Talk:Vigenère cipher

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	To-do list for Vigenère cipher:	edit - history - watch - refresh
	Fix and add to External links Describe relationship to the Simple XOR cipher and One-time pad Describe Vigenère's version of the cipher in greater detail Describe how Babbage broke Vigenere's cipher in greater detail

Contents 1 New wording for le chiffre 2 pronunciation? 3 Adding 26 in the Integers, Modulo 26? 4 Historical Notes

[edit] New wording for le chiffre

I changed the wording to make it more prominant because I just redirected le chiffre indéchiffrable here. If someone types it in and it is redirected, I think it would be best if it is in the lead and bolded. Rewording it (yeah starting sentences with because are akward and bad- i think even ungrammatical) would be fine by me if it is still at the top. Broken S

The Charles Babbage article contradicts this:

The autokey cipher was generally called "the undecipherable cipher", though owing to popular confusion, many thought that the weaker polyalphabetic cipher was the "undecipherable" one.

If that's correct then le chiffre indéchiffrable should redirect to Autokey cipher and this section corrected.

[edit] pronunciation?

"Vigenère"?

Also, the Vigenère table shown is interesting because of its selective use of borders. It only has borders where they matter, unlike 90% of Wikipedia tables which look like ugly grid messes.

I don't know IPA but it is roughly pronounced "vi-je-nair". Also that table is an image not a table, that's why it look different. BrokenSegue 04:24, 22 February 2006 (UTC)

[edit] Adding 26 in the Integers, Modulo 26?

I don't understand why anyone would say:

and decryption,

since 26 is congruent to 0, modulo 26.

It should simply be written:

I removed the useless zero in the previous version.

- Dr. Morelos

Adding 26 ensures that C_i-K_i is positive; computing residues of negative numbers looks weird to computer scientists. Lunkwill 18:12, 3 June 2006 (UTC)

By using the sign as well as the 26, we're confusing two things: one is the compsci use of mod as an operator, and the other is the maths syntax for expressing modular equivalence. I suggest we use the maths syntax — the need for adding 26 is an implementation detail. — Matt Crypto 22:55, 3 June 2006 (UTC)

I removed the 26. BrokenSegue 17:41, 17 June 2006 (UTC)

[edit] Historical Notes

Is it worth pointing out that the Tabula Recta is a tabular representation of Alberti's cipher disk?

The article states that the Vigenère Cipher is miscalled by that name, and it mentions that the Vigenère table is also known as a Tabula Recta - but doesn't say why.

A Tabula Recta contains the shifted alphabets in their normal alphabetical order. There were other tables with disarranged alphabets that looked like a Sudoku problem.

Trithemius described a 24-letter table in his Polygraphia:

In hoc tabula literarum canonica sive recta tot ex uno et usuali nostro latinarum ipsarum per mutationem seu transpositionem habes alphabeta, quot in ea per totum sunt monogrammata ... Even if you can't read Latin, you can see English words: (roughly) In this regular table of letters in the usual order of our Latin alphabet although altered and shifted ... NOTE: a 'transposition' to Trithemius was a Caesar shift.

There are a couple of small points about the table. Line D is the standard Caesar Shift where A becomes D (by adding Caesar's initial).

Line N is latter-day ROT13. It's also the basis of della Porta's polygraphic table that appeared in De Furtivis Literarum Notis in 1563. The pattern is from the Hebrew ALBAM, the brother of ATBASH, dating from ca 500 BC (?) Della Porta produced a sort of half-size tableau with two key letters assigned to each line and intended the table to be used with a keyword.

Trithemius did suggest how to use the table as a Progressive Cipher. You moved down a line for each letter of your message; which reflected Alberti's proposal of turning his disk by one letter at the start of each word.

Incidentally, Alberti disks were still issued to the U. S. Signals Corps in the 20th century. The cipher system was called 'Larabee' and it was essentially Vigenère.

The name sticks to a cipher which Vigenère did not claim as his own. His invention was an AUTOKEY wherein the text enciphered itself. This, strictly, is the original sense of le chiffre indéchiffrable - though it often proved equally indecipherable to its users because a single error produces garbage in the rest of the encipherment.

Lewis Carrol described the Vigenère Variant, which is sometimes called The Lewis Carrol Cipher. In the variant, the key is subtracted from plain text instead of added. The Beaufort subtracts plain from key. The Gronsfeld uses numbers instead of letters (or you can think of it as A - J where A is zero).

I've never come across the term 'Beaufort Variant' which could be misleading. Today the gentleman is remembered for his Wind Scale rather than his cipher.

These literal ciphers can be reproduced in a spreadsheet. Using upper case makes it simple as you restrict the range to 65-90. For plain vanilla Vigenère =MOD(PlainChar + KeyChar,26)+65

Modern English only acquired 26 letters in the 18th century. --Steve 04:41, 15 June 2006 (UTC)

Neat! Thanks for the exposition. Lunkwill 06:16, 15 June 2006 (UTC)