Cryptography Reference
In-Depth Information
This implementation is lazy in this respect, but I don't thinkmuch
is sacrificed. It is relatively easy to place the grammars in the correct
format. It could be modified to read:
Start
→
noun verb
noun
→
Fred and Barney
what
Fred
what
what
→
went fishing
where
went bowling
where
where
→
in
direction state
direction
→
northern
southern
state
→
Iowa.
Minnesota.
Any context-free grammar can be placed in Greibach Normal
Form. It is also possible for the grammar in Greibach Normal Form
to be expanded so that there are no ambiguities. Alas, sometimes
n
needs to be made quite large to accomplish this. Another solution is
to implement more complicated parsing algorithms.
7.3.2 Suggestions for Building Grammars
Creating a grammar that can be used to effectively turn data into in-
nocuous text can be time-consuming if you want to do it well. More
words and phrases mean more choices, and more choices mean
more data that can be packed into place. The grammars that have
long phrases and few choices can be pretty inefficient. Here are some
suggestions:
David McKellar created
one grammar that
encodes message in
spam-like phrases
removed from his
collection of spam
messages. You can see it
in action at
spammimic.com
.
Think about the Plot and Narrative
The grammar for the baseball
game voice-over in Figure 7.1 is an excellent example of a genre
that can successfully stand plenty of repeating. These genres
make the best choice for context-free grammars because the
repeating effect saves you plenty of effort. You don't need to
come up with production after production to make the system
work. The same choices can be used over and over again.
There are other good areas to explore. Stock market analysis is
generally content-free and filled with stream-of-consciousness
ramblings about a set of numbers flowing throughout the world.
No one can summarize why millions of people are buying and
selling. Sports reporting usually amounts to coming up with
different ways of saying “X smashed Y” or “X stopped Y.” Amore
sophisticated version can be built that would use actual news
feeds tomodify the grammars so the data was correct and filled
with hidden bits.
There are other genres that are naturally plot-free. Modern po-
etry and free verse are excellent genres to exploit. People don't
