Schneier on Security

Clive Robinson • November 11, 2017 12:50 PM

@ Mike Barno,

Maybe his bromides

Hmm I shall try not to get into a “brown study” over that 😉

@ Wael,

Tried to read the paper – gave me a headache. Is this Cryptography or Steganography? Sounds like both?

If it relies on “being out of sight” to the uninformed then it’s steganography.

@ ALL,

There is a problem with “secret inks” and it’s been known since before the second world war.

An ink has to be visable at the surface of the paper. Which by the laws of physics means it either has to “soak in” to the paper like old fashioned fountain pen ink, or it has to “adhere to” the surface of the paper in some way.

Either way the surface of the paper where the secret ink is, is different to other parts of the paper. That is a real physical diffrence not just chemical.

In the case of “soaking in” the fibers in the paper would have swollen prior to the ink drying, and this leaves a permanent physical mark on the fibers which can be seen via various quite simple techniques. In the case of adhering to the surface of the paper it to will cause a physical change that again can be seen by the same or similar simple techniques…

The simplest test is realy quite simple you need a bright light source, a darkish room and a slightly curved surface. If you arange the light to come at near right angles in a thin “sheet beam” from a slit or similar. You then put the sheet of paper on the curved surface such that it just touches into the beam and rock it slowly backwards and forwards so you get the beam to just kiss the surface of the paper… Any physical changes in the surface of the paper casts shadows on the surface. Think of it as the same process of dawn breaking across a mountain range that photographas love to put into photo competitions because of the contrasts etc.

But… There is something else all substances have frequency responses in the various light spectra from far infra-red through to the far ultra-violet. Which means they stand out under spectroscopic analysis as the ink will have a different frequency response to that of the underlying paper finish (often a calcium compound these days).

Lastly there is the method David Khan mentioned in his crypto book that was allegedly developed at the FBI laboratories (though there are mentions of others doing the same thing in Europe at the same time and earlier). It’s the iodine vapour test and the modern equivalent the “super glue” vapour test. You hang the paper in a fish tank size vapour chamber and by the use of an electrically heated dish you form a thin vapour of iodine or super glue, which then with a little static magic adhears to the surface of the paper at varying thicknesses. Which with the appropriate light source highlights the ink or paper at a different level to the other. Certainly enough for it to be photographed.

The real way to do secret writing these days is to apply a very thin uniform coating to the paper, such there are no physical differences. In this film you use an appropriate halide emulsion much as Silver bromide and silver chloride were used in gelatin for photographic materials. These chemicals form electron traps when exposed to light but as they are in effect embeded in the emulsion they don’t show up to the simple tests. As with photogrphic film you have to fix and later develop the image.

However promising modern chemicals are those that also need a magnetic field as well as light, though they do tend to be quite poisonous… Similar chemicals are in the likes of rewritable CDs / DVDs etc. Oh and those that use UV etc like those on your children’s teeth to protect them from modern diets.