The most difficult part concerning sections 2.3 - 2.4 for me was the understanding of the reasoning behind finding the key length concerning the Vinegere cipher. The book indicates that to find the appropriate key length for the Vinegere cipher, it is necessary to choose various shifts and then find the shift with the greatest "coincidences" pertaining to identical ciphertext letter matchups. This was confusing at first but was eventually explained using dot products later in the section. Despite this explanation, however, I'm still not entirely sure of the reasoning behind its usefulness and will hope for clarification in lecture.
The farthest-reaching and probably most relevant part of this reading was demonstrated in both the vinegere cipher and substitution cipher sections; namely that frequency analysis is a powerful device for cryptanalysis. The vast majority of ciphers prior to enigma were solvable in this fashion. This also explains why linguists and crossword enthusiasts were so successful at code-breaking.
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