made friedman_method

Author: nikos258

ciphers/break_vigenere.py

@@ -4,8 +4,8 @@
     'N': 6.95, 'O': 7.68, 'P': 1.82, 'Q': 0.11, 'R': 6.02, 'S': 6.28,
     'T': 9.10, 'U': 2.88, 'V': 1.11, 'W': 2.09, 'X': 0.17, 'Y': 2.11, 'Z': 0.07
 }
-
 LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+PARAMETER = 0.0665  # index of confidence of the entire language (for english 0.0665)
 
 
 def index_of_coincidence(frequencies: dict, length: int) -> float:
@@ -17,7 +17,7 @@ def index_of_coincidence(frequencies: dict, length: int) -> float:
     """
     index = 0.0
     for value in frequencies.values():
-        index += (value/length)**2
+        index += (value / length) ** 2
     return index
 
 
@@ -37,7 +37,7 @@ def calculate_indexes_of_coincidence(ciphertext: str, step: int) -> list:
     for j in range(step):
         frequencies = dict()
         c = 0
-        for i in range(0+j, length, step):
+        for i in range(0 + j, length, step):
             c += 1
             try:  # in case the frequencies dictionary does not already have this key
                 frequencies[ciphertext[i]] += 1
@@ -48,6 +48,40 @@ def calculate_indexes_of_coincidence(ciphertext: str, step: int) -> list:
     return indexes_of_coincidence
 
 
+def friedman_method(ciphertext: str, max_keylength: int=None) -> int:
+    """
+    Implements Friedman's method for finding the length of the key of a Vigenere cipher. It finds the length with an
+    index of confidence closer to that of an average text in the english language.
+    :param ciphertext: a string (text)
+    :param max_keylength: the maximum length of key that Friedman's method should check, if None then it defaults to the
+    length of the cipher
+    :return: the length of the key
+    """
+    # sets the default value of MAX_KEYLEBGTH
+    if max_keylength is None:
+        max_keylength = len(ciphertext)
+
+    frequencies = [1.5]  # the zeroth position should not be used: length of key is greater than zero
+
+    # for every length of key
+    for i in range(1, max_keylength + 1):
+
+        # for a specific length it finds the minimum index of coincidence
+        min1 = 15.0
+        for val in calculate_indexes_of_coincidence(ciphertext, i):
+            if abs(val - PARAMETER) < abs(min1 - PARAMETER):
+                min1 = val
+        frequencies.append(min1)
+
+    # finds which length of key has the minimum difference with the language PARAMETER
+    li = (15.0, -1)  # initialization
+    for i in range(len(frequencies)):
+        if abs(frequencies[i] - PARAMETER) < abs(li[0] - PARAMETER):
+            li = (frequencies[i], i)
+
+    return li[1]
+
+
 def find_key_from_vigenere_cipher(ciphertext: str) -> str:
     clean_ciphertext = list()
     for symbol in ciphertext:
@@ -61,4 +95,4 @@ def find_key_from_vigenere_cipher(ciphertext: str) -> str:
 
 
 if __name__ == '__main__':
-    print(index_of_coincidence(LETTER_FREQUENCIES_DICT, 1000))
+    print(index_of_coincidence(LETTER_FREQUENCIES_DICT, 1000))