Schneier on Security

Reversing the past • May 22, 2024 1:06 PM

@ALL

This is not a new issue, just a new use.

Those who have been involved with communications and signal processing whilst not exactly eating this stuff for breakfast have been munching on it seriously since the end of the 1950’s and begining of the 1960’s when solid state electronics became small enough and fast enough to make it practically useful in real time.

Most people get to hear two or three things about communications,

1. Signal to noise ratio.
2. Noise is random.
3. Noise is Gaussian in characteristic.

Thus you get ‘Additive White Gaussian Noise'(AWGN) and an explanation such as

https://wirelesspi.com/additive-white-gaussian-noise-awgn/

The reality is AWGN is a simple model of reality that gets widely used and is for many things not a particularly good model.

Noise in reality is the ‘Root Mean Square'(RMS) of many signals.

N^2 = S1^2 + S2^2 + S3^2 + … Si^2

Where each signal S is in reality multipled by a ‘channel gain’ which is like an information carrying signal varying with time.

But also there is another fun thing to consider. Each signal consists of multiple time dispersed copies of it’s self. And gets called ‘Inter Symbol Interference'(ISI) which if used with care can improve the information capacity of a channel

https://www.tutorialspoint.com/digital_communication/digital_communication_pulse_shaping.htm

Knowing that noise is actually multiple sources of information not true randomness, tells you why you can ‘de-pixelate’ images that are either not random or the randomness can be modelled in some -usually- out of band way thus removed layer by layer.

If you think of pixelation as being a very low grade substitution cipher with at best poor chaining, as opposed to a One Time Pad. You can also see why it can be removed layer by layer.

As is seen with the ‘Crypto-Tux’ or ‘ECB-Penguin’ image where the Linux Tux Penguin gets encrypted using AES-128 in ‘Electronic Cipher Book’ simple substitution Mode

https://github.com/robertdavidgraham/ecb-penguin

The actual moral is

“Fully Determanistic algorithms have inverse algorithms.”

(Even supposed “One Way Functions” are invertible to a dictionary attack, especially if the actual input data has a very limited input set).

Reversing the past • May 22, 2024 7:33 PM

@lurker

There is a problem with

“If you don’t want stuff to be seen, remove it completely with a big sharp knife”

It’s proportional fonts that are all to common in documents these days.

Unlike monospaced, fixed-pitch, fixed-width, or non-proportional fonts, the characters are all different widths with lowercase ‘i’ usually being the most narrow and upper case ‘W’ the widest.

If you cut out a single word or a suspected phrase the an unredacting process can find out if the width matches the hole that’s been left with quite some probability.

There is also another issue with proportional fonts. Few people by eye can see if the width of the white space changes.

You can hide a DRM / canary-trap serial / ID number in a printed document in such a way that even cutting out 9/10ths of the text on a page will still not remove the number thus the ‘whistle blower’ will be identifiable.

So one way to make it more anonymous is to scan the document using ‘Optical Character Recognition'(OCR) then “spell check” it etc then save it as an ASCII text file.

Take the file to another computer and read the text file into an editor and reformat the sentences paragraphs and other basic document format back in using a monotype font

Replace redacted words/sections with minus signs in blocks of five with a single space. Not to match words but fill out to approximately the basic space.

This way you keep the basic structure of the document thus limit the usual ploy of people saying

“This can not be a valid document as it looks nothing like any of our documents.”

Or

“We have never produced a document that looks like that.”

“I’ve never seen a document that looks like that in our organisation.”

Etc. Thus not denying if the text is true or not but denying the “style” or “look” of it.