Computer security

1. Computer security Section 3

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 • fill rest of matrix with other letters • eg. using the keyword MONARCHY

4. 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: • 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)

5. 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.

6. 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.

7. Playfair Cipher Con., • Encipher the following word “ RENAISSANCE” using Playfair key as shown.

8. 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

9. 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

11. 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

12. 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

13. 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