frequency_analysis.py

# Exercise from Udacity course. 
# Crypto Analysis: Frequency Analysis
# To analyze encrypted messages, to find out information about the possible 
# algorithm or even language of the clear text message, one could perform 
# frequency analysis. This process could be described as simply counting 
# the number of times a certain symbol occurs in the given text. 
# For example:
# For the text "test" the frequency of 'e' is 1, 's' is 1 and 't' is 2.
#
# The input to the function will be an encrypted body of text that only contains 
# the lowercase letters a-z. 
# As output a list of the normalized frequency for each of the letters a-z. 
# The normalized frequency is simply the number of occurrences, i, 
# divided by the total number of characters in the message, n.

def freq_analysis(message):
    alphabet = [0.0] * ((ord('z') - ord('a')) + 1)
    for i in message:
        counter = message.count(i)
        freq = counter / len(message)
        alphabet[ord(i)-ord('a')] = freq
                
    return alphabet


def main():
    print(freq_analysis("abcd"))
    #>>> [0.25, 0.25, 0.25, 0.25, 0.0, ..., 0.0]

    print(freq_analysis("adca"))
    #>>> [0.5, 0.0, 0.25, 0.25, 0.0, ..., 0.0]

    print(freq_analysis('bewarethebunnies'))
    #>>> [0.0625, 0.125, 0.0, 0.0, ..., 0.0]

if __name__ == '__main__':
    main()