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

[Peter Fairbrother]

On 26/01/2026 02:21, zeb--- via cryptography wrote:
>  > On 2026-01-25 06:00, Peter Fairbrother wrote:
>  >
>  > Problems: you can't know what these codes are, in case you are caught
>  > and tortured.
>  >
>  > you can't give the agent anything physical in case the Gestapo find it.
>  >
>  > you can't say to the agent, make up something random for the first
>  > message and repeat thereafter, because the first message needs to be
>  > authenticated.
>  >
>  >
>  > So how do you tell the agent what his secure and distress codes are?
> 
> 
> That seems impossible.
> 
> I suppose you actually _could_ give each agent something physical (and 
> individual), like a sealed envelope, to be burned or eaten or self- 
> destruct, after remembered.
> 
> So what's the answer?

I don't know - though it was done verbally. Afaik it's an unsolved mystery.

It's the only thing which Marks was "asked" by GCHQ, MI5 etc to leave 
out of his book.

Peter Fairbrother