Done keyword ciphers and breaking

[cipher-tools.git] / cipher.py

diff --git a/cipher.py b/cipher.py
index 3f9e29087408202e91b0294f231bea9edfc0054a..5a8cfa22bd788981557c5fe733467e5ec2dc116e 100644 (file)
--- a/cipher.py
+++ b/cipher.py
@@ -6,7 +6,7 @@ import logging
 logger = logging.getLogger(__name__)
 logger.addHandler(logging.FileHandler('cipher.log'))
 logger.setLevel(logging.WARNING)
-logger.setLevel(logging.INFO)
+#logger.setLevel(logging.INFO)
 
 english_counts = collections.defaultdict(int)
 with open('count_1l.txt', 'r') as f:
@@ -15,6 +15,10 @@ with open('count_1l.txt', 'r') as f:
         english_counts[letter] = int(count)
 normalised_english_counts = norms.normalise(english_counts)        
 
+keywords = []
+with open('words.txt', 'r') as f:
+    keywords = [line.rstrip() for line in f]
+
 
 modular_division_table = [[0]*26 for x in range(26)]
 for a in range(26):
@@ -42,6 +46,13 @@ def sanitise(text):
     return ''.join(sanitised)
 
 def ngrams(text, n):
+    """Returns all n-grams of a text
+    
+    >>> ngrams(sanitise('the quick brown fox'), 2)
+    [('t', 'h'), ('h', 'e'), ('e', 'q'), ('q', 'u'), ('u', 'i'), ('i', 'c'), ('c', 'k'), ('k', 'b'), ('b', 'r'), ('r', 'o'), ('o', 'w'), ('w', 'n'), ('n', 'f'), ('f', 'o'), ('o', 'x')]
+    >>> ngrams(sanitise('the quick brown fox'), 4)
+    [('t', 'h', 'e', 'q'), ('h', 'e', 'q', 'u'), ('e', 'q', 'u', 'i'), ('q', 'u', 'i', 'c'), ('u', 'i', 'c', 'k'), ('i', 'c', 'k', 'b'), ('c', 'k', 'b', 'r'), ('k', 'b', 'r', 'o'), ('b', 'r', 'o', 'w'), ('r', 'o', 'w', 'n'), ('o', 'w', 'n', 'f'), ('w', 'n', 'f', 'o'), ('n', 'f', 'o', 'x')]
+    """
     return [tuple(text[i:i+n]) for i in range(len(text)-n+1)]
 
 def letter_frequencies(text):
@@ -61,6 +72,11 @@ def letter_frequencies(text):
         counts[c] += 1
     return counts
 
+def deduplicate(text):
+    return list(collections.OrderedDict.fromkeys(text))
+
+
+
 def caesar_encipher_letter(letter, shift):
     """Encipher a letter, given a shift amount
 
@@ -197,6 +213,17 @@ def affine_decipher(message, multiplier=1, adder=0, one_based=True):
     return ''.join(enciphered)
 
 
+def keyword_encipher(message, keyword):
+    cipher_alphabet = ''.join(deduplicate(sanitise(keyword) + string.ascii_lowercase))
+    cipher_translation = ''.maketrans(string.ascii_lowercase, cipher_alphabet)
+    return message.lower().translate(cipher_translation)
+
+def keyword_decipher(message, keyword):
+    cipher_alphabet = ''.join(deduplicate(sanitise(keyword) + string.ascii_lowercase))
+    cipher_translation = ''.maketrans(cipher_alphabet, string.ascii_lowercase)
+    return message.lower().translate(cipher_translation)
+
+
 def caesar_break(message, metric=norms.euclidean_distance, target_frequencies=normalised_english_counts, message_frequency_scaling=norms.normalise):
     """Breaks a Caesar cipher using frequency analysis
     
@@ -248,6 +275,21 @@ def affine_break(message, metric=norms.euclidean_distance, target_frequencies=no
     return (best_multiplier, best_adder, best_one_based), best_fit
 
 
+def keyword_break(message, metric=norms.euclidean_distance, target_frequencies=normalised_english_counts, message_frequency_scaling=norms.normalise):
+    best_keyword = ''
+    best_fit = float("inf")
+    for keyword in keywords:
+        plaintext = keyword_decipher(message, keyword)
+        frequencies = message_frequency_scaling(letter_frequencies(plaintext))
+        fit = metric(target_frequencies, frequencies)
+        logger.info('Keyword break attempt using key {0} gives fit of {1} and decrypt starting: {2}'.format(keyword, fit, plaintext[:50]))
+        if fit < best_fit:
+            best_fit = fit
+            best_keyword = keyword
+    logger.info('Keyword break best fit with key {0} gives fit of {1} and decrypt starting: {2}'.format(best_keyword, best_fit, keyword_decipher(message, best_keyword)[:50]))
+    return best_keyword, best_fit
+
+
 if __name__ == "__main__":
     import doctest
     doctest.testmod()