Coursera Cryptography 1
Week 1 homework
1。
Many Time Pad
Let us see what goes wrong when a stream cipher key is used more than once. Below
are eleven hex-encoded ciphertexts that are the result of encrypting eleven plaintexts
with a stream cipher, all with the same stream cipher key. Your goal is to decrypt the last
ciphertext, and submit the secret message within it as solution.
Hint: XOR the ciphertexts together, and consider what happens when a space is XORed
with a character in [a-zA-Z].
text #1
we can factor the number 15 with quantum computers. we can also factor the number 15 with a do
text #2
euler would probably enjoy that now his theorem becomes a corner stone of crypto – Annonymous
text #3
the nice thing about Keeyloq is now we cryptographerS can drive a lot of fancy cars – Dan Bone
text #4
the ciphertext produced by a weak encryption algorithm looks as good as ciphertext produced by
text #5
you don’t want to buy a set of car keys from a guy who specializes in stealing cars – Marc Rot
text #6
there are two types of cryptograph so that which will keep secrets safe from your little sister
text #7
there are two types of cyptography: one that allows the government to use brute force to break
text #8
We can see the point where the chip is unhappy if a wrong bit is sent and consumes more power
text #9
a (private-key) encryption scheme states 3 algorithms, namely a procedure for generating keys
text #10
the concise oxforddictionary (2006) defi nes crypto as The art of writing o r solving codes.
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# coding=utf-8 import codecs def main(): ciphertext=[0]*11 ciphertext[0]="32510ba9babebbbefd001547a810e67149caee11d945cd7fc81a05e9f85aac650e9052ba6a8cd8257bf14d13e6f0a803b54fde9e77472dbff89d71b57bddef121336cb85ccb8f3315f4b52e301d16e9f52f904" ciphertext[1]="315c4eeaa8b5f8aaf9174145bf43e1784b8fa00dc71d885a804e5ee9fa40b16349c146fb778cdf2d3aff021dfff5b403b510d0d0455468aeb98622b137dae857553ccd8883a7bc37520e06e515d22c954eba5025b8cc57ee59418ce7dc6bc41556bdb36bbca3e8774301fbcaa3b83b220809560987815f65286764703de0f3d524400a19b159610b11ef3e" ciphertext[2]="234c02ecbbfbafa3ed18510abd11fa724fcda2018a1a8342cf064bbde548b12b07df44ba7191d9606ef4081ffde5ad46a5069d9f7f543bedb9c861bf29c7e205132eda9382b0bc2c5c4b45f919cf3a9f1cb74151f6d551f4480c82b2cb24cc5b028aa76eb7b4ab24171ab3cdadb8356f" ciphertext[3]="32510ba9a7b2bba9b8005d43a304b5714cc0bb0c8a34884dd91304b8ad40b62b07df44ba6e9d8a2368e51d04e0e7b207b70b9b8261112bacb6c866a232dfe257527dc29398f5f3251a0d47e503c66e935de81230b59b7afb5f41afa8d661cb" ciphertext[4]="32510ba9aab2a8a4fd06414fb517b5605cc0aa0dc91a8908c2064ba8ad5ea06a029056f47a8ad3306ef5021eafe1ac01a81197847a5c68a1b78769a37bc8f4575432c198ccb4ef63590256e305cd3a9544ee4160ead45aef520489e7da7d835402bca670bda8eb775200b8dabbba246b130f040d8ec6447e2c767f3d30ed81ea2e4c1404e1315a1010e7229be6636aaa" ciphertext[5]="3f561ba9adb4b6ebec54424ba317b564418fac0dd35f8c08d31a1fe9e24fe56808c213f17c81d9607cee021dafe1e001b21ade877a5e68bea88d61b93ac5ee0d562e8e9582f5ef375f0a4ae20ed86e935de81230b59b73fb4302cd95d770c65b40aaa065f2a5e33a5a0bb5dcaba43722130f042f8ec85b7c2070" ciphertext[6]="32510bfbacfbb9befd54415da243e1695ecabd58c519cd4bd2061bbde24eb76a19d84aba34d8de287be84d07e7e9a30ee714979c7e1123a8bd9822a33ecaf512472e8e8f8db3f9635c1949e640c621854eba0d79eccf52ff111284b4cc61d11902aebc66f