfair
• Uses a key matrix [5X5] to encrypt the plaintext
• Operation / working

 Key generation

 Diagram formation

 Encryption of diagram using key matrix

Example

• Cipher Text
• UZQSOVUOHXMOPVGPOZPEVSGWSZOPFPESXUDBMETSXAIZVUE PHZHMDSHZOWSFPAPPDTSVPQUZWYMXUZUHSXEPYEPOPDZSUF POMBZWPFUPZHMDJUDTMOHMQ
• Relative frequencies of letters in cipher text can be compared with the

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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

Playfair Cipher

• Multiple-letter encryption cipher
• Treats the set of plain text as a single unit
• Invented by sir Charles Wheatstone in 1854
• Known by the his friend Baron Playfair
• Uses a key matrix [5X5] to encrypt the plaintext
• Operation / working

 Key generation

 Diagram formation

 Encryption of diagram using key matrix

Key generation

• Rules  Convert all the alphabet to upper case  Punctuations and space will not be considered  Place the keyword in a 5 by 5 matrix starting from top left corner  Repeating letters are written once  I/J are treated as one letter  Remaining matrix is populated with rest of the alphabets  The letters already appeared will not be written again
• Our key word  “MANCHESTER”
• Key Matrix M A N C H E S T R B D F G I/J K L O P Q U V W X Y Z

Encryption Rules

• Pick up a pair from prepared plaintext
• Find the letters in the key matrix
• Rule-  Replace the letters by the letters that lies in opposite corners of the rectangle they form
• Rule-  If both letters are in same row then replace each by the letter to the right  Wrap the text from right to left
• Rule-  If both letters are in same column then replace each by the letter beneath  Wrap the text from top to bottom
• Rule-04 docsity.com

Rule-

Plaint Text TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX Key M A N C H E S T R B D F G I/J K L O P Q U V W X Y Z

• Replace the letters by the letters that lies in opposite corners of the rectangle they form .. N. H .. T. B ..... ..... ..... Cipher Text BN FR

Rule-

Plaint Text

TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX

Key

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

• If both letters are in same column then replace each by the letter beneath ... C. ... R. ... I/J. ..... .....

Cipher Text BN FR TS RI

Rule-0X

• Complete the rest of the conversion

Plaint Text

TH IS SE CR ET ME SX SA GE IS EN CR YP TE DX

Key

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

Cipher Text BN FR TS RI

Hill Cipher (2)

• P 13. standard frequency distribution for English
• Z 11.
• S 8.
• U 8.
• O 7.
• M 6.
  • H 5.
  • D 5.
  • E 5.
  • V 4.
  • X 4.
    • F 3.
    • W 3.
    • Q 2.
    • T 2.
    • A 1.
      • B 1.
      • G 1.
      • Y 1.
      • I 0.
      • J 0.
        • C 0.
        • K 0.
        • L 0.
        • N 0.
        • R 0.
• C 1 =(k 11 p 1 +k 12 p 2 +k 13 p 3 )mod
• C 2 =(k 21 p 1 +k 22 p 2 +k 23 p 3 )mod
• C 3 =(k 31 p 1 +k 32 p 2 +k 33 p 3 )mod - k 11 p 1 +k 12 p 2 +k 13 p - k 21 p 1 +k 22 p 2 +k 23 p - k 31 p 1 +k 32 p 2 +k 33 p
  • C
  • C
  • C - P - P - P

Vigenere Cipher…

• The character in the keywords defines the row in table
• The character in the plaintext defines the column in the table
• The intersection of the two generates the cipher letter
• To make the key as long as the plain text the key is repeated
• The punctuations are eliminated
• Example

“we are discovered save yourself”

Key

Painttext

Ciphertext

Vernam Cipher

• Operates on bits rather than letters
• Use a key that provide no statistical relationship between

plaintext and ciphertext

• The cipher text is generated by bitwise XOR of plain text

and cipher text

• Decryption is reverse of the encryption
• The process is actually using a very long key
• The key is eventually repeated
• More strong than Vigenere cipher but not secure

completely