Implementing-cryptography-using-python

Chapter 4 ■ Cryptographic Math and Frequency Analysis 121
would give you enough details to attempt to break the cipher. With the Caesar
cipher, you will be able to brute-force a solution within 25 rounds. More advanced
substitution ciphers, like the Vigenère cipher, offer a little more of a challenge.
We will first take a look at understanding how frequency analysis works; we
will then apply the analysis to both the Caesar and the Vigenère cipher so that
you can truly understand the power of frequency analysis. Frequency analysis
isn’t always helpful. Take the case of the novel Gadsby. Gadsby is a 1939 novel
by Ernest Vincent Wright written as a lipogram, which does not include words
that contain the letter E. Our first example will take a small set of text and count
the occurrence of each letter, as shown here:
# initializing string
test_str = "We hold these truths to be self-evident, that all men are
created equal, "
test_str += "that they are endowed by their Creator with certain
unalienable Rights, "
test_str += "that among these are Life, "
test_str += "Liberty and the pursuit of Happiness."
# get count of each element in string
all_freq = {}
for i in test_str:
if i in all_freq:
all_freq[i] += 1
else:
all_freq[i] = 1
# printing result
print()
print ("Count of all characters in the provided text is :\n " + str(all_freq))
# frequency using collections.Counter()
from collections import Counter
# initializing string
test_str = "We hold these truths to be self-evident, that all men are created equal, "
test_str += "that they are endowed by their Creator with certain unalienable Rights, "
test_str += "that among these are Life, "
test_str += "Liberty and the pursuit of Happiness."
# using collections.Counter() to get
# count of each element in string
res = Counter(test_str)