Institute of Southern Punjab, Multan

Transcription

1 Institute of Southern Punjab, Multan Network Security Substitution Techniques Lecture#4 Mazhar Hussain

2 Lecture 4: Substitution Techniques Polybius Cipher Playfair Cipher Vernam Cipher 2

3 Polybius Cipher 3

4 Polybius Cipher A Polybius Square is a table that allows someone to translate letters into numbers To give a small level of encryption, this table can be randomized and shared with the recipient In order to fit the 26 letters of the alphabet into the 25 spots created by the table The letters I and J are usually combined but it also depends on the sender/cryptographer which can make changes into the hierarchy of table 4

5 To encipher a message you replace each letter with the row and column in which it appears For example, D should be replaced with 14 To decipher a message you find the letter that intersects the specified row and column Example: Plain Text: Continued THIS IS A SECRET MESSAGE Cipher Text:

6 Decrypt the Following given Cipher Text and generate the meaningful Plain Text Cipher Text: What will be they Key?? 6

7 The Polybius Square is essentially identical to the simple substitution cipher, except that each plaintext character is enciphered as 2 cipher text characters It can usually be detected if there are only 5 or 6 different characters in the cipher text This algorithm offers very little communication security, and can be easily broken even by hand, especially as the messages become longer (more than several hundred cipher text characters) 7

8 Keys for the Polybius Square usually consist of a 25 letter 'key square also (the letters along the top and side can be chosen arbitrarily) An example encryption using the given key: Cipher Text: CCBACBBABECCEDABBABABBDDEDEABBBDBD CACB EDABBADBBBDDEDBDBA Plain Text: defend the east wall of the castle 8

9 It is easy to see how each character in the plaintext is replaced with 2 characters in the cipher alphabet Decryption is just as easy, by using 2 cipher characters as the row and column into the key square to get the original plaintext character back When generating keys it is popular to use a key word, e.g. 'zebra' to generate it, since it is much easier to remember a key word compared to a random jumble of 25 characters Cipher Alphabet: zebracdfghiklmnopqstuvwxy 9

10 Here we have written out the key as a single string instead of a square Cipher Alphabet: zebracdfghiklmnopqstuvwxy Cipher Text Alphabets: abcde To create the square, the first 5 characters make the first row, the second 5 characters make the second row etc If your keyword has repeated characters, be careful not to include the repeated characters in the cipher alphabet 10

11 Playfair Cipher 11

12 Playfair Cipher The Playfair cipher was the first practical digraph substitution cipher The scheme was invented in 1854 by Charles Wheatstone, but was named after Lord Playfair who promoted the use of the cipher The technique encrypts pairs of letters (digraphs), instead of single letters as in the simple substitution cipher 12

13 The Playfair is significantly harder to break since the frequency analysis used for simple substitution ciphers does not work with it The Playfair algorithm is based on the use of a 5 5 matrix of letters constructed using a keyword Frequency analysis can still be undertaken, but on the 25*25=625 possible digraphs rather than the 25 possible monographs 13

14 It was used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the Australians during World War II. This was because Playfair is reasonably fast to use and requires no special equipment A typical scenario for Playfair use would be to protect important but non-critical secrets during actual combat. By the time the enemy cryptanalysts could break the message the information was useless to them 14

15 Perhaps the most famous cipher of 1943 involved the future president of the U.S., J. F. Kennedy, Jr. On 2 August 1943 The KEY' for a Playfair cipher is generally a word, for the sake of example we will choose 'monarchy'. This is then used to generate a 'key square', e.g. 15

16 Any sequence of 25 letters can be used as a key, so long as all letters are in it and there are no repeats. Note that there is no 'j', it is combined with 'i' We now apply the encryption rules to encrypt the plaintext 1. Identify any double letters in the plaintext and replace the second occurrence with an 'x' e.g. 'hammer' -> 'hamxer' 2. If the plaintext has an odd number of characters, append an 'x' to the end to make it even 16

17 3. Break the plaintext into pairs of letters, e.g. 'hamxer' -> 'ha mx er' 4. The algorithm now works on each of the letter pairs 5. If the letters are in different rows and columns, replace the pair with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair. The order is important the first encrypted letter of the pair is the one that lies on the same row as the first plaintext letter 'ha' -> 'bo', 'es' -> 'il' 17

18 6. If the letters appear on the same row of the table, replace them with the letters to their immediate right respectively (wrapping around to the left side of the row if a letter in the original pair was on the right side of the row) 'ma' -> 'or', 'lp' -> 'pq' 7. If the letters appear on the same column of the table, replace them with the letters immediately below respectively (wrapping around to the top side of the column if a letter in the original pair was on the bottom side of the column) 'rk' -> 'dt', 'pv' -> 'vo' 18

19 Clarification with pictures. There are three general cases: 19

20 PROBLEM 1 Key Square: monarchybdefgiklpqstuvwxz Cipher Text: ugrmkcsxhmufmkbtoxgcmvatluiv What will be the Plain Text of the given Cipher and Key?? Plain Text: wearediscoveredsaveyourselfx We are discovered save yourself 20

21 PROBLEM 2 The KEY' for a Playfair cipher is : PROBLEMS Use it to generate a Key Square Key Square: PROBLEMSACDFGHIKNQTUVWXYZ Cipher Text: AGMVMKQYQBYTMAQBPM What will be the Plain Text of the given Cipher and Key Square?? 21

22 The Plain Text of the given Cipher and Key Square must be: SHEWENTXTOTHESTORE SH EW EN TX TO TH ES TO RE SHE WENT TO THE STORE 22

23 PROBLEM 3 The KEY' for a Playfair cipher is : NETWORKSECURITY Use it to generate a Key Square Key Square: NETWORKSCURITYABCDFGHQVXZ Plain Text: THIS SUBJECT IS HEADACHE What will be the Cipher Text of the given Plain Text and Key Square?? 23

24 The Cipher Text of the given Plain Text and Key Square must be:- NVTKCRCRWKETRVOIGTRXWQ NV TK CR CR WK ET RV OI GT RX WQ 24

25 Key Square: baklxernydhmiuqvsgtcfwopz Cipher Text: VUMGTMKHDVGUVMRBRXVQDB What will be the Plain Text of the given Cipher Text and Key Square?? THISSUBJECTISHEADACHEX THIS SUBJECT IS HEADACHE 25

26 Vernam Cipher 26

27 Vernam Cipher The ultimate defense against such a cryptanalysis is to choose a keyword that is as long as the plaintext and has no statistical relationship to it Such a system was introduced by an engineer named Gilbert Vernam in 1918 His system works on binary data (bits) rather than letters The system can be expressed briefly in upcoming slides 27

28 In modern terminology, a Vernam cipher is a symmetrical stream cipher in which the plaintext is combined with a random or pseudorandom stream of data (the "key stream") of the same length, to generate the cipher text, using the Boolean "exclusive or" (XOR) function This is symbolized by and is represented by the following "truth table", where + represents "true" and represents "false 28

29 29

30 Spirit of this technique is the means of construction of the key Vernam proposed the use of a running loop of tape that eventually repeated the key, so that in fact the system worked with a very long but repeating keyword Although such a scheme, with a long key, presents to challenge cryptanalyst. But, it can be broken with sufficient cipher text, the use of known or likely plaintext sequences, or both 30

31 END OF LECTURE 4 31