You are to write a program called {vigenereic.py} that takes as input a Vigenre ciphertext, uses the code you wrote in assignment 1 for finding the key length, and then implements the algorithm given at the end of section 2.3.3 to find the key. Its only output should be the key length and the key itself. Previous code:

def getKeyLength(ciphertext):

max_shift = -1

max_coinc = -1

for shift in range(1,15):

coinc_count = 0

for i in range(len(ciphertext)):

shifted_index = (i + shift) % len(ciphertext)

if ciphertext[i] == ciphertext[shifted_index]:

coinc_count += 1

print("Shift: {0}, Coincidences: {1}".format(shift, coinc_count))

if coinc_count > max_coinc:

max_shift = shift;

max_coinc = coinc_count

return (max_shift, max_coinc)

v_shift, v_coinc_count = getKeyLength(ciphertext)

print("v_shift: {0}, v_coinc_count: {1}".format(v_shift, v_coinc_count))

To make this a little easier, here is the A₀ vector referred to in the algorithm in both Java and Python format. These contain the frequencies of letters in English:

 EnglishFrequencies = [0.082, 0.015, 0.028, 0.043, 0.127, 0.022, 0.020, 0.061, 0.070, 0.002, 0.008, 0.040, 0.024, 0.067, 0.075, 0.019, 0.001, 0.060, 0.063, 0.091, 0.028, 0.010, 0.023, 0.001, 0.020, 0.001] 

From these you can form the the A₁, ..., A₂₅ vectors.

You will have to determine a way to translate character ASCII codes to and from the range 0 to 25.

Try out your program on the Vigenre ciphertext:

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