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Correl Roush
359c4b18a0
Might try fixing this up more later, the test case still failed. Probably because it's so simple.
99 lines
3.5 KiB
Python
99 lines
3.5 KiB
Python
"""
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Each character on a computer is assigned a unique code and the preferred
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standard is ASCII (American Standard Code for Information Interchange). For
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example, uppercase A = 65, asterisk (*) = 42, and lowercase k = 107.
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A modern encryption method is to take a text file, convert the bytes to ASCII,
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then XOR each byte with a given value, taken from a secret key. The advantage
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with the XOR function is that using the same encryption key on the cipher text,
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restores the plain text; for example, 65 XOR 42 = 107, then 107 XOR 42 = 65.
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For unbreakable encryption, the key is the same length as the plain text
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message, and the key is made up of random bytes. The user would keep the
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encrypted message and the encryption key in different locations, and without
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both "halves", it is impossible to decrypt the message.
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Unfortunately, this method is impractical for most users, so the modified method
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is to use a password as a key. If the password is shorter than the message,
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which is likely, the key is repeated cyclically throughout the message. The
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balance for this method is using a sufficiently long password key for security,
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but short enough to be memorable.
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Your task has been made easy, as the encryption key consists of three lower case
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characters. Using cipher1.txt (right click and 'Save Link/Target As...'), a file
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containing the encrypted ASCII codes, and the knowledge that the plain text must
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contain common English words, decrypt the message and find the sum of the ASCII
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values in the original text.
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"""
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def encrypt(text, key):
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encrypted = []
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key_n = 0
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for char in text:
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key_c = ord(key[key_n])
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encrypted.append(ord(char) ^ key_c)
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key_n += 1
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if key_n >= len(key):
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key_n = 0
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return encrypted
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def decrypt(encrypted, key):
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decrypted = []
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key_n = 0
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for char in encrypted:
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key_c = ord(key[key_n])
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decrypted.append(chr(char ^ key_c))
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key_n += 1
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if key_n >= len(key):
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key_n = 0
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return ''.join(decrypted)
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def get_key(encrypted, key_len=3):
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keys = []
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chars = []
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valid_chars = range(ord('a'), ord('z') + 1)
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for i in range(key_len):
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chars.append(list(valid_chars))
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# Loop through the encrypted text, clearing out invalid key chars
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key_n = 0
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for value in encrypted:
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for c in chars[key_n]:
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result = value ^ c
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#if result not in valid_results:
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if result < 0x20 or result > 0x7e:
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chars[key_n].remove(c)
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key_n += 1
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if key_n >= key_len:
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key_n = 0
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#print chars
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# gather keys
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keys = [chr(c) for c in chars[0]]
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for n in range(1, key_len):
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_keys = []
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for last in keys:
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for c in chars[n]:
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_keys.append(last + chr(c))
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keys = _keys
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words = ['there', 'who', 'the', 'is', 'in', 'it']
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scored = []
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for k in keys:
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d = decrypt(encrypted, k)
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score = 1
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for word in words:
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if word in d:
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score *= len(word)
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scored.append((score, k))
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return sorted([s for s in scored if s[0] > 1])
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if __name__ == '__main__':
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#e = encrypt('Myles is a ridiculous dog who loves his bone', 'amz')
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#print get_key(e)
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with open('cipher1.txt', 'r') as codefile:
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codes = [int(c) for c in codefile.readline().split(',')]
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best = get_key(codes)[-1][1]
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print 'Using', best
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d = decrypt(codes, best)
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print 'Decrypted:'
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print d
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print 'Sum:', sum([ord(c) for c in d])
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