Polyalphabetic Substitution Cipher

Polyalphabetic Substitution Cipher

1. Prof. Neeraj Bhargava
Mrs. Shubha Chaturvedi
Department of Computer Science, School of Engineering &
System Sciences
MDS University Ajmer, Rajasthan

2. Apolyalphabeticcipherisanycipherbasedon
substitution,usingmultiplesubstitutionalphabets.
Aletterintheplaintextisnotalwaysreplacedbythe
sameletterintheciphertext.
Wecanusemorethen1substitutionforthesame
letter.
Theselectionofthenextalphabeticcloudbe
selectedinanyofthethreeways:-
(i) Sequential
(ii)Key word coded
(iii) Auto encoded

3. Thevigenerecipherisanalgorithmthatisusedto
encryptinganddecryptingthetext.
Thevigenerecipherisanalgorithmofencryptingan
alphabetictextthatusesaseriesofinterwovencaesar
ciphers.
Itisbasedonakeyword'sletters.Itisanexampleofa
polyalphabeticsubstitutioncipher.
Thisalgorithmiseasytounderstandandimplement.
Thisalgorithmwasfirstdescribedin1553byGiovan
BattistaBellaso.
ItusesaVigeneretableorVigeneresquarefor
encryptionanddecryptionofthetext.Thevigenere
tableisalsocalledthetabularecta.

5. EXAMPLE: The plaintext is "JAVATPOINT", and the key is
"BEST”
Encryption
Thefirstletteroftheplaintextiscombinedwiththefirstletterofthekey.
Thecolumnofplaintext"J"androwofkey"B"intersectsthealphabetof"K"
inthevigeneretable,sothefirstletterofciphertextis"K".
Similarly,thesecondletteroftheplaintextiscombinedwiththesecond
letterofthekey.Thecolumnofplaintext"A"androwofkey"E"intersects
thealphabetof"E"inthevigeneretable,sothesecondletterofciphertextis
"E".
Thisprocesscontinuescontinuouslyuntiltheplaintextisfinished.
Ciphertext=KENTUTGBOX

6. Decryption
Decryptionisdonebytherowofkeysinthevigeneretable.First,select
therowofthekeyletter,findtheciphertextletter'spositioninthatrow,
andthenselectthecolumnlabelofthecorrespondingciphertextasthe
plaintext.
Example:
Intherowofthekeyis"B"andtheciphertextis"K"andthisciphertextletter
appearsinthecolumn"J",thatmeansthefirstplaintextletteris"J".
Next,intherowofthekeyis"E"andtheciphertextis"E"andthisciphertext
letterappearsinthecolumn"A",thatmeansthesecondplaintextletteris"A".
Thisprocesscontinuescontinuouslyuntiltheciphertextisfinished.
Plaintext=JAVATPOINT

7. Method -2
Whenthevigeneretableisnotgiven,
theencryptionanddecryptionaredone
byVigenaralgebraicallyformulainthis
method(converttheletters(A-Z)into
thenumbers(0-25)).
Formulaofencryptionis,
Ei=(Pi+Ki)mod26
Formulaofdecryptionis,
Di=(Ei-Ki)mod26

8. Example: The plaintext is "JAVATPOINT", and the key is "BEST".
Encryption:E
i= (P
i+ K
i) mod 26
Plain text : J A V A T P O I N T
Plaintext value: 09 00 21 00 19 15 14 08 13 19
Key : B E S T B E S T B E
Key value: 01 04 18 19 01 04 18 19 01 04
Ciphertext value (E) :
10 04 13 19 20 19 06 01 14 23
Cipher text: K E N T U T G B O X

9. Decryption:
D
i= (E
i-K
i) mod 26
If any case (Di) value becomes negative (-ve), in this case, we
will add 26 in the negative value. Like, the third letter of the
ciphertext;
N = 13 and S = 18
D
i= (E
i-K
i) mod 26
D
i= (13 -18) mod 26
D
i= -5 mod 26
D
i= (-5 + 26) mod 26
D
i= 21
Ciphertext : K E N T U T G B O X
Ciphertext value (E): 10 04 13 19 20 19 06 01 14 23
Key : B E S T B E S T B E
Key value (K) : 01 04 18 19 01 04 18 19 01 04
Plaintext: J A V A T P O I N T
Plaintext value (P): 09 00 21 00 19 15 14 08 13 19

10. TheHomophonic Substitutioncipherisa
substitutioncipherinwhichsingleplaintextletters
canbereplacedbyanyofseveraldifferent
ciphertextletters.Theyaregenerallymuchmore
difficulttobreakthanstandardsubstitution
ciphers.
Thenumberofcharacterseachletterisreplaced
byispartofthekey,e.g.theletter'E'mightbe
replacedbyanyof5differentsymbols,whilethe
letter'Q'mayonlybesubstitutedby1symbol.

11. ThePolybiussquare,alsoknownasthePolybius
checkerboard.
Itis5*5gridthatallowstoencryptlettersinto
numbers.isadeviceinventedbytheancient
GreeksCleoxenusandDemocleitus,andmade
famousbythehistorianandscholarPolybius.

12. Each letter is then represented by its coordinates in the grid. For example,
"BAT" becomes "12 11 44". The 26 letters of the Latin/English alphabet do
not fit in a 5 ×5 square, two letters must be combined (usually I and J as
above, though C and K is an alternative). Alternatively, a 6 ×6 grid may be
used to allow numerals or special characters to be included as well as letters.
EXAMPLE :
Find out the polybius square cipher for the plain text ?
STEP-1
First we will create a matrix 5*5 ,in order to fit 26 letters of the English
Alphabet into the 25 spots letter I and J usually combined.
1 2 3 4 5
1
2
3
4
5
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

13. STEP-2
Assign the number to letter in the plain text
from the about to get cipher text.
PLAINTEXT :
U N I V E R S I T Y
CIPHER TEXT :
45 33 24 51 15 42 43 24 44 54

14. Q. Find out the Vigenere Cipher for the plain
text message ”MaharashiUniversity” by using
the Keyword “Orange”.