2b2e436434eacc0aba938220b084800c2ca4e693522643573b2c4ce35050b0cf774201f0fe52ac9f26d71b6cf61a711cc229f77ace7aa88a2f19983122b11be87a59c355d25f8e4" ciphertext[7]="32510bfbacfbb9befd54415da243e1695ecabd58c519cd4bd90f1fa6ea5ba47b01c909ba7696cf606ef40c04afe1ac0aa8148dd066592ded9f8774b529c7ea125d298e8883f5e9305f4b44f915cb2bd05af51373fd9b4af511039fa2d96f83414aaaf261bda2e97b170fb5cce2a53e675c154c0d9681596934777e2275b381ce2e40582afe67650b13e72287ff2270abcf73bb028932836fbdecfecee0a3b894473c1bbeb6b4913a536ce4f9b13f1efff71ea313c8661dd9a4ce" ciphertext[8]="315c4eeaa8b5f8bffd11155ea506b56041c6a00c8a08854dd21a4bbde54ce56801d943ba708b8a3574f40c00fff9e00fa1439fd0654327a3bfc860b92f89ee04132ecb9298f5fd2d5e4b45e40ecc3b9d59e9417df7c95bba410e9aa2ca24c5474da2f276baa3ac325918b2daada43d6712150441c2e04f6565517f317da9d3" ciphertext[9]="271946f9bbb2aeadec111841a81abc300ecaa01bd8069d5cc91005e9fe4aad6e04d513e96d99de2569bc5e50eeeca709b50a8a987f4264edb6896fb537d0a716132ddc938fb0f836480e06ed0fcd6e9759f40462f9cf57f4564186a2c1778f1543efa270bda5e933421cbe88a4a52222190f471e9bd15f652b653b7071aec59a2705081ffe72651d08f822c9ed6d76e48b63ab15d0208573a7eef027" ciphertext[10]="466d06ece998b7a2fb1d464fed2ced7641ddaa3cc31c9941cf110abbf409ed39598005b3399ccfafb61d0315fca0a314be138a9f32503bedac8067f03adbf3575c3b8edc9ba7f537530541ab0f9f3cd04ff50d66f1d559ba520e89a2cb2a83" cipher=[] for i in range(11): cipher.append(''.join([chr(x) for x in split_hex(ciphertext[i])])) for j in range(1,11): result=[] for i in range(1,11): if i==j: continue raw=strxor(cipher[j], cipher[i]) raw=list(raw) for i,letter in enumerate(raw): letter=ord(letter) if (letter>=0x41 and letter<=0x5a) or (letter>=0x61 and letter<=0x7a): pass else: raw[i]='_' result.append(''.join(raw)) print(''.join(raw)) length=len(cipher[j])//2 plain=['=']*length for i in range(length): count_=0 countL=0 mark='' only=True for letter in [string[i] for string in result]: if letter=='_': count_+=1 else: countL += 1 if mark!='' and mark == letter.lower(): pass elif mark!='' and mark != letter.lower(): only=False mark=letter.lower() if countL==1: plain[i]=mark elif countL>1: if only: plain[i] = mark else: plain[i]='#' print() print(''.join(plain)) plaintext="The nice thing about Keeyloq is now we cryptographers can drive a lot of fancy cars - Dan Bone" print(strxor(strxor(plaintext,cipher[3]),cipher[6])) def list2hex(lo): h=["{:02x}".format(i) for i in lo] h="".join(h) return h def split_hex(h, dtype='int'): ''' :param h: A hex string. Example: "315c4eea". Its length should be an even number. :return: A list of integer corresponding to every two letters in h ''' if dtype == 'int': split = [int(h[2 * i:2 * i + 2], 16) for i in range(len(h) // 2)] elif dtype == 'str' or 'string': split = [h[2 * i:2 * i + 2] for i in range(len(h) // 2)] else: split = [int(h[2 * i:2 * i + 2], 16) for i in range(len(h) // 2)] return split def strxor(a, b): # xor two strings of different lengths if len(a) > len(b): return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a[:len(b)], b)]) else: return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b[:len(a)])]) def encrypt(key, msg): c = strxor(key, msg) c=codecs.encode(bytes(c,encoding='utf-8'), 'hex') return c def decrypt(key, cipher): cipher=''.join([chr(x) for x in split_hex(cipher)]) msg=strxor(key, cipher) return msg if __name__=="__main__": main() |