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Computer security

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Computer security

Section 3

- 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

- 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

- 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:
- BA LX LO ON
- 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)

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.

- 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.

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

- 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

ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ

- 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

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
- Plain: THE SIMPLEST POSSIBLE TRANSPOSITIONSXX
- 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

- Cipher: STIEH EMSLP STSOP EITLB SRPNA TOIIS XOXSN

Example

- We can use a word, with letter order giving sequence: to write in the plaintext; or read off the cipher
- Plain: A,CONVENIENT,WAY,TO,EXPRESS,
THE,PERMUTATION

- 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

-Cipher: ANOCN IEVET TNWOY AESPX RTSEH UPEET MRAZI TOZZN