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Computer security. Section 3. 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

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Playfair cipher
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
Playfair Key Matrix

  • a 5X5 matrix of letters based on a keyword

  • fill in letters of keyword

  • fill rest of matrix with other letters

  • eg. using the keyword MONARCHY

Playfair cipher con
Playfair Cipher Con.,

  • Plaintext is encrypted two letters at a time

  • If a pair is a repeated letter, insert filler like 'X’.

  • e.g. BALLOON will be treated as:


    • If both letters m1 and m2 fall in the same row, then c1 and c2 replace each with letter to the right of m1 and m2, respectively.

  • (wrapping back to start from end)

Computer security

Playfair Cipher Con.,

  • If both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom)

  • If the plaintext has an odd number of characters, a null letter is appended to the end of plaintext.

Playfair cipher con1
Playfair Cipher Con.,

  • Otherwise each letter is replaced by the letter in the same row and in the column of the other letter of the pair.

  • Ex. Encrypt the message “ playfair invented by Charles” using Playfair cipher.

Playfair cipher con2
Playfair Cipher Con.,

  • Encipher the following word “ RENAISSANCE” using Playfair key as shown.

Vigen re cipher
Vigenère Cipher

  • Simplest polyalphabetic substitution cipher is the Vigenère Cipher

  • Effectively multiple caesar ciphers

  • Given a key letter X and a plain text Y, the cipher text letter is at inspection of the row labelled x and the column labelled y in this case the cipher text is V.

  • Decryption simply works in reverse


  • 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


Security of vigen re ciphers
Security of Vigenère Ciphers

  • Have multiple ciphertext letters for each plaintext letter

  • Hence letter frequencies are obscured

  • But not totally lost

  • Start with letter frequencies

    • see if look monoalphabetic or not

Computer security

Row Transposition ciphers

  • In general write message in a number of columns and then use some rule to read off from these columns.

  • Key could be a series of number being the order to: read off the cipher; or write in the plaintext


  • Key (R Cipher): 41532

    Key (W Plain) : 4 1 5 3 2

    1 2 3 4 5 1 2 3 4 5

    T H E S I S T I E H

    M P L E S E M S L P

    T P O S SS T S O P

    I B L E T E I T L B

    R A N S P S R P N A

    O S I T I T O I I S

    O N S X XX O X S N


Computer security


  • We can use a word, with letter order giving sequence: to write in the plaintext; or read off the cipher



  • Key (W): C O M P U T E R

  • Key (W): 1 4 3 5 8 7 2 6

    A C O N V E N I A N O C N I E V

    E N T W A Y T O E T T N W O Y A

    E X P R E S S T E S P X R T S E

    H E P E R M U T H U P E E T M R

    A T I O N Z Z Z A Z I T O Z Z N