[Cryptography] Leo Marks' 1998 talk about WW2 SOE code-making and breaking

[Peter Fairbrother]

On 19/01/2026 17:16, Kent Borg wrote:

> Question: What is a "worked out key"?

"Worked out keys" weren't actually worked out at all, they were randomly 
generated subkeys similar in use to the non-random subkeys agents worked 
out from a poem.

Before WOKs the agents choose some words or words from a poem - the 
choice was coded as line x word y, and x and y were sent at the 
beginning of the transmission. The agent then numbered the letters in 
the words, rearranged the letters in alphabetical order, then used the 
numbering orders of the rearranged letters as subkeys to order the 
transpositions.

Whew!

Iirc, WOKs were groups of numbers for the agents to do the 
transpositions with. Eg if the WOK was 3,5,4,1,2,7,6  8,4,3,6,1,5,2,7 
the first transposition would be to rearrange the columns so the first 
column moved to third, the second column fifth, and so on.

I might have gotten that a little wrong, it's been a long time, but it 
was something pretty close to that.


Being random they couldn't be used to decrypt earlier massages, unlike 
poem codes/subkeys, where if the poem was known all the traffic 
previously encrypted in that code was toast.

Their use also meant that agents didn't have to generate the subkeys, 
which was prone to mistakes and misuse (reusing the same random words 
from the poem). Also also, the agents didn't have to learn a new code.


Peter Fairbrother