Added 2014/03/07 - Vigenere Cipher

http://www.problemotd.com/problem/vigenere-cipher/

20140306.py

@@ -0,0 +1,63 @@
+# Vigenere Cipher brute forcer
+#
+# Solution to http://www.problemotd.com/problem/vigenere-cipher/
+# By Andrew Williams (github.com/nikdoof)
+ 
+import itertools, string
+ 
+alpha = string.ascii_lowercase
+ 
+def vigenere_encode(text, key):
+    k = key * (int(len(text) / len(key)) + 1)
+    k = k[:len(text)]
+    return ''.join([alpha[(alpha.index(x) + alpha.index(y)) % 26] for x, y in zip(text, k)])
+ 
+ 
+def vigenere_decode(cypher, key):
+    k = key * (int(len(cypher) / len(key)) + 1)
+    k = k[:len(cypher)]
+    return ''.join([alpha[(alpha.index(x) - alpha.index(y)) % 26] for x, y in zip(cypher, k)])
+ 
+ 
+freq = {
+    'E': 12.70, 'T': 9.06, 'A': 8.17, 'O': 7.51, 
+    'I': 6.97, 'N': 6.75, 'S': 6.33, 'H': 6.09, 
+    'R': 2.78, 'U': 2.76, 'M': 2.41, 'W': 2.36, 
+    'F': 1.93, 'B': 1.29, 'V': 0.98, 'K': 0.77, 
+    'J': 0.07
+}
+ 
+def count_letters(s):
+    count = dict([(c, 0) for c in alpha])
+    for l in s:
+        count[l] += 1
+    return count
+ 
+def score_letters(s):
+    score = 0
+    c = count_letters(s)
+    for x in c:
+        if x.upper() in freq:
+            score += (c[x] * freq[x.upper()])
+    return score
+ 
+if __name__ == '__main__':
+    # Self test
+    print 'Encode Test Passed? {}'.format(vigenere_encode('todayismybirthday', 'reddit') == 'KSGDGBJQBEQKKLGDG'.lower())
+    print 'Decode Test Passed? {}'.format(vigenere_decode('KSGDGBJQBEQKKLGDG'.lower(), 'reddit') == 'todayismybirthday')
+ 
+    # Source
+    ct = 'ZEJFOKHTMSRMELCPODWHCGAW'.lower()
+ 
+    # Process the keys and store the results
+    res = []
+    for i in range(1,5):
+        for p in itertools.product(string.ascii_lowercase, repeat=i):
+            c = 0
+            p = ''.join(p)
+            dec = vigenere_decode(ct, p)
+            res.append((p, dec, score_letters(dec)))
+ 
+    # Show the top 200 scoring results
+    for x, y, z in sorted(res, key=lambda tup: tup[2])[-200:]:
+        print '{} - {} ({})'.format(x, y, z)