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

[Peter Fairbrother]

On 21/01/2026 22:55, Jon Callas wrote:

> A thing to remember about it, though, is that he never just comes out and says they were doing one-time pads. 

In the book he sure does, many times. He call them LOPs, letter one-time 
pads. He even gives an example of a substitution/decryption square for 
OTPs in the book. Chapter 30.


He says that they developed some unbreakable system, so unbreakable that 
he really shouldn't talk about it.

In the book the only things he dances around or doesn't talk about are 
why he got interested in cryptography and the method of verbally giving 
agents distress codes which the giver couldn't remember.


(By the way, the background banner photo in my LinkedIn profile is an 
SOE silk pad, and there are lots of images available on the web, 
including <https://people.duke.edu/~ng46/collections/crypto-silks.htm>. 
I got it by searching for "SOE one time pad on silk".)

That, the first picture, is a decryption square, not an OTP or LOP. 
There were a few versions, but mostly they were all the same. I don't 
know what the second picture is, maybe one of the silks SOE gave to OSS, 
as afaik SOE didn't use number keys. The third is a set of WOKs. I have 
never seen a LOP or an image of one.



Incidentally in an earlier post I mildly castigated Marks for using 
substitution squares rather than Captain Crunch type alphabetic 
addition, but on doing some research I find that Bletchley also used 
substitution squares, so I can't blame Marks for not seeing that.

I also skimmed the book again, and the way he made ciphertexts from 
double transposition (poem and WOK) codes look like the output from an 
OTP was:

"to choose five words from poem A, and obtain a transposition-key in the 
normal way. But instead of encoding a message in it, he'd encode the 
whole of poem B and use the resulting code-group as his one-time pad!" 
(chapter 63).

He called them MOPs or mental one-time pads. I don't know how the pad 
was used if silk substitution squares weren't available.

Peter Fairbrother