Cryptography Reference
In-Depth Information
0
1
0
1
0
1
0
1
0
1
0
1
Fred and Barney went shooting in Kathmandu.
Fred and Barney went shooting in Kalamazoo.
0
1
Fred and Barney went shooting in Timbuktu.
Fred and Barney went shooting in Minnesota.
0
1
Thelma and Louise went bungee-jumping where
Thelma and Louise went fishing where
0
1
Bob and Ray went shooting where
Bob and Ray went fishing where
0
1
Thelma and Louise verb3
0
1
Laverne and Shirley verb
Fred and Barney verb4
Bob and Ray verb2
Figure 7.5: A Huffman tree that converts bits into productions for the
variable noun .
Table 7.1:
Phrase
Bits
Fred and Barney went shooting in Katmandu.
0000
Fred and Barney went shooting in Minnesota.
0011
Fred and Barney went fishing in Minnesota.
110100
Fred and Barney went bungee-jumping in Minnesota.
1100100
Thelma and Louise went bungee-jumping in Minnesota.
010000
Thelma and Louise went flying in Timbuktu.
100101
the last two bits. In fact, the first two bits seem to mean “Fred and
Barney went shooting” and the last two bits choose the location. This
pattern could easily be erased if the order of the productions were
permuted. It could also be affected by any weighting given to the
phrases.
But this same pattern does not hold for the other sentences. If
the sentence begins “Fred and Barney went fishing” or if they go
“bungee-jumping”, then a different pattern holds. The location is de-
termined by the choice made when the variable where is expanded.
In this case, the relationship between the bits and the location is dif-
ferent. “Minnesota” is produced by the bits 00 in this case.
This is a good illustration of the effect that is the basis for all of the
security of this system. The meaning of the phrase “Minnesota” de-
pends on its context. In most cases it is generated by the bits 00, but
in a few cases it emerges from the bits 11. This is somewhat ironic be-
cause the grammars are called “context-free.” The term is still correct
Search WWH ::




Custom Search