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
Priority 4  

Contents

[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:

C_i \equiv (P_i + K_i) \mod 26
and decryption,
P_i \equiv (C_i - K_i + 26) \mod 26

since 26 is congruent to 0, modulo 26.

It should simply be written:

P_i \equiv (C_i - K_i) \mod 26

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 \equiv 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)
Correct is either writing this as a congruence relation:
P_i \equiv C_i-K_i\pmod{26}
or using the modulo operation
P_i = (C_i-K_i)\, \bmod 26
where in the second case we have to explicitly specify that modulo operation always returns a nonnegative integer, because the modulo operation does not have a clear definition. 85.0.100.59 05:57, 30 October 2007 (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).

--Doesn't sound like much of a variant to me! It is equivalent to using a different key. 86.154.82.20 00:49, 24 August 2007 (UTC)

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)

[edit] Translation

The French word for unbreakable is "incassable" according to my French volumes. Casser (to break) is the root, as in "casse-tete", for "break-head" which is used sometimes to describe puzzles. "Le chiffre indéchiffrable" translates directly as "the indecipherable cipher," since the root of "indéchiffrable" is "chiffre." I haven't changed this since I may not know the precedent for using the "unbreakable" translation.--WPaulB 02:32, 3 March 2007 (UTC)

yeah, I agree undecipherable seems more correct, but for a cipher what's the difference between unbreakable and undecipherable? BrokenSegue 23:43, 10 June 2007 (UTC)
We want to use an idiomatic translation, to capture the meaning. In current American English usage, "undecipherable" could connote "illegible" or "incorrectly enciphered" as well as "unbreakable". I think "unbreakable" captures the intent more precisely. — DAGwyn 20:23, 13 June 2007 (UTC)

[edit] comments

sorry to intervene this neat todo-list, but I have something to say (I'm not a skilled/trained/experienced wikipedia-contributor, so i don't know how to put this right :P): the picture of "The Vigenère square or Vigenère table" overlaps the paragraph left of it, speaking of 26! possibilites and so on. screen-resolution:1280x1024. using firefox.

Doesn't do that for me... Don't know why it would break like that. BrokenSegue 23:44, 10 June 2007 (UTC)
It seems okay under Firefox 2.0.0.3 on Solaris. Perhaps you have a PNG viewer enabled that mispositions images? — DAGwyn 20:19, 13 June 2007 (UTC)
I get this error too. Firefox 2.0.0.6 on Kubuntu. Perhaps it is a font thing. That often messes up layouts. 86.154.82.20 00:52, 24 August 2007 (UTC)

[edit] Babbage: possible reason for not publishing his method

What are people's views on adding the likely reason for Babbage not publishing his method, i.e. that he had contacts with the government of the day and may have been asked not to publish? This is mentioned in the Simon Singh book already referenced (albeit in the English edition entitled "The Science of Secrecy"). --TraceyR 17:56, 6 September 2007 (UTC)

It appears to be pure speculation on Singh's part, therefore not something worth reporting in an encyclopedia. I note that Singh has definitely got several other things wrong in his book. — DAGwyn 06:09, 7 September 2007 (UTC)

[edit] Consequently??

From the article:
This cipher is well known because while it is easy to understand and implement, it often appears to beginners to be unbreakable; this earned it the description le chiffre indéchiffrable (French for 'the unbreakable cipher'). Consequently, many people have tried to implement obfuscation or encryption schemes that are essentially Vigenère ciphers, only to have them broken[1]

The way I interpret this paragraph is that BECAUSE the Vingenere cipher was thought to be unbreakable, folks went out and wrote their own schemes that were (a) sometimes obfuscation so not even related to Vingenere Cipher and (b) ciphers that were based on the same principal as Vingerere and shared the weakness. Certainly (a) does not follow. And I don't believe the reference [1] would make that statement, but it could be illogical also ;-) —Preceding unsigned comment added by 24.99.85.51 (talk) 05:42, 24 January 2008 (UTC)

I fixed the sentence. Obfuscation isn't relevant and merely, um, obfuscates the issue. — DAGwyn (talk) 21:29, 24 January 2008 (UTC)

[edit] Friedman test

Test was invented in 1920, not in 1925, according to http://www.nsa.gov/cch/cch00005.cfm.

It should be changed, I think. I'm not sure, because there are other sources with 1922.

My native language is Serbian, and I'll put 1920. on sr.wikipedia.

Senior-sr (talk) 13:06, 30 January 2008 (UTC)

The kappa test is just a simple application of the index of coincidence, and probably evolved over some period of time. I changed the article to say "in the 1920s". — DAGwyn (talk) 19:20, 30 January 2008 (UTC)