Cryptography With Python 简明教程

Simple Substitution Cipher

简单替换密码是最常用的密码，包括用于将每个明文字符替换为每个密文字符的算法。在此过程中，与凯撒密码算法相比，字母顺序混乱。

Simple substitution cipher is the most commonly used cipher and includes an algorithm of substituting every plain text character for every cipher text character. In this process, alphabets are jumbled in comparison with Caesar cipher algorithm.

Example

简单替换密码的密钥通常由 26 个字母组成。示例密钥为 −

Keys for a simple substitution cipher usually consists of 26 letters. An example key is −

plain alphabet : abcdefghijklmnopqrstuvwxyz
cipher alphabet: phqgiumeaylnofdxjkrcvstzwb

使用上述密钥的一个示例加密为 −

An example encryption using the above key is−

plaintext : defend the east wall of the castle
ciphertext: giuifg cei iprc tpnn du cei qprcni

以下代码显示了一个实现简单替换密码的程序−

The following code shows a program to implement simple substitution cipher −

import random, sys

LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
def main():
   message = ''
   if len(sys.argv) > 1:
      with open(sys.argv[1], 'r') as f:
         message = f.read()
   else:
      message = raw_input("Enter your message: ")
   mode = raw_input("E for Encrypt, D for Decrypt: ")
   key = ''

   while checkKey(key) is False:
      key = raw_input("Enter 26 ALPHA key (leave blank for random key): ")
      if key == '':
         key = getRandomKey()
      if checkKey(key) is False:
		print('There is an error in the key or symbol set.')
   translated = translateMessage(message, key, mode)
   print('Using key: %s' % (key))

   if len(sys.argv) > 1:
      fileOut = 'enc.' + sys.argv[1]
      with open(fileOut, 'w') as f:
         f.write(translated)
      print('Success! File written to: %s' % (fileOut))
   else: print('Result: ' + translated)

# Store the key into list, sort it, convert back, compare to alphabet.
def checkKey(key):
   keyString = ''.join(sorted(list(key)))
   return keyString == LETTERS
def translateMessage(message, key, mode):
   translated = ''
   charsA = LETTERS
   charsB = key

   # If decrypt mode is detected, swap A and B
   if mode == 'D':
      charsA, charsB = charsB, charsA
   for symbol in message:
      if symbol.upper() in charsA:
         symIndex = charsA.find(symbol.upper())
         if symbol.isupper():
            translated += charsB[symIndex].upper()
         else:
            translated += charsB[symIndex].lower()
				else:
               translated += symbol
         return translated
def getRandomKey():
   randomList = list(LETTERS)
   random.shuffle(randomList)
   return ''.join(randomList)
if __name__ == '__main__':
   main()

Output

当您实现上面给出的代码时，可以看到以下输出−

You can observe the following output when you implement the code given above −

simple substitution