Re: Seriation

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.
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.
Peter Fairbrother