凯撒

穷举法：在密钥空间较小的情况下，采用暴力破解方式攻击方法。
频率统计：在密文长度足够长的时候，可使用词频分析。
爬山法：选择性地尝试不同解密密钥，然后给每一个解密出来的明文标记上一个适应度。若解密出来的明文越接近我们的日常用的英语，它的适应度就越高；若解密出来的明文越难读懂或者越不像我们日常用的英语，则其适应度越低。

# -*- coding: UTF-8 -*-
import random
from ngram_score import ngram_score
#参数初始化
ciphertext = 'JGRMQOYGHMVBJWRWQFPWHGFFDQGFPFZRKBEEBJIZQQOCIBZKLFAFGQVFZFWWEOGWOPFGFHWOLPHLRLOLFDMFGQWBLWBWQOLKFWBYLBLYLFSFLJGRMQBOLWJVFPFWQVHQWFFPQOQVFPQOCFPOGFWFJIGFQVHLHLROQVFGWJVFPFOLFHGQVQVFIEOGQILHQFQGIQVVOSFAFGBWQVHQWIJVWJVFPFWHGFIWIHZZRQGBABHZQOCGFHX'
 
parentkey = list('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
#只是用来声明key是个字典
key = {'A':'A'}
#读取quadgram statistics
fitness = ngram_score('quadgrams.txt')
parentscore = -99e9
maxscore = -99e9
j = 0
 
print('---------------------------start---------------------------')
while 1:
  j = j+1
  #随机打乱key中的元素
  random.shuffle(parentkey)
  #将密钥做成字典
  for i in range(len(parentkey)):
    key[parentkey[i]] = chr(ord('A')+i)
    #用字典一一映射解密
  decipher = ciphertext
  for i in range(len(decipher)):
    decipher = decipher[:i] + key[decipher[i]] + decipher[i+1:]
  parentscore = fitness.score(decipher)#计算适应度
  #在当前密钥下随机交换两个密钥的元素从而寻找是否有更优的解
  count = 0
  while count < 1000:
    a = random.randint(0,25)
    b = random.randint(0,25)
    #随机交换父密钥中的两个元素生成子密钥，并用其进行解密
    child = parentkey[:]
    child[a],child[b] = child[b],child[a]
    childkey = {'A':'A'}
    for i in range(len(child)):
      childkey[child[i]] = chr(ord('A')+i)
    decipher = ciphertext
    for i in range(len(decipher)):
      decipher = decipher[:i] + childkey[decipher[i]] + decipher[i+1:]
    score = fitness.score(decipher)
    #此子密钥代替其对应的父密钥，提高明文适应度
    if score > parentscore:
      parentscore = score
      parentkey = child[:]
      count = 0
    count = count+1
  #输出该key和明文
  if parentscore > maxscore:
    maxscore = parentscore
    maxkey = parentkey[:]
    for i in range(len(maxkey)):
      key[maxkey[i]] = chr(ord('A')+i)
    decipher = ciphertext
    for i in range(len(decipher)):
      decipher = decipher[:i] + key[decipher[i]] + decipher[i+1:]
 
    print ('Currrent key: '+''.join(maxkey))
    print ('Iteration total:', j)
    print ('Plaintext: ', decipher)

原文：https://blog.csdn.net/qq_35964497/article/details/78985332