Codes And Ciphers

#10 Vigenere Cipher

#Cryptograhy

LECTURE #10: VIGENÈRE CIPHER

[INTRODUCTION]

This cipher was originally described by Giovan Battista Bellaso in his 1553 book "La cifra del. Sig. Giovan Battista Bellaso" but later, the scheme was misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the "Vigenère cipher".

[ALGORITHM]

The Vigenère Cipher uses tabula recta, a 26x26 table contains 26 possible shifts of alphabet arranged into rows and columns. (Refer to the picture in the comment box)

The Vigenère cipher requires a keyword, which is repeated so that the total length of it is in equal length to the plaintext.

For example, the plaintext is:
QED CLUB PROJECT LOKI

And the keyword is:
LORILEI

It will be:
QED—Club—Project—Loki
LOR—ILEI—LORILEI—LORI

Following the tradition, we will remove all spaces and punctuation, convert all letters to upper case, and divide the result into 5-letter blocks like this:

QEDCL—UBPRO—JECTL—OKI
LORIL—EILOR—ILEIL—ORI

[ENCRYPTION]

Using the characters of the plaintext and the keyword as the column and row index, respectively, we will get the encrypted letter from their intersection.

Let us use the example above.

The first plaintext letter is Q and the first keyword letter is L.

Using the Tabula Recta, find plaintext Q at the left most row (when you are facing the tabula) and keyword L at the column heading (top part of the tabula). Now its intersection will give you the cipher text which is letter B.

Proceeding:
Plaintext E + keyword O = S
Plaintext D + keyword R = U
Plaintext C + keyword I = K

Thus,
Plaintext: QEDCL—UBPRO—JECTL—OKI
with Keyword: LORIL—EILOR—ILEIL—ORI
will have a Cipher text: BSUKW—YJAFF—RHGBW—CBQ

So, your encrypted message will be:
"BSUKWYJAFFRHGBWCBQ"­.

[DECRYPTION]

Let's use the cipher text and keyword above:

Cipher text: BSUKWYJAFFRHGBWCBQ
Keyword: LORILEILORILEILORI

To decrypt the code, find keyword L in the column heading of tabula recta, move down to the its cipher text which is letter B. Now, letter Q, that is on the left most row, is the plaintext.

Following:
Keyword O + cipher text S = E
Keyword R + cipher text U = D
Keyword I + cipher text K = C

Do the same procedure in the remaining letters and tadah!~ You have decoded it!~

[GAME!]
Try to decode this and PM Watson's Swordcane for your answer. Good luck!

"HSGQWZCMPMYCLHDPPEO­LXSOBFD"
Key: God of Mischief

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