Cryptography Reference
In-Depth Information
it conforms to our natural impression of the structure. This is dan-
gerous because an attackermight be savvy enough to exploit this pat-
tern. A sequence of expansions can fix this. Here is the grammar after
several changes:
→
noun
Bob and Ray
verb2
Fred and Barney
verb4
Laverne and Shirley
verb
Thelma and Louise
verb3
Bob and Ray went shooting
where
Bob and Ray went fishing
where
Thelma and Louise went fishing
where
Thelma and Louise went bungee-jumping
where
Fred and Barney went shooting in Minnesota.
Fred and Barney went shooting in Timbuktu.
Fred and Barney went shooting in Katmandu.
Fred and Barney went shooting in Kalamazoo.
verb
→
went fishing
where
went shooting
where
went flying
where
went bungee-jumping in Minnesota.
went bungee-jumping in Timbuktu.
went bungee-jumping in Katmandu.
went bungee-jumping in Kalamazoo.
verb2
→
went flying
where
went bungee-jumping
where
verb3
→
went shooting
where
went flying
where
verb4
→
went fishing
where
went flying
where
went bungee-jumping
where
where
→
in Minnesota.
in Timbuktu.
in Katmandu.
in Kalamazoo.
The productions for the variable
noun
have been expanded in a
number of different ways. Some have had the variable
verb
rolled
into them completely while others have had only a partial combina-
tion. There are now four different versions of the variable
verb
that
were created to handle the productions that were not expanded.
The effect of the contractions is immediately apparent. Figure 7.5
shows the Huffman tree that converts bits into productions for the
variable
noun
. The relationships among nouns, verbs, and locations
and the bits that generated them is now much harder to detect. The
first two bits don't correspond to the noun any more.
Table 7.1 shows phrases and the bits that generated them:
2
There are still some correlations between sentences. The first two
sentences in Table 7.1 have different endings which are reflected in
2
Some of the relationships between bits and the choice of production are left un-
explained and are for the reader to discover.
