X-Git-Url: https://git.njae.me.uk/?a=blobdiff_plain;f=cipher.py;h=55d99b6773bef3aac5b752ee8e0395856e3c75a8;hb=5141b041bd618e0339ce04f6abef56665d829b95;hp=053635008df52ccdaba6b354dd5c4ddd11462ba5;hpb=26f5de0a23dd94ded412f6f507910ac5e26ea2b6;p=cipher-tools.git

diff --git a/cipher.py b/cipher.py
index 0536350..55d99b6 100644
--- a/cipher.py
+++ b/cipher.py
@@ -21,6 +21,9 @@ def sanitise(text):
     sanitised = [c.lower() for c in text if c in string.ascii_letters]
     return ''.join(sanitised)
 
+def ngrams(text, n):
+    return [tuple(text[i:i+n]) for i in range(len(text)-n+1)]
+
 def letter_frequencies(text):
     """Count the number of occurrences of each character in text
     
@@ -105,10 +108,21 @@ def caesar_decipher(message, shift):
     return caesar_encipher(message, -shift)
 
 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
+    
+    >>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrhecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm')
+    (4, 0.3186395289018361)
+    >>> caesar_break('jhzhuhfrqilqhgwrdevwudfwuhdvrqlqjwkhqkdylqjvxemhfwhgwrfulwlflvpwkhhasodqdwlrqrisrzhuwkdwmxulglfdovfl')
+    (3, 0.3290204286173084)
+    >>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgteeraxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert')
+    (19, 0.4215290123583277)
+    >>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurersvaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur')
+    (13, 0.31602920807545154)
+    """
     sanitised_message = sanitise(message)
     best_shift = 0
     best_fit = float("inf")
-    for shift in range(1, 25):
+    for shift in range(26):
         plaintext = caesar_decipher(sanitised_message, shift)
         frequencies = message_frequency_scaling(letter_frequencies(plaintext))
         fit = metric(target_frequencies, frequencies)