Problem - Kevin Tafuro

The previous code is subtly incompatible with the S/KEY dictionary
because their dictionary is not in alphabetical order. (S/KEY is an
authentication system using one-time passwords.) Be sure to use the
right dictionary!
The code is written in such a way that you can use dictionaries of different sizes if
you wish to encode a different number of bits per word. Currently, the dictionary
encodes 11 bits of data (by having exactly 211 words), where no word is more than 4
characters long. The web site also provides a dictionary that encodes 13 bits of data,
where no word is more than 6 letters long. The previous code can be modified to use
the larger dictionary simply by changing the two appropriate preprocessor definitions
at the top.
The algorithm takes 11 bits of the binary string, then finds the word that maps to the
unique 11-bit value. Note that it is rare for the number of bits represented by a single
word to align exactly to a byte. For example, if you were to encode a 2-byte
binary string, those 16 bits would be encoded by 2 words, which could represent up
to 22 bits. Therefore, there will usually be leftover bits. In the case of 2 bytes, there
are 6 leftover bits. The algorithm sets all leftover bits to 0.
Because of this padding scheme, the output doesn’t always encode how many bytes
were in the input. For example, if the output is 6 words long, the input could have
been either 7 or 8 bytes long. Therefore, you need to manually truncate the output to
the desired length.
See Also
Recipe 4.8
4.8 Converting Text Keys to Binary Keys
Problem
A user enters a textual representation of a key or other binary data (see Recipe 4.7).
You need to convert it to binary.
Solution
Parse out the words, then look them up in the dictionary to reconstruct the actual
bits, as shown in the code included in the next section.
130 | Chapter 4: Symmetric Cryptography Fundamentals
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