Implementing-cryptography-using-python

Chapter 4 ■ Cryptographic Math and Frequency Analysis 123
modifiedText = modifiedText.lower()
plaintext = ""
for c in modifiedText:
if (c.isalpha()):
plaintext = plaintext + c
# Determine the frequency analysis of the plaintext
frequency = {}
for ascii in range(ord('a'), ord('a')+26):
frequency[chr(ascii)] = float(plaintext.count(chr(ascii)))/
len(plaintext)
sum_freqs_squared = 0.0
for ltr in frequency:
sum_freqs_squared += frequency[ltr]*frequency[ltr]
# Results
print()
print ("The frequency should be near .065 if plaintext in English: " +
str(sum_freqs_squared))
Figure 4.7 shows the frequency analysis of the text of the Declaration of
Independence. Since the document is in English, the frequency analysis should
be near the value .065. When you are checking to see if your cryptanalysis is
complete, your result should be similar.
Figure 4.7: declaration_freq.py
Cryptanalysis with Python
Now that you understand the importance of frequency analysis (FA), you can
start to explore how to use FA to start cracking classical ciphers. The techniques
you learn here can be applied to a number of substitution-type ciphers such as
the Caesar, ROT-13, and Vigenère ciphers. As your understanding grows, you
will also be able to apply these techniques to the other classical ciphers that are
outlined in Chapter 3.
The next cryptanalysis code listing we will examine will take the Declaration
of Independence and encrypt it using a randomly generated key. It will then use
FA to examine all key possibilities; remember that when using a letter-shifting
cipher such as the Caesar cipher, there are only 26 possible keys. The code listing