Implementing-cryptography-using-python

126 Chapter 4 ■ Cryptographic Math and Frequency Analysis
Here's the example code to read in the words using the .split() method
to break them into a list of words. Using the following Python code, you will
download a file from the Internet called common_en_words.txt, which contains
10,000 common words in the English language. The output will show each of
these words on their own line:
import urllib.request, ssl
# URL to Common English Words
commonWordsPath = "https://raw.githubusercontent.com/noidentity29/
AppliedCryptoPython/master/common_en_words.txt"
# create URL request
response = urllib.request.urlopen(commonWordsPath, context=ssl._create_
unverified_context())
readText = response.read()
# the file is in a binary format, decode
fileOfWords = readText.decode('utf-8')
# create an array for each word
words = fileOfWords.split()
# print word list
for word in words:
print (word)
# filter out the shorter words
longerwords = list(filter(lambda x: len(x)> 2, words))
# print out longer words
for word in longerwords:
print (word)
Determining the Frequency
The trick to decrypting historical ciphers using cryptanalysis relies on being
able to determine the frequency of letters regardless of their encryption state.
In the next example, you will see how to examine the frequency of letters
used in text. The online provided file, ch4_encrypted.txt, is encrypted with
a computer-generated key that represents the letters of the alphabet. From our
previous lessons, you learned that the most common letters include E, T, A, O,
I, N. Armed with this, you can start examining the frequency of letters in text