X-Git-Url: https://git.njae.me.uk/?a=blobdiff_plain;f=caesar.py;fp=caesar.py;h=4d9a7220320067d372e53275011c4d1a8283ec44;hb=21c390a77d42729afa23844ef2f1295106bed3de;hp=0000000000000000000000000000000000000000;hpb=d0a53e974970bc915d94280b5158b50f93054dc3;p=cipher-tools.git

diff --git a/caesar.py b/caesar.py
new file mode 100644
index 0000000..4d9a722
--- /dev/null
+++ b/caesar.py
@@ -0,0 +1,120 @@
+from utilities import *
+from language_models import *
+from logger import logger
+
+def caesar_encipher_letter(accented_letter, shift):
+    """Encipher a letter, given a shift amount
+
+    >>> caesar_encipher_letter('a', 1)
+    'b'
+    >>> caesar_encipher_letter('a', 2)
+    'c'
+    >>> caesar_encipher_letter('b', 2)
+    'd'
+    >>> caesar_encipher_letter('x', 2)
+    'z'
+    >>> caesar_encipher_letter('y', 2)
+    'a'
+    >>> caesar_encipher_letter('z', 2)
+    'b'
+    >>> caesar_encipher_letter('z', -1)
+    'y'
+    >>> caesar_encipher_letter('a', -1)
+    'z'
+    >>> caesar_encipher_letter('A', 1)
+    'B'
+    >>> caesar_encipher_letter('é', 1)
+    'f'
+    """
+    # letter = unaccent(accented_letter)
+    # if letter in string.ascii_letters:
+    #     if letter in string.ascii_uppercase:
+    #         alphabet_start = ord('A')
+    #     else:
+    #         alphabet_start = ord('a')
+    #     return chr(((ord(letter) - alphabet_start + shift) % 26) + 
+    #                alphabet_start)
+    # else:
+    #     return letter
+
+    letter = unaccent(accented_letter)
+    if letter in string.ascii_letters:
+        cipherletter = unpos(pos(letter) + shift)
+        if letter in string.ascii_uppercase:
+            return cipherletter.upper()
+        else:
+            return cipherletter
+    else:
+        return letter
+
+def caesar_decipher_letter(letter, shift):
+    """Decipher a letter, given a shift amount
+    
+    >>> caesar_decipher_letter('b', 1)
+    'a'
+    >>> caesar_decipher_letter('b', 2)
+    'z'
+    """
+    return caesar_encipher_letter(letter, -shift)
+
+def caesar_encipher(message, shift):
+    """Encipher a message with the Caesar cipher of given shift
+    
+    >>> caesar_encipher('abc', 1)
+    'bcd'
+    >>> caesar_encipher('abc', 2)
+    'cde'
+    >>> caesar_encipher('abcxyz', 2)
+    'cdezab'
+    >>> caesar_encipher('ab cx yz', 2)
+    'cd ez ab'
+    >>> caesar_encipher('Héllo World!', 2)
+    'Jgnnq Yqtnf!'
+    """
+    enciphered = [caesar_encipher_letter(l, shift) for l in message]
+    return cat(enciphered)
+
+def caesar_decipher(message, shift):
+    """Decipher a message with the Caesar cipher of given shift
+    
+    >>> caesar_decipher('bcd', 1)
+    'abc'
+    >>> caesar_decipher('cde', 2)
+    'abc'
+    >>> caesar_decipher('cd ez ab', 2)
+    'ab cx yz'
+    >>> caesar_decipher('Jgnnq Yqtnf!', 2)
+    'Hello World!'
+    """
+    return caesar_encipher(message, -shift)
+
+
+def caesar_break(message, fitness=Pletters):
+    """Breaks a Caesar cipher using frequency analysis
+
+    >>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrh' \
+          'ecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm') # doctest: +ELLIPSIS
+    (4, -130.849989015...)
+    >>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgtee' \
+          'raxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert') # doctest: +ELLIPSIS
+    (19, -128.82410410...)
+    >>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurer' \
+          'svaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur') # doctest: +ELLIPSIS
+    (13, -126.25403935...)
+    """
+    sanitised_message = sanitise(message)
+    best_shift = 0
+    best_fit = float('-inf')
+    for shift in range(26):
+        plaintext = caesar_decipher(sanitised_message, shift)
+        fit = fitness(plaintext)
+        logger.debug('Caesar break attempt using key {0} gives fit of {1} '
+                     'and decrypt starting: {2}'.format(shift, fit,
+                                                        plaintext[:50]))
+        if fit > best_fit:
+            best_fit = fit
+            best_shift = shift
+    logger.info('Caesar break best fit: key {0} gives fit of {1} and '
+                'decrypt starting: {2}'.format(best_shift, best_fit, 
+                    caesar_decipher(sanitised_message, best_shift)[:50]))
+    return best_shift, best_fit