added exam exercise 19

exam/ex19/caesar.py

@@ -0,0 +1,32 @@
+
+
+def internal_ord(c):
+	c = c.lower()
+	internal_ord = ord(c) - ord("a")
+	if(internal_ord < 0 or internal_ord > 25):
+		raise ValueError("'{}' is an unsupported character".format(c))
+	return internal_ord
+
+def internal_chr(i):
+	return chr(ord("a") + i)
+
+
+def encode_or_keeps_space(c):
+	if(c == " "):
+		return (False, c)
+	return (True, internal_ord(c))
+
+def prepare_string(s):
+	return (encode_or_keeps_space(c) for c in s)
+
+
+def _caesar(s, K):
+	for encode, i in prepare_string(s):
+		if(encode):
+			yield internal_chr((i + K) % 26)
+		else:
+			yield i
+
+def caesar(s, K):
+	return "".join(_caesar(s, K))
+	

exam/ex19/main.py

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+from caesar import caesar
+from statistical_attack import get_statistical_key
+
+text1 = "hello world this is a test"
+text2 = "this is a message that is obfuscated"
+
+K2 = 4
+K1 = 13
+
+print(caesar(text1, K1))
+print(caesar(caesar(text1, K1), K1))
+
+print(caesar(text2, K2))
+print(caesar(caesar(text2, K2), abs(K2 - 26)))
+
+text4 = "In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence"
+
+text4 = "".join((s for s in text4 if s in " abcdefghijklmnopqrstuvwxyz"))
+
+print(get_statistical_key(caesar(text4, K2)))
+print(get_statistical_key(caesar(text2, K2)))

exam/ex19/statistical_attack.py

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+from collections import Counter
+
+alphabet = "abcdefghijklmnopqrstuvwxyz"
+
+reference = Counter((c for c in open("sample.tx").read().lower() if c in alphabet))
+
+def get_statistics(text):
+	return Counter([c for c in text.lower() if c in alphabet])
+
+def relative_most_common(statistics):
+	c, abs_probability = statistics.most_common(1)[0]
+
+	total_chrs = sum([v for k,v in statistics.most_common()])
+	return abs_probability / total_chrs
+	
+
+def get_statistical_key(text):
+	statistics = get_statistics(text)
+
+	quality = relative_most_common(statistics) / relative_most_common(reference)
+
+	c, abs_probability = statistics.most_common(1)[0]
+
+	K = abs(ord("e") - ord(c))
+
+	if(quality > 1):
+		quality = relative_most_common(reference) / relative_most_common(statistics)
+	
+	return K, quality