Using some of the possible variations of K4, I attempted to use a digraphic substitution cipher on K4. It didn’t work even though I had high hopes that it would.
The earliest practical digraphic cipher (pairwise substitution), was the so-called Playfair cipher, invented by Sir Charles Wheatstone in 1854. In this cipher, a 5 x 5 grid is filled with the letters of a mixed alphabet (two letters, usually I and J, are combined). A digraphic substitution is then simulated by taking pairs of letters as two corners of a rectangle, and using the other two corners as the ciphertext (see the Playfair cipher main article for a diagram). Special rules handle double letters and pairs falling in the same row or column. Playfair was in military use from the Boer War through World War II.
Several other practical polygraphics were introduced in 1901 by Felix Delastelle, including the bifid and four-square ciphers (both digraphic) and the trifid cipher (probably the first practical trigraphic). (Source:Wikipedia)
Because a digraph ciphertext to digraph plaintext requires a known table or a known keyworded-table, I used a recent observation that the third and fourth panels of the Kryptos Copperplate are more than just a Vigenere table but act as a digraphic substitution table where 2 letters of ciphertext can be translated into one letter of plaintext. I assumed the first coordinate was the row letter as per standard conventions.
I translated the first two lines of each variation on K4 (V1 is the original text) to see if a clear message was revealed and then attempted the next in order to not spend all day on something that wasn’t working.
Here are my results:
V1 (original Copperplate K4 text)
B E G O N Z G T R Y E O T J D
V2 (KRYPTOS letters removed)
B E G O N Z G T R Y E O T J
v3 (K4 transposed to align keyword KRYPTOS)
G I C R B V G U N M O E V U R
V4 (K4 transposed to align keyword KRYPTOS, these letters then removed)
I B Z D H Y V Y U W B T M H
V5 (K4 transposed to align keyword KRYPTOS, digraphs arranged in columns under KR, YPT, OS, null headings)
I I Y H
C N O
R R T
B S Z
V E Y
V6 (Similar to V5 but letters under YPT in triplets)
not applicable with digraphic substitution
I really liked this idea as it combined several of my long-standing assumptions about how to solve K4 but it didn’t pan out. C’est la vie, no?