Implementing-cryptography-using-python

Chapter 4 ■ Cryptographic Math and Frequency Analysis 125
frequency = {}
for ascii in range(ord('a'), ord('a')+26):
frequency[chr(ascii)] = float(encrypted.count(chr(ascii)))/len(plaintext)
sum_freqs_squared = 0.0
for ltr in frequency:
sum_freqs_squared += frequency[ltr]*frequency[ltr]
print ("Will be near .065 despite Caesar: " + str(sum_freqs_squared))
for possible_key in range(1, 26):
sum_f_sqr = 0.0
for ltr in normal_freqs:
caesar_guess = shiftBy(ltr, possible_key)
sum_f_sqr += normal_freqs[ltr]*frequency[caesar_guess]
if abs(sum_f_sqr - .065) < .005:
print ("Key is probably: ", possible_key, " f_sqr is ",sum_f_sqr)
Figure 4.8 shows the key that was randomly generated to encrypt the text.
The code will then return the FA of the encrypted code, which will be relatively
close to .065 since it is a substitution cipher. The code looks for which key gets
the closest to .065 and will then recommend that result as a key. Each time you
execute your solution, you will generate a new key.
Figure 4.8: Cearsar_FA.py
Using an Online Word List
One useful technique to help identify when you have successfully decrypted a
historical cipher is to use known word lists. In the following example, we simply
load a text file containing 10,000 common English words. In the technique shown
we will read in the words using the .split() method to break them into a list
of words. Additionally, you will see that we are interested only in the words
that are at least three letters long. The quickest way to filter these words is to
use a filter with the lambda function. Lambda functions are functions that do not
require a name like the defined functions do. The lambda function returns a
function object that can be assigned to a variable. In Python, the most common
use for lambda functions is for a simple one-line function. You will typically
see the lambda functions used in conjunction with a map or filter function.