Feistel

Feistel 结构算法解密

题目重述

拿到题目，题目先是给了一点密文，打开之后，发现是一段乱码：

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先来看看题目代码：

题目代码

import random


K = ？？？
KSIZE = ？？？
SIZE = 96
Divisor = [2,3,4,6,8,12,16,24,32,48]
PSIZE = Divisor[random.randint(0,9)]



# get test msg

def clear(ttmsg):
    tmsg = ''
    for i in range(len(ttmsg)):
        if (ttmsg[i].isalpha()) :
            tmsg += ttmsg[i]
    return tmsg.lower()



fp = open('tmsg.txt','r')
tmsg = ''
text = fp.readlines()
for i in text:
    tmsg += i  



# devide str into part (each part's size = size)
def str_size(string,size):
    s = []
    for i in range(0,len(string),size):
        c = string[i:i+size]
        for j in range(len(c),size):
            c += ' '
        s.append(c)
    return s


# Encode function
def fence(message,n):
    part = ['' for i in range(n)]
    C = ''
    for i in range(len(message)):
        part[i%n] += message[i]
    for i in range(n):
        C += part[i]
    return C

def enCaeser (message,key):
    j = 1
    C = ''
    for i in range(len(message)):
        C += chr( (ord(message[i]) - ord('a') + j ) %26 + ord('a') )
        j += key
    return C

def make_key(message):
    last = ''
    key = ''
    for i in range(len(message)-1,len(message)-KSIZE-1,-1):
        last += message[i]
    key = STUPID(last,K)
    return key

def X1(message,key):
    klen = len(key)
    result = ''
    for i in range(len(message)-KSIZE):
        result += chr(ord(message[i]) ^ ord(key[i%klen]) ^ ord ('a'))
    return result + key


def part_Encrypt (message):
    
    C = ''
    key = ''
    key = make_key(message)
    # C =  X1(message,key)
    C = fence(C,32)
    return C

def Encode (message):
    
    Cipher = ''
    part = str_size(message,SIZE)  
    for i in range(len(part)):
        key = ''
        c = ''
        key = make_key(part[i])
        # c = part[i]
        c = X1(part[i],key)
        c = fence (c,PSIZE)
        Cipher += c
    return Cipher
C = Encode(tmsg)

解题思路

出题者提示了我们这个结构采用了栅栏密码加密，所以我们肯定要熟悉并破解栅栏密码。可是如何破解呢？后来我们经过讨论，知道了我们可以利用分组长度对应的因子，实现快速解密缩小范围。而且对于Feistel结构，它的子密钥是密文C1接起来的，这让我们有了思路，我们最后再利用重合指数检验爆破的结果。

实验结果

for j in Divisor:
    p1 = fence(part[0],j)
    for k in range(96):
        key = p1[-k:]
        p2 = X1(p1,key)
        gp.append((p2,key))  
    CL = []
    for i in gp:
        (ppp,gk) = i
        clp = count_CL1(ppp)
        diff = abs(clp-0.065)
        CL.append( (diff,i) ) 
        CL = sorted(CL,key=(lambda x:x[0]))
    (d,(p,result_key)) = CL[0]

然后再破解分组长度：

def guess_key(message):
    pre = message[:KSIZE]
    last = message[KSIZE:]
    possible = []
    for k in range(26):
        key = deCaeser(last,k)
        rev = key[::-1]
        possible.append(((pre+rev),k))
    CL =[]
    for i in possible:
        (ppp,gk) = i
        clp = count_CL1(ppp)
        diff = abs(clp-0.065)
        CL.append( (diff,i,gk ) )
        CL = sorted(CL,key=(lambda x:x[0]))
    return CL

解密函数：

def Decipher(message):
    gp=[]
    part=str_size(message,SIZE)
    for j in Divisor:
        p1=fence(part[0],j)
        for k in range(96):
            key=p1[-k:]
            p2=X1(p1,key)
            gp.append((p2,key))
    CL=[]
    for i in gp:
        (ppp,gk)=i
        clp=count_CL1(ppp)
        diff=abs(clp-0.065)
        CL.append((diff,i))
        CL=sorted(CL,key=(lambda x:x[0]))
    (d,(p,result_key))=CL[0]
    rest=guess_key(p)
    (d,(p,result_key))=rest[0]
    return (len(result_key),result_ksize)

实验总结

通过本次实验，我们确实深刻地掌握了Feistel密码结构，知道了讨论和团队合作的重要性！！！