Implementing-cryptography-using-python

136 Chapter 4 ■ Cryptographic Math and Frequency Analysis
normal_freqs = {'a': 0.080642499002080981, 'c': 0.026892340312538593, 'b':
0.015373768624831691, 'e': 0.12886234260657689, 'd': 0.043286671390026357, 'g':
0.019625534749730816, 'f': 0.024484713711692099, 'i': 0.06905550211598431, 'h':
0.060987267963718068, 'k': 0.0062521823678781188, 'j': 0.0011176940633901926, 'm':
0.025009719347800208, 'l': 0.041016761327711163, 'o': 0.073783151266212627, 'n':
0.069849754102356679, 'q': 0.0010648594165322703, 'p': 0.017031440203182008, 's':
0.063817324270355996, 'r': 0.06156572691936394, 'u': 0.027856851020401599, 't':
0.090246649949305979, 'w': 0.021192261444145363, 'v': 0.010257964235274787, 'y':
0.01806326249861108, 'x': 0.0016941732664605912, 'z': 0.0009695838238376564}
lowerMessage = message.lower()
sampling = lowerMessage[keyPos::keyLength]
for ascii in range(ord('a'), ord('a')+26):
frequency[chr(ascii)] = float(sampling.count(chr(ascii)))/
len(sampling)
sum_freqs_squared = 0.0
for ltr in frequency:
sum_freqs_squared += frequency[ltr]*frequency[ltr]
for possible_key in range(1, 26):
sum_f_sqr = 0.0
for ltr in normal_freqs:
caesar_guess = shiftBy(ltr, possible_key)
freqCalc = normal_freqs[ltr]*frequency[caesar_guess]
sum_f_sqr += freqCalc
engValue = abs(sum_f_sqr - .065)
if engValue < tolerance:
allKeys.append(possible_key)
return allKeys
def getKey(encrypted, kl, dictionary):
keys = []
testKey = []
defaultKey = []
for i in range (0,kl):
keyPos = findKeyPos(encrypted,16,i)
answerLen = len(keyPos)
answerIndex = 0
if answerLen > 1:
defaultKey = keys[:]
testKey = keys[:]
defaultKey.append(keyPos[0])
decrypted = decryptIndex(defaultKey,encrypted)