Is K4 of Kryptos a Werftschlüssel Cipher?

Maybe but it would be ridiculously hard to break with only 97 letters and no knowledge of the table used for enciphering the digraphs.  We would need some indication from “clues” hidden in Kryptos that would allow us to form the table to reverse engineer K4.

This one falls under possible but too hard for me personally to attempt.

I’ll repost from Just Nick:

The Werftschlüssel is more generally known by its anglicized name “Dockyard Cipher” or just “Dockyard.” It was the standard mid-grade code used by the German navy during WW2, a companion to the famous Enigma machine that was better suited to auxiliary vessels, small units, and others situations where the expensive and valuable Enigmas could not reasonably be issued. Dockyard is best known for the role it played in routine breaking of Enigma ciphers. Always ready to help out their foes with some bad practice, the German navy would often send identical (or at the very largely similar) messages in the two systems. The easier-to-break Dockyard would yield the known-plaintext crib that was necessary for the folks at Bletchley Park to break that day’s Enigma settings.

Dockyard was simple to implement — the plaintext was written out in five columns (apparently there were specially printed tables to help with this process).

Each vertical pair of letters (or digraph) would be encrypted together according to one of a set of tables published on a monthly or bimonthly basis. IT would be looked up and replaced with its cipher pair, TH with its pair, WE with its, and so on. Each column would be enciphered with using the keypairs from a separate table. These columns were selected from the book of 20 or 30 columns mentioned earlier — I don’t know if the procedure was to use a daily set of columns or for the operator to select five for each message and somehow communicate that in an indicator group.

The idea of using 20 or 30 different digraph substitutions was an interesting one — presumably intending to reduce the amount of traffic that was generated for each. The seriation step also helped break up common digraphs (e.g. the “ch” that is as common in German as the “th” is in English). Digraphic substitution is a nice way of diffusing the statistical predictability of language — no single digraph has the same obvious statistical dominance as “e” for example. There still are trends, and with a large enough sample, they are noticeable.

The seriation of Dockyard works to further diffuse the common elements of language, but the resulting digraphs still do not have a random distribution, much as we might think that they would. The statistical frequency of individual letters and certain patterns of letter use still result in digraphs appearing more often than others. The EE pairing is more common than any other, for example, because there are simply more e’s going around to possibly end up stacked on top of each other.

As a result, the Naval Section at Bletchley could slowly compile up a key for each of the tables in a set. Using the statistical procedures discussed together with some guesses about message content (all the tools of classical cryptanalysis), the tables would be recovered. Once the tables were recovered, identifying which five were in use on a given message was a relatively simple matter of frequency counts. From all of this, the Dockyard cipher was probably the greatest source of intercept information to the Allies about German naval movements — particularly considering its role as a source of Enigma plaintext cribs. From 1941 to 1945, about 33,000 dockyard messages were intercepted. 90% of them were read the same day they were intercepted…

Go check out Just Nick the Noodler (noodlebook.blogspot) as one man’s obscure knowledge is another man’s obsession.