one-time pad(一次性密码本)

详见wiki one time pad
大概意思是说
只要满足一下4条即可无解加密:

1. The key must be truly random.
2. The key must be at least as long as the plaintext.
3. The key must never be reused in whole or in part.
4. The key must be kept completely secret.

一般用来加密较短的信息, 否则秘钥长度太长, 不适用于实际情况

0529242a631234122d2b36697f13272c207f2021283a6b0c7908
2f28202a302029142c653f3c7f2a2636273e3f2d653e25217908
322921780c3a235b3c2c3f207f372e21733a3a2b37263b313012
2f6c363b2b312b1e64651b6537222e37377f2020242b6b2c2d5d
283f652c2b31661426292b653a292c372a2f20212a316b283c09
29232178373c270f682c216532263b2d3632353c2c3c2a293504
613c37373531285b3c2a72273a67212a277f373a243c20203d5d
243a202a633d205b3c2d3765342236653a2c7423202f3f652a18
2239373d6f740a1e3c651f207f2c212a247f3d2e65262430791c
263e203d63232f0f20653f207f332065262c3168313722367918
2f2f372133202f142665212637222220733e383f2426386b

这题中给了11串密文, 用了重复秘钥加密不同文本, 可以用Many Time Pad Attack破解

结论是有10串使用了相同秘钥加密的密文, 可以破解出秘钥, 然后解决所有密文

用现成脚本

稍微修改一下

import string
import collections
import sets

# XORs two string
def strxor(a, b):     # xor two strings (trims the longer input)
    return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b)])

# 10 unknown ciphertexts (in hex format), all encrpyted with the same key
c1 = "0529242a631234122d2b36697f13272c207f2021283a6b0c7908"
c2 = "2f28202a302029142c653f3c7f2a2636273e3f2d653e25217908"
c3 = "322921780c3a235b3c2c3f207f372e21733a3a2b37263b313012"
c4 = "2f6c363b2b312b1e64651b6537222e37377f2020242b6b2c2d5d"
c5 = "283f652c2b31661426292b653a292c372a2f20212a316b283c09"
c6 = "29232178373c270f682c216532263b2d3632353c2c3c2a293504"
c7 = "613c37373531285b3c2a72273a67212a277f373a243c20203d5d"
c8 = "243a202a633d205b3c2d3765342236653a2c7423202f3f652a18"
c9 = "2239373d6f740a1e3c651f207f2c212a247f3d2e65262430791c"
c10 = "263e203d63232f0f20653f207f332065262c3168313722367918"
ciphers = [c1, c2, c3, c4, c5, c6, c7, c8, c9, c10]
# The target ciphertext we want to crack
target_cipher = "2f2f372133202f142665212637222220733e383f2426386b"

# To store the final key
final_key = [None]*150
# To store the positions we know are broken
known_key_positions = set()

# For each ciphertext
for current_index, ciphertext in enumerate(ciphers):

	counter = collections.Counter()
	# for each other ciphertext
	for index, ciphertext2 in enumerate(ciphers):
		if current_index != index: # don't xor a ciphertext with itself
			for indexOfChar, char in enumerate(strxor(ciphertext.decode('hex'), ciphertext2.decode('hex'))): # Xor the two ciphertexts
				# If a character in the xored result is a alphanumeric character, it means there was probably a space character in one of the plaintexts (we don't know which one)
				if char in string.printable and char.isalpha(): counter[indexOfChar] += 1 # Increment the counter at this index
	knownSpaceIndexes = []

	# Loop through all positions where a space character was possible in the current_index cipher
	for ind, val in counter.items():
		# If a space was found at least 7 times at this index out of the 9 possible XORS, then the space character was likely from the current_index cipher!
		if val >= 7: knownSpaceIndexes.append(ind)
	#print knownSpaceIndexes # Shows all the positions where we now know the key!

	# Now Xor the current_index with spaces, and at the knownSpaceIndexes positions we get the key back!
	xor_with_spaces = strxor(ciphertext.decode('hex'),' '*150)
	for index in knownSpaceIndexes:
		# Store the key's value at the correct position
		final_key[index] = xor_with_spaces[index].encode('hex')
		# Record that we known the key at this position
		known_key_positions.add(index)

# Construct a hex key from the currently known key, adding in '00' hex chars where we do not know (to make a complete hex string)
final_key_hex = ''.join([val if val is not None else '00' for val in final_key])
# Xor the currently known key with the target cipher
output = strxor(target_cipher.decode('hex'),final_key_hex.decode('hex'))
# Print the output, printing a * if that character is not known yet
print ''.join([char if index in known_key_positions else '*' for index, char in enumerate(output)])

'''
Manual step
'''
# From the output this prints, we can manually complete the target plaintext from:
# The secuet-mes*age*is: Wh** usi|g **str*am cipher, nev***use th* k*y *ore than onc*
# to:
# The secret message is: When using a stream cipher, never use the key more than once

# We then confirm this is correct by producing the key from this, and decrpyting all the other messages to ensure they make grammatical sense
target_plaintext = "The secret message is: When using a stream cipher, never use the key more than once"

print ' '
print final_key_hex

print target_plaintext
key = strxor(target_cipher.decode('hex'),target_plaintext)
for cipher in ciphers:
	print strxor(cipher.decode('hex'),key)

py2运行, 得到结果
拿到秘钥的16进制

414c45580054467b48005200004700455300544845004b45007d

转字符串

key = "414c45580054467b48005200004700455300544845004b45007d"
print(key.decode('hex'))

ALEX TF{H R G ES THE KE }

不完整的秘钥, 但是已经可读了, 猜测也是解密得一部分, 所以接下来继续猜测完整秘钥即可
根据题目

This time Fady learned from his old mistake and decided to use onetime pad as his encryption technique, but he never knew why people call it one time pad! Flag will start with ALEXCTF{.

以及空格处, 补全单词得到

ALEXCTF{HERE_GOES_THE_KEY}