5.2 Caesar cipher - decrypt python

Task for this level: Please program to calculate the offset, and use the obtained offset to decrypt the read ciphertext.

code show as below:

import string

def caesar_decrypt(text, offset):
    """接收一个加密的字符串text和一个整数offset为参数，采用字母表和数字中前面第offset个字符
    代替当前字符的方法对字符串中的字母和数字进行替换，实现解密效果，返回值为解密的字符串。"""
    ###############################Begin#############################################
    lower = string.ascii_lowercase          # 小写字母
    upper = string.ascii_uppercase          # 大写字母
    digit = string.digits                # 数字
    before = string.ascii_letters + digit
    after = lower[offset:] + lower[:offset] + upper[offset:] + upper[:offset] + digit[offset:] + digit[:offset]
    table = ''.maketrans(after, before)
    decrypt_text = text.translate(table)
    return decrypt_text  


    ################################End##############################################

def find_offset(key_text, ciphertext):
    """接收一个单词和一个加密字符串为参数，尝试用[0,25]之间的数为偏移量进行解密。
        如果key_text 在解密后的明文里，则说明解密成功。
    找出偏移量数值并返回这个整数偏移量。"""
    ###############################Begin#############################################
    for i in range(26):
        plain_text = caesar_decrypt(ciphertext, i) #使用i作为偏移量，尝试解密得到明文
        if key_text in plain_text : # 如果key_text在明文里，说明偏移值为i时能成功解密
            return i
    ################################End##############################################


if __name__ == '__main__':
    key_message = 'question'                                   #密文中的已知单词
    cipher_text = 'Yt gj,tw sty yt gj,ymfy nx f vzjxynts.'      #截获的密文
    secret_key = find_offset(key_message, cipher_text)   #破解密码，得到密匙
    print(f'密钥是{secret_key}')
    
    target_text = input() #读入新密文，进行解密
    #'Fyyfhp ts Ujfwq Mfwgtw ts Ijhjrgjw 2, 6496'   #新密文，需要解密
    print(caesar_decrypt(target_text, secret_key))  #解密，打印明文

caesar cipher

The Caesar encryption method can produce new changes according to different shifts. For example, by shifting each letter to the 19left , such a clear and secret comparison table (taking uppercase letters as an example):

明：A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

密：T U V W X Y Z A B C D E F G H I J K L M N O P Q R S

Under this encryption table, the comparison relationship between plaintext and ciphertext becomes:

明文：THE FAULT, DEAR BRUTUS, LIES NOT IN OUR STARS BUT IN OURSELVES.

密文：MAX YTNEM, WXTK UKNMNL, EBXL GHM BG HNK LMTKL UNM BG HNKLXEOXL.

In this way, every time an offset value is changed, a different encryption result can be obtained, and this method can improve the encryption strength to a certain extent.

Python String module built-in attributes

string.ascii_lowercase Lower case letters
string.ascii_uppercase uppercase letter
string.digits numbers 0-9

def caesar_decrypt(text, offset):
"""接收一个加密的字符串text和一个整数offset为参数，采用字母表和数字中前面第offset个字符
代替当前字符的方法对字符串中的字母和数字进行替换，实现解密效果，返回值为解密的字符串。"""
lower = string.ascii_lowercase # 小写字母
upper = string.ascii_uppercase # 大写字母
digit = string.digits # 数字
before = string.ascii_letters + digit
after = lower[offset:] + lower[:offset] + upper[offset:] + upper[:offset] + digit[offset:] + digit[:offset]
table = ''.maketrans(after, before)
decrypt_text = text.translate(table)
return decrypt_text

Python common functions

（1）range

range()The function returns an iterable object (the type is object), not a list type, so the list will not be printed when printing.

grammar:

range(stop)
range(start, stop[, step])

Parameter Description

start: Counting starts from start. The default is to start from 0. For example range(3) is equivalent to range(0, 3);
stop: Count up to stop, but not including stop. For example: range (0, 3) is [0, 1, 2] without 3

step: step size, the default is 1. For example: range(0, 3) is equivalent to range(0, 3, 1)

def find_offset(key_text, ciphertext):
"""接收一个单词和一个加密字符串为参数，尝试用[0,25]之间的数为偏移量进行解密。
如果key_text 在解密后的明文里，则说明解密成功。
找出偏移量数值并返回这个整数偏移量。"""
for i in range(26):
plain_text = caesar_decrypt(ciphertext, i) #使用i作为偏移量，尝试解密得到明文
if key_text in plain_text : # 如果key_text在明文里，说明偏移值为i时能成功解密
return i

（2）maketrans

maketrans()The method is used to create the conversion table of the character map. For the simplest calling method that accepts two parameters, the first parameter is a string, indicating the character to be converted, and the second parameter is also a string indicating the conversion target.

grammar:

str.maketrans(intab, outtab)

Parameter Description

intab -- A string of characters to substitute in the string.
outtab – A string of corresponding mapped characters.

5.2 Caesar cipher - decrypt python

Python common functions

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