How to encode & decode the Vigenère cipher

The Vigenère cipher is a method of encryption that uses a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution..

How to encode and decode the Vigenère cipher

The Vigenère cipher is based on a key encryption. It's pretty much the ceasar cipher but more difficult to crack because frequeny analysis is not gonna work here.

If you want to encrypt the message "divert troops to east ridge" with the key "white"
you should do this:

keyword W H I T E W H I T E W H I T E W H I T E W H I
plaintext d i v e r t t r o o p s t o e a s t r i d g e
Ciphertext Z P D X V P A Z H S L Z B H I W Z B K M Z N M

write the message and type the keyword above it, and repeat the key to fit it with
the length of the text.

Here we have a table:
....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

_1 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 A
_2 C D E F G H I J K L M N O P Q R S T U V W X Y Z A B
_3 D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
_4 E F G H I J K L M N O P Q R S T U V W X Y Z A B C D
_5 F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
_6 G H I J K L M N O P Q R S T U V W X Y Z A B C D E F
_7 H I J K L M N O P Q R S T U V W X Y Z A B C D E F G
_8 I J K L M N O P Q R S T U V W X Y Z A B C D E F G H
_9 J K L M N O P Q R S T U V W X Y Z A B C D E F G H I
10 K L M N O P Q R S T U V W X Y Z A B C D E F G H I J
11 L M N O P Q R S T U V W X Y Z A B C D E F G H I J K
12 M N O P Q R S T U V W X Y Z A B C D E F G H I J K L
13 N O P Q R S T U V W X Y Z A B C D E F G H I J K L M
14 O P Q R S T U V W X Y Z A B C D E F G H I J K L M N
15 P Q R S T U V W X Y Z A B C D E F G H I J K L M N O
16 Q R S T U V W X Y Z A B C D E F G H I J K L M N O P
17 R S T U V W X Y Z A B C D E F G H I J K L M N O P Q
18 S T U V W X Y Z A B C D E F G H I J K L M N O P Q R
19 T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
20 U 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
21 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
22 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 V
23 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
24 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
25 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 Y
26 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


To encrypt the first letter, "d" , begin by identifying the key letter above it, "W", which in turn defines a particular row in the Vigenère cipher . The row beginning with "W", row 22, is the cipher alphabet that will be used to find the substitute letter for the plaintext "d". We look to see where the column headed by "d" intersects the row beginning with "W", which turns out to be at the letter "Z". So the letter "d" is replaced by the letter "Z".

Continue with the rest of the letters of the plaintext to make your encypted message.

To decode it when you have the ciphertext and keyword you can simply reverse the process. You should be able to do the challenge by now !!

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