Re: Seriation

Peter Fairbrother wrote:

On 07/12/2024 15:27, Stefan Claas wrote:
 

Seriation was used in the German WWII cipher Doppelkastenschlüssel
and I thought it is pretty cool. However, I did not managed yet to
understand how encryption and decryption works.

 
Doppelkastenschlüssel was a hand cipher of the type known as double
Playfair, or horizontal two square, used by the German Army throughout
WW2 as a field cipher.
 
It works pretty much like Playfair except there are two 5x5 Polybius
squares; the first letter of a plaintext digraph goes in the left square
and the second letter goes in the right square. As in Playfair, the
opposite corners of the rectangle form the ciphertext, with the letter
from the right hand box first.
 
If both letters were on the same horizontal line the previous letters in
the line were used, also usually with the letter from the right hand box
first.
 
The squares were random, not based on keywords (so they seldom ended
XYZ), and were distributed using a key network.
 
21-letter (usually) seriation was used to make both digraph frequency
analysis and transparency analysis harder.

According to the German WWII documentaion 17-letters per two lines,
a blank line and then again 17-letters per two lines and so on.

 
Cascaded substitution, where the results of the digraph substitution
were used as inputs to a second substitution, was also used (sometimes).
 
It was broken by both US and UK cryptanalysts:
 
media.defense.gov/2021/Jun/29/2002751757/-1/-1/0/WORLD_WAR_II.PDF
 
 
A similar two-square cypher was also used by the Germans later in WW1,
but based on keywords and without seriation.

Thanks for the info. I have read the following German docs, one original
from WWII[1] and the Wikipedia[2] article.

[1] <https://cryptocellar.org/wmc/schluesselanleitung-dk-1940.pdf>
[2] <https://de.wikipedia.org/wiki/Doppelkastenschl%C3%BCssel>

Interesting also Klaus Schmeh's articles on this subject.

<https://scienceblogs.de/klausis-krypto-kolumne/tag/doppelkasten-schluessel/>

--
Regards
Stefan