python实现Virginia无密钥解密

本文实例为大家分享了virginia无密钥解密的具体代码，供大家参考，具体内容如下

加密

virginia加密是一种多表替换加密方法，通过这种方法，可以有效的解决单表替换中无法应对的字母频度攻击。这种加密方法最重要的就是选取合适的密钥，一旦密钥被公开，保密性也就无从谈起。结合virginia加密原理，给出使用python实现的代码

plaintext = "whenigotthethemeithoughtofgooglesartificialintelligencealphagothisprogramoverthebestofhumanplayeriwanttoaskwhenscienceandtechnologycontinuetodevelopwehumanbeingswillbewhatpositionweshouldrealizethatthedevelopmentofscienceandtechnologyisirreversibleanditconstituteaprimaryprductiveforcebutmanmustkeeppacewiththetimestoenhancetheablitytocontrol" # 密文
alphabet = "abcdefghijklmnopqrstuvwxyz" # 26个字母
ciphertext = "";
key = "helloworld" # 密钥
keylen = len(key)
plaintextlen = len(plaintext)
j = 0
for i in range(0,plaintextlen):
 j = i%keylen
 keynum = alphabet.index(key[j])
 plainnum = alphabet.index(plaintext[i])
 plaintemp = alphabet[(keynum*plainnum)%26] # 密钥对明文作用
 ciphertext += plaintemp
print(ciphertext)

解密

重点谈谈解密部分。这里的解密主要分为获取密钥长度，根据密钥长度获取密钥，根据密钥获取明文三个部分。

获取密钥长度

使用暴力破解密钥长度的方法，循环遍历可能的密钥长度。每次循环中，记录在这种密钥长度下重复相隔密钥长度密文的次数，从理论上来讲，次数最多的那个密钥长度，最有可能正确。当密文长度足够长时，正确的可能性很高。给出获取密钥长度的python函数代码：

def getkeylen(ciphertext): # 获取密钥长度
 keylength = 1
 maxcount = 0
 for step in range(3,18): # 循环密钥长度
  count = 0
  for i in range(step,len(ciphertext)-step):
   if ciphertext[i] == ciphertext[i+step]: 
     count += 1
  if count>maxcount: # 每次保存最大次数的密钥长度
   maxcount = count 
   keylength = step
 return keylength # 返回密钥长度

获取密钥

当已经获取密钥长度之后，我们可以通过分组将相同密钥作用下的密文进行分组，在每一组中，都是一个简单的单表替换加密。在这种情况下，我们通过重合指数法破解密钥，给出获取密钥部分的python函数代码：

def getkey(text,length): # 获取密钥
 key = [] # 定义空白列表用来存密钥
 alpharate =[0.08167,0.01492,0.02782,0.04253,0.12705,0.02228,0.02015,0.06094,0.06996,0.00153,0.00772,0.04025,0.02406,0.06749,0.07507,0.01929,0.0009,0.05987,0.06327,0.09056,0.02758,0.00978,0.02360,0.0015,0.01974,0.00074]
 matrix =texttolist(text,length)
 for i in range(length):
  w = [row[i] for row in matrix] #获取每组密文
  li = countlist(w) 
  powli = [] #算乘积
  for j in range(26):
   sum = 0.0
   for k in range(26):
    sum += alpharate[k]*li[k]
   powli.append(sum)
   li = li[1:]+li[:1]#循环移位
  abs = 100
  ch = ''
  for j in range(len(powli)):
    if abs(powli[j] -0.065546)<abs: # 找出最接近英文字母重合指数的项
     abs = abs(powli[j] -0.065546) # 保存最接近的距离，作为下次比较的基准
     ch = chr(j+97)
  key.append(ch)
 return key 

 破解明文

在已知密钥和明文的基础上，我们很容易就可以得到明文，给出python代码：

def virginiacrack(ciphertext): # 解密函数
 length = getkeylen(ciphertext) #得到密钥长度
 key = getkey(ciphertext,length) #找到密钥
 keystr = ''
 for k in key:
  keystr+=k
 print('the key is:',keystr)
 plaintext = ''
 index = 0
 for ch in ciphertext:
  c = chr((ord(ch)-ord(key[index%length]))%26+97)
  plaintext += c
  index+=1
 return plaintext # 返回明文

代码

这是解密部分的全部代码，注意需要自己添加密文文件的位置

def virginiacrack(ciphertext): # 解密函数
 length = getkeylen(ciphertext) #得到密钥长度
 key = getkey(ciphertext,length) #找到密钥
 keystr = ''
 for k in key:
  keystr+=k
 print('the key:',keystr)
 plaintext = ''
 index = 0
 for ch in ciphertext:
  c = chr((ord(ch)-ord(key[index%length]))%26+97)
  plaintext += c
  index+=1
 return plaintext
def openfile(filename): # 读文件
 file = open(filename,'r')
 text = file.read()
 file.close();
 text = text.replace('\n','')
 return text

def getkeylen(ciphertext): # 获取密钥长度
 keylength = 1
 maxcount = 0
 for step in range(3,18): # 循环密钥长度
  count = 0
  for i in range(step,len(ciphertext)-step):
   if ciphertext[i] == ciphertext[i+step]:
     count += 1
  if count>maxcount:
   maxcount = count
   keylength = step
 return keylength

def getkey(text,length): # 获取密钥
 key = [] # 定义空白列表用来存密钥
 alpharate =[0.08167,0.01492,0.02782,0.04253,0.12705,0.02228,0.02015,0.06094,0.06996,0.00153,0.00772,0.04025,0.02406,0.06749,0.07507,0.01929,0.0009,0.05987,0.06327,0.09056,0.02758,0.00978,0.02360,0.0015,0.01974,0.00074]
 matrix =texttolist(text,length)
 for i in range(length):
  w = [row[i] for row in matrix] #获取每组密文
  li = countlist(w) 
  powli = [] #算乘积
  for j in range(26):
   sum = 0.0
   for k in range(26):
    sum += alpharate[k]*li[k]
   powli.append(sum)
   li = li[1:]+li[:1]#循环移位
  abs = 100
  ch = ''
  for j in range(len(powli)):
    if abs(powli[j] -0.065546)<abs: # 找出最接近英文字母重合指数的项
     abs = abs(powli[j] -0.065546) # 保存最接近的距离，作为下次比较的基准
     ch = chr(j+97)
  key.append(ch)
 return key    

def countlist(lis): # 统计字母频度
 li = []
 alphabet = [chr(i) for i in range(97,123)]
 for c in alphabet:
  count = 0
  for ch in lis:
   if ch == c:
    count+=1
  li.append(count/len(lis))
 return li

def texttolist(text,length): # 根据密钥长度将密文分组
 textmatrix = []
 row = []
 index = 0
 for ch in text:
  row.append(ch)
  index += 1
  if index % length ==0:
   textmatrix.append(row)
   row = []
 return textmatrix

if __name__ == '__main__':
 ciphertext = openfile(r'') # 这里要根据文档目录的不同而改变
 plaintext= virginiacrack(ciphertext)
 print('the plaintext:\n',plaintext)

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。