Python密码学编程(1)反转加密法

1 反转加密法

主要内容：

• len()函数
• while循环
• 布尔函数类型
• 比较运算符
• 条件
• 代码块

1.1 反转加密法描述

• 反转加密法通过反向输出消息进行加密

1.2 反转加密法程序的源代码

# Reverse Cipher

message = 'There can keep a secret, if two of them are dead.'
translated = ''

i = len(message) - 1
while i>=0:
    translated = translated + message[i]
    i = i - 1
    
print(translated)

.daed era meht fo owt fi ,terces a peek nac erehT

1.3 运行反转加密法程序

• 运行程序得到输出为：.daed era meht fo owt fi ,terces a peek nac erehT
• 要想解密这条消息，把“.daed era meht fo owt fi ,terces a peek nac erehT”粘贴到message中

# Reverse Cipher2

message = '.daed era meht fo owt fi ,terces a peek nac erehT'
translated = ''

i = len(message) - 1
while i>=0:
    translated = translated + message[i]
    i = i - 1
    
print(translated)

There can keep a secret, if two of them are dead.

1.4 这个程序如何工作

# Reverse Cipher2

注释说明这个程序做什么

message = 'There can keep a secret, if two of them are dead.'

我们把希望保存的字符串存入message向量，每当希望加密或解密一个新的字符串时，直接输入进去即可

translated = ''

translated变量是程序保存反转的字符串的地方，开始时保存一个空字符串。

• 空字符串是两个单引号，而不是一个双引号

1.5 len()函数

i = len(message) - 1

• len()函数接收一个字符串值参数，返回一个整数值，表示这个字符串中有多少字符，也就是这个字符串的长度。
  • 空格和标点符号也算入字符串长度

1.6 while循环简介

while i>=0:

• 这称为while循环或称为while语句。一个while循环由4个部分组成：
  • while关键字
  • 一个计算得到True或False布尔值的表达式（又称为条件）
  • 一个冒号（：）
  • 一块缩进的代码

1.7 布尔数据类型

• 布尔数据类型只有True和False两个值，这些值是大小写敏感的。
• 和其他数据类型 的值一样，布尔值也可以保存在变量里。

spam = True
spam

True

spam = False
spam

False

1.8 比较运算符

• 比较运算符用来比较两个值，计算得到True或False布尔值
  • < 小于
  • $>$ 大于
  • <= 小于或等于
  • $>=$ 大于或等于
  • == 等于
  • != 不等于

1.9 条件

• 条件是在while语句或if语句中使用的表达式的别称。条件通常包含比较运算符，但条件仍然只是表达式。

1.10 代码块

• 一个代码块是一行或多行代码组合起来，有着相同的最小缩进量，可以通过观察这行的缩进来判断一个代码块的起止。

1.11 while循环语句

while i>=0:
    translated = translated + message[i]
    i = i - 1
    
print(translated)

• 如果条件计算得到True，那么程序的执行就会进入while语句后面的代码块
• 如果条件计算得到False，那么程序的执行就会跳过后面这个代码块里的代码，跳到这个代码块后面的第一行代码
• 如果条件是True，程序的执行将从这个代码块的顶部开始，向下依次执行每行。在到达这个代码块底部时，程序的执行会回到前面的while语句，再次检查这个条件。如果依然是True，执行会再次跳入代码块。如果是False，程序的执行将会跳过它。

1.12 “增长”一个字符串

在while循环里加入新的代码：

while i>=0:
    translated = translated + message[i]
    print(i, message[i], translated)
    i = i - 1
    
print(translated)

通过循环的每次迭代输出i, message[i], translated三个表达式，看translated表示式是怎么增长的：

message = '.daed era meht fo owt fi ,terces a peek nac erehT'
translated = ''

i = len(message) - 1
while i>=0:
    translated = translated + message[i]
    print(i, message[i], translated)
    i = i - 1
    
print(translated)

48 T T
47 h Th
46 e The
45 r Ther
44 e There
43   There 
42 c There c
41 a There ca
40 n There can
39   There can 
38 k There can k
37 e There can ke
36 e There can kee
35 p There can keep
34   There can keep 
33 a There can keep a
32   There can keep a 
31 s There can keep a s
30 e There can keep a se
29 c There can keep a sec
28 r There can keep a secr
27 e There can keep a secre
26 t There can keep a secret
25 , There can keep a secret,
24   There can keep a secret, 
23 i There can keep a secret, i
22 f There can keep a secret, if
21   There can keep a secret, if 
20 t There can keep a secret, if t
19 w There can keep a secret, if tw
18 o There can keep a secret, if two
17   There can keep a secret, if two 
16 o There can keep a secret, if two o
15 f There can keep a secret, if two of
14   There can keep a secret, if two of 
13 t There can keep a secret, if two of t
12 h There can keep a secret, if two of th
11 e There can keep a secret, if two of the
10 m There can keep a secret, if two of them
9   There can keep a secret, if two of them 
8 a There can keep a secret, if two of them a
7 r There can keep a secret, if two of them ar
6 e There can keep a secret, if two of them are
5   There can keep a secret, if two of them are 
4 d There can keep a secret, if two of them are d
3 e There can keep a secret, if two of them are de
2 a There can keep a secret, if two of them are dea
1 d There can keep a secret, if two of them are dead
0 . There can keep a secret, if two of them are dead.
There can keep a secret, if two of them are dead.