code: def decipher(self,string):
                        
          string = self.remove_punctuation(string)  
          if len(string)%2 == 1: string = string + 'X'
          ret = ''
          for c in range(0,len(string.upper()),2):
              a,b = self.decipher_pair(string[c],string[c+1])
              ret += a + b
          return ret
docs: Decipher string using Foursquare cipher according to initialised key. Punctuation and whitespace
          are removed from the input. The ciphertext should be an even number of characters. If the input ciphertext is not an even number of characters, an 'X' will be appended.
  
          Example::
  
              plaintext = Foursquare(key1='zgptfoihmuwdrcnykeqaxvsbl',key2='mfnbdcrhsaxyogvituewlqzkp').decipher(ciphertext)     
  
          :param string: The string to decipher.
          :returns: The deciphered string.