Python实现Vigenère密码无密钥自动解密详解

云舒

发布时间：2026-10-13 | 6人浏览过

原创

本文详细介绍使用Python实现Vigenère（维吉尼亚）密码无密钥解密的全过程。内容涵盖通过重复间隔统计法确定密钥长度，利用重合指数法（Index of Coincidence）破解具体密钥字符，以及最终还原明文。代码包含获取密钥长度、分组统计及完整解密函数，适用于密码学学习与算法实践。

本文分享一段使用Python编写的Vigenère（维吉尼亚）密码无密钥解密示例代码，供读者参考学习。

Vigenère加密原理与Python实现

加密部分

Vigenère加密属于多表替换加密算法，其核心优势在于能有效抵御针对单表替换加密的字母频率分析攻击。该算法的安全性高度依赖于密钥的选择与保密，一旦密钥泄露，加密即失效。基于Vigenère加密原理，下文提供Python实现代码。


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)

Vigenère密码解密核心步骤

解密部分

解密过程主要包含三个关键步骤：首先确定密钥长度，其次根据密钥长度推导密钥内容，最后利用密钥还原明文。

密钥长度的暴力破解方法

确定密钥长度

采用暴力破解法确定密钥长度，即遍历所有可能的密钥长度值。在每次循环中，统计密文中相隔距离等于当前假设密钥长度的重复字符对数量。理论上，重复次数最多的长度即为真实密钥长度的概率最高，尤其在密文足够长时，该结论更为可靠。以下是获取密钥长度的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

基于重合指数法的密钥推导

确定密钥内容

在已知密钥长度的基础上，将密文按密钥长度进行分组，每组内的字符均由同一个密钥字符加密，因此每组内部表现为单表替换加密。针对每组密文，利用重合指数法（Index of Coincidence）来破解对应的密钥字符。以下是获取密钥的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

明文还原与完整代码示例

还原明文

在已知密钥和密文的情况下，通过逆向运算即可轻松还原明文。以下是Python解密代码：


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

完整代码

上述为解密部分的完整代码。请注意，运行前需自行指定密文文件的路径。


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)

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