WW2(?) steganography

[Peter Fairbrother]

Just been reading "Garbo - the spy who saved d-day", which is an MI5 account
released by the public records office of the Garbo (real name Juan Pujol)
case (the Germans thought Garbo, a Spaniard, ran a ring of spies in Britain
and was one of their best agents - in fact Garbo was a double agent, and
there were no spies, just a lot of MI5 officers).

They communicated largely by using secret inks in letters to Spain/Portugal,
and the book gives some tales of inks and developers, "striping", which is a
technique whereby the censor applies a stripe of reagent to test for
invisible inks - but the striping can be detected from examining innocent
mail, so you use better secret inks - or you stripe with ineffective
developers - and so on, in a maze of deception and double-cross.

The interesting thing is that, even though the report, written in 1945, has
now been released, the only things redacted (so far - I'm half way through)
are the names of the inks and developers...

-- Peter Fairbrother