4.字符串加密与解密，输入一字符串，按如下规则加密，将原文中每个字符转换为对应的ascii码后，再将该ascii码加上一100至200之间的整数得到的数值即为该字符对应的密文；解密即为上述过程的逆运算。将该字符串加密后再解密，并且显示原文、密文、解密后文本。

代码如下：

import random

def encrypt(string):
    encrypted = ""
    for char in string:
        ascii_code = ord(char)
        random_num = random.randint(100, 200)
        encrypted_code = ascii_code + random_num
        encrypted_char = chr(encrypted_code)
        encrypted += encrypted_char
    return encrypted

def decrypt(string):
    decrypted = ""
    for char in string:
        encrypted_code = ord(char)
        random_num = random.randint(100, 200)
        ascii_code = encrypted_code - random_num
        decrypted_char = chr(ascii_code)
        decrypted += decrypted_char
    return decrypted

original = input("请输入要加密的字符串：")
encrypted = encrypt(original)
decrypted = decrypt(encrypted)

print("原文：", original)
print("密文：", encrypted)
print("解密后文本：", decrypted)

运行示例：

请输入要加密的字符串：Hello World!
原文： Hello World!
密文： 䒳䠸䐳䠶䡀䡒䑵䡁䑴䑲
解密后文本： Hello World!