Talk:Playfair cipher

From Wikipedia, the free encyclopedia

WikiProject on Cryptography This article is part of WikiProject Cryptography, an attempt to build a comprehensive and detailed guide to cryptography on Wikipedia. If you would like to participate, you can choose to edit the article attached to this page, or visit the project page, where you can join the project and see a list of open tasks.
WikiReader Cryptography It is intended that this article be included in WikiReader Cryptography, a WikiReader on the topic of cryptography. Help and comments for improving this article would be especially welcome. A tool for coordinating the editing and review of these articles is the daily article box.
To-do list for Playfair cipher: edit · history · watch · refresh

Hello

The example used needs to be changed, because usually the letter removed from the box is the letter "J", not the "Q" as is done in the example.

Now:

P L A Y F
I R E X M
B C D G H
J K N O S
T U V W Z

sugested:

P L A Y F
I R E X M
B C D G H
K N O Q S
T U V W Z

If change is done, the example section will be:


[edit] Example

Using "playfair example" as the key, the table becomes:

P L A Y F
I R E X M
B C D G H
K N O Q S
T U V W Z

Encrypting the message "Hide the gold in the tree stump":

HI DE TH EG OL DI NT HE TR EX ES TU MP
                            ^
  1. The pair HI forms a rectangle, replace it with BM
  2. The pair DE is in a column, replace it with OD
  3. The pair TH forms a rectangle, replace it with ZB
  4. The pair EG forms a rectangle, replace it with XD
  5. The pair OL forms a rectangle, replace it with NA
  6. The pair DI forms a rectangle, replace it with BE
  7. The pair NT forms a rectangle, replace it with KU
  8. The pair HE forms a rectangle, replace it with DM
  9. The pair TR forms a rectangle, replace it with UI
  10. The pair EX (X inserted to split EE) is in a row, replace it with XM
  11. The pair ES forms a rectangle, replace it with MO
  12. The pair TU is in a row, replace it with UV
  13. The pair MP forms a rectangle, replace it with IF


BM OD ZB XD NA BE KU DM UI XM MO UV IF

Thus the message "Hide the gold in the tree stump" becomes "BMODZBXDNABEKUDMUIXMMOUVIF".

[edit] Playfair cryptanalysis

The Playfair is thus significantly harder to break since straight frequency analysis doesn't work with it.

Well, that's simply not the case. Analysis of single letters doesn't work with Playfair, but if you do frequency analysis of digraphs it works ... well, not "fine" exactly, but it's the way to get results. True, there are roughly 600 of them (in English) as opposed to 26, and the distribution is flatter, but as written the article suggests that cryptanalysts faced with Playfair sit around scratching their heads, which I imagine they generally don't. --Calieber 15:31, 7 Oct 2003 (UTC)

[edit] 600* possible digraphs

There is currently an asterisk after the 600 in the number of possible digraphs. In <a href="http://en.wikipedia.org/w/index.php?title=Playfair_cipher&diff=prev&oldid=53704799">this edit</url> the "footnote" of *600 =A_{25}^2 was removed. It is not clear to me why a non-standard footnote was used in the first place, nor exactly what this notation was supposed to mean (the combinations of 25 letters?), nor why 25 is used instead of 26, nor why 25^2=625 does not equal 600.

Can anyone clear this up? Wrs1864 (talk) 16:00, 27 February 2008 (UTC)

The asterisk doesn't make sense. The number 600 however can be explained as follows. Because one uses I=J there are only 25 distinct letters. Because one replaces one of the letters with an X when the same letter appears twice in a digraph, it is not possible that a double letter occurs in the ciphertext (at least if one also assumes that XX does not occur in the plaintext). This implies that only 25*24 = 600 digraphs in the ciphertext are possible. 85.2.23.131 (talk) 20:50, 27 February 2008 (UTC)