Cryptography Reference
In-Depth Information
Many of these spreading techniques hide information in the
noise of an image or sound file, but there is no reason why they
can't be used with other forms of data as well.
Many of the techniques
are closely related to the
process of generating
cryptographically secure
random numbers- that
is, a stream of random
numbers that can't be
predicted. Some
algorithms use this
number stream to
choose locations, others
blend the random
values with the hidden
information, still others
replace some of the
random values with the
message.
Adopt a Statistical Profile
Data often falls into a pattern and com-
puters often try to make decisions about data by looking at the
pattern. English text, for instance, uses the letter 'p' for more
often than the letter 'q' and this information can be useful for
breaking ciphers. If data can be reformulated so it adopts the
statistical profile of the English language, then a computer pro-
gramminding ps and qs will be fooled.
Adopt a Structural Profile
Mimicking the statistics of a file is just
the beginning. More sophisticated solutions rely on complex
models of the underlying data to better mimic it. Chapter 7, for
instance, hides information bymaking it look like the transcript
of a baseball game. The bits are hidden by using them to choose
between the nouns, verbs and other parts of the text. The data
are recovered by sorting through the text and matching up the
words with the bits that selected them. This technique can
produce startling results, although the content of the messages
often seems a bit loopy or directionless. This is often good
enough to fool humans or computers that are programmed to
algorithmically scan for particular words or patterns.
Replace Randomness
Many software programs use random num-
ber generators to add realism to scenes, sounds, and games.
Monsters look better if a randomnumber generator adds blotches,
warts, moles, scars and gouges to a smooth skin defined by
mathematical spheres. Information can be hidden in the place
of the random number. The location of the splotches and scars
carries the message.
Change the Order
A grocery list may be just a list, but the order of
the items can carry a surprisingly large amount of information.
Split Information
Data can be split into any number of packets that
take different routes to their destination. Sophisticated algo-
rithms can also split the information so that any subset of
k
of
the
n
parts are enough to reconstruct the entire message.
Hide the Source
Some algorithms allow people to broadcast infor-
mation without revealing their identity. This is not the same as
hiding the information itself, but it is still a valuable tool. Chap-
ters 10 and 11 show how to use anonymous remailers and more
