H4ckIT CTF 2016 : ninja-scheme-195

ethereel

题目内容：
General Tompson welcomes you...again! We have some crypto-problem here...again. Our scouts have intercepted this enemy cryptogram: dd67ca82d358f0c8479e118addcec2f8ce086c0f6f239f9b66d7226a38c68198dbd777f366fb9fd83b60d11109be174759c75ea56a4866c2 Some time later our IT-ninjas have broken into the enemy computer system and grabbed something pretty much similar to undefined encryption algorithm scheme. Look at this grabbed scheme and help us to understand how it works. Yours, Gen. Tompson




writeup:

1. #!/usr/bin/env python
   
2. # coding=utf-8
   
3. def slice(s,size):
   
4. return [s[i:i+size] for i in range(0,len(s),size)]
   
5. #print slice("abcdefghijklmnop",4)
   
6. #['abcd'.'efgh','ijkl','mnop']
   
7. def xor(a,b):
   
8. return "".join([chr(ord(a[i]) ^ ord(b[i%len(b)])) for i in xrange(len(a))])
   
9. #print xor('AAAABBBB','11112222')
   
10. #pppppppp
    
11. def f(L,n):
    
12. ans = ""
    
13. for i in range(len(L)):
    
14. ans += chr((ord(L[i]) + n) % 256)
    
15. return ans
    
16. def decrypt(cipher,rounds):
    
17. assert len(cipher) == 8
    
18. r = cipher[4:]
    
19. l = cipher[:4]
    
20. tmp = l
    
21. l = r
    
22. r = tmp
    
23. for n in reversed(range(1,rounds + 1)):
    
24. tmp = l
    
25. l = r
    
26. r = xor(tmp,f(r,n))
    
27. return l + r
    
28. cipher = "dd67ca82d358f0c8479e118addcec2f8ce086c0f6f239f9b66d7226a38c68198dbd777f366fb9fd83b60d11109be174759c75ea56a4866c2"
    
29. cipher = slice(cipher.decode("hex"),8)
    
30. for i in range(1000):
    
31. plain = decrypt(cipher[0],i)
    
32. if plain.find("h4ck1t") >= 0:
    
33. print "rounds = ",i
    
34. rounds = i
    
35. flag = ""
    
36. for c in cipher:
    
37. flag += decrypt(c,rounds)
    
38. print flag

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