Cryptography Reference
In-Depth Information
Humans also provide some error correction for basic spelling and
grammatical errors. A hidden message can be encoded by introduc-
ing seemingly randommisspellings from time to time.
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15.3.1 Positioning
Another possible solution is to simply adjust the positions of letters,
words, lines and paragraphs. Typesetting is as much of an art as a
job and much care can be devoted to algorithms for arranging these
letters on a page. Information can always be hidden when making
this decision.
The L
A
T
E
XandT
E
X typesetting systems used in creating this topic
justify lines by inserting more white space after a punctuation mark
than after a word. American typesetters usually put three times as
much space after punctuation. The French, on the other hand,
avoid this distinction and set both the same. This mechanism is easy
to customize and it is possible to change the “stretchability” of white
space following any character. This paragraph was typeset so the
whitespace after words ending in 'e' or 'r' received three times as
much space with the
sfcode
macro.
Changing these values throughout a document to smaller values
is relatively simple to do. In many cases, detecting these changes
is also relatively simple. Many ommercial OCR programs continue
to make minor errors on a page, but steganographic systems using
white space can often be more accurate. Detecting the size of the
whitespace is often easier than sussing out the differences between
the ink marks.
Jack Brassil, Steve Low, NicholasMaxemchuk and Larry O'Gorman
experimented with many techniques for introducing small shifts to
the typesetting algorithms. [BO96, LMBO95, BLMO95, BLMO94]
This can be easy to do with open source tools like T
E
Xthatalsoin-
clude many hooks for modifying the algorithms.
Dima Profrock, Mathias
Schlauweg and Erika
Muller suggest that
actual objects in
digitized video can be
moved slightly to encode
watermarks.[PSM06]
One of their most successful techniques is moving the individual
lines of text. They successfully show that entire lines can be moved
up or down one or two six-hundredths of an inch. Moving a line is
easy to detect if any skew can be eliminated from the image. As long
as the documents are close to horizontal alignment, the distance
between the individual lines can be measured with enough precision
to identify the shifted lines.
The simplest mechanism for measuring a line is to “flatten” it
into one dimension. That is, count the number of pixels with ink
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Some might be tempted to blame me and the proofreader for any errors that crept
into the text. But perhaps I was sending a secret message.
