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# Vigeneré Cipher

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by

## Eva Blackthorne

on 21 November 2015

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#### Transcript of Vigeneré Cipher

My Cipher?
One-Time Pad Cipher
The One-Time Pad cipher, as we went over during class, solves this problem by using a long chain of random letters (or numbers) instead of a repeated codeword.
Playfair addition
For an added level of encryption, one could use the playfair cipher. The previous use of the vigenère cipher would not alter the way the playfair encrypts, and it would add another level of code that would be hard to decrypt. If one wanted to leave the book reference as a number as well, the playfair could be expanded to be a 6x6 square (like how the ADFGVX contains 36 characters).
Vigenère Cipher
As we all (hopefully) know, the Vigenère Cipher is where we repeat the code word multiple times on top of the plaintext, and then encrypt each plaintextletter with the caesar cipher of the codeletter on top.
Long Code Word
For a one-time pad cipher, you need a code word the same length as the message. Thus, to send multiple messages using different codewords each time, one needs a large set of codewords (aka a pad) that could be easily lost or stolen by the cracker. Also, if one mistakenly uses the wrong codeword, that would be a large problem.
Thus, instead of holding onto large pads, one could carry around an innocent-looking book, and reference that, starting from a specific character, until the end of the message. This specific character would be referenced using the first three letters of the codetext, after transcribing them to numbers.
Flaws
However, the Vigenère cipher is crackable by first finding the distance between repeated sets of letters from repeated words in the plaintext corresponding to the repeated codeword, and then finding each letter of the codeword by frequency analysis.
Book Reference
Advantages
With so many books in the world, especially nondescript ones, using a book as a reference would both be hard to figure out and an easy way to execute the one-time pad cipher.
Encryption

ilikecatsandcookies
code-number:
plaintext:
code-number: 86:2:1 (HFBA)
hiswasanimpenetrabl
hiswasanimpenetrabl
ilikecatsandcookies
PTAGEUAGAMCHPSHBIFD
encoded message:
HFBAPTAGEUAGAMCHP
SHBIFD
Disadvantages
Having the number reference in letters could cause some confusion; instead of looking on page 2 for the 3rd paragraph with character 45, one might end up looking on page 23, paragraph 4, character 5.
Decryption
ilikecatsandcookies
code-number:
plaintext:
code-number: 86:2:1 (HFBA)
hiswasanimpenetrabl
hiswasanimpenetrabl
PTAGEUAGAMCHPSHBIFD
encoded message:
HFBAPTAGEUAGAMCHP
SHBIFD
ilikecatsandcookies
Full transcript