Suppose that the cycling pattern of a polyalphabetic encryption system is M1, M2

Question

Suppose that the cycling pattern of a polyalphabetic encryption system is M1, M2, M1, M3. Simple Caesar ciphers with shift of 2, 3, and 5 are 3 substitution ciphers. Use the following polyalphabetic cipher table Plaintext: 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

M1 (k=2): y z a b c d e f g h i j k l m n o p q r s t u v w x

M2 (k=3): x y z a b c d e f g h i j k l m n o p q r s t u v w

M3 (k=5): v w x y z a b c d e f g h i j k l m n o p q r s t u

to encrypt the following message: the quick brown fox jumps over the lazy dog

Explanation / Answer

---------------------------------ans ------------------

divide the given text into 3 different parts so using the Caesar cipher shift values given of starting and ending position of these parts (because 3 Caesar cipher are given that’s why we broke the message into 3 parts). spaces will be ignored for encryption

the quick brown     fox jumps over      the lazy dog

Y                         X                               V

we have used Y,X,V because in the Caesar cipher M1 “a” is encrypted with y

and in M2 “a” is encrypted with x and in M3 “ a” encrypted with v . so we taken uppercase alphabet to identify different caesar cipher to decrypt the message.

part1 = the quick brown  

part 2 = fox jumps over   

part 3 = the lazy dog

so we start the ciphertext with a uppercase "Y". We then encrypt the first part as a Caesar Shift, using the ciphertext alphabet given M1(k=2).

so

Plaintext:   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

M1 (k=2): y z a b c d e f g h i j k l m n o p q r s t u v w x

Plain text : Thequickbrown  

Cipher text: YrfcosgahzpmulX

The lowercase letters above encrypt the plaintext letters given. The "Y" indicates the starting position of the ciphertext, and the "X" indicates that we need to change the position so that "x" is beneath "a". We then get the new ciphertext alphabet as ceaser cipher M2(k=3)

Plain text : 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

M2 (k=3):   x y z a b c d e f g h i j k l m n o p q r s t u v w

Part2 will be encrypted with Caesar cipher with shift 3

Plaintext : fox jumps over

Ciphertext: XclugrkmplsboV

This time the plaintext letters are encrypted to the ciphertext letters using the ciphertext alphabet above, and the "V" indicates that we are changing alphabet again to get the final ciphertext alphabet below with cease cipher M3(k=5).

Part 3 will be encrypted with Caesar cipher with shift 5

Plain text : 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

M3 (k=5):    v w x y z a b c d e f g h i j k l m n o p q r s t u

Plaintext: thelazydog

Ciphertext: Voczgutyjb

So final cipher text for the message

the quick brown fox jumps over the lazy dog

YrfcosgahzpmulXclugrkmplsboVoczgutyjb

Where uppercase alphabets indicates that we need to change the position so that uppercase alphabet is beneath "a". We then get the new ciphertext for other part of the message

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