Compression - Disappearing Cryptography - page 78

than 5, it will hide one bit. If the bit is zero, then the token is left
unchanged. If the bit is one, then the number of characters to be
copied is shortened by one. Here's the same quote with the two bits
11 encoded:
The famous Baltimore Oriole, Cal Ripken Jr., is the son of
(30,9) nSr.whocoachedforthe (46,5) es in the past.
In both cases, the size of the copying was cut by one. This does
reduce the amount of compression to a small extent.
The greatest advantage of this approach is that the file format is
unchanged. A standard GZIP programwill be able to decompress the
data without noticing that information was hidden in the process.
Information could be left around without attracting suspicion. A
quick analysis, however, could also reveal that data was hidden in
such a manner. If you scan the file and examine the tokens, you can
easily determine which ones are just a character too small. There
is no way to deny that the program that did the GZIP compression
failed.
5.4 Summary
Compression algorithms are normally used to reduce the size of a
file without removing information. This can increase their entropy
and make the files appear more random because all of the possible
bytes become more common. The compression algorithms can also
be useful when they're used to producemimicry by running the com-
pression functions in reverse. This is described in Chapter 6.
The Disguise Compression algorithms generally produce data that
looks more random. That is, there is a more even distribution
of the data.
How Secure Is It? Not secure at all. Most compression algorithms
transmit the table or dictionary at the beginning of the file. This
may not be necessary because both parties could agree on such
a table in advance. Although I don't know how to figure out
the mapping between the letters and the bits in the Huffman
algorithm, I don't believe it would be hard to figure out.
How to Use It Many compression programs available for all com-
puters. They often use proprietary algorithms that are better
than the versions offered here and make an ideal first pass for
any encryption program.