主要为两个函数的实现：
加密函数 : y = ax+b (mod m), (一般m设置为26)，a为密钥，并且a与m互质，b为另一密钥，为任意数
解密函数：x = a^-1 (y - b) (mod m) , a^-1 为a 的逆元，在用其他语言实现求逆元时需要注意mod 取整问题；
参考代码如下，

#coding = utf-8
import math

CryptoText = ''
PlainText = ''
txt = ''
a,b,m=0,0,0

# 加密
def encode():
    global a,b,CryptoText,txt
    if math.gcd(26,a) == 1:
        for i in txt:
            tmp = a * (ord(i)-97) + b
            CryptoText += chr((tmp % m) + 97)
        print("[+]密文为----->",CryptoText)
        return 0
    else:
        print("[-]加密失败，密钥 a 与 26 不互质,请重新输入a")
        return 1

# 求乘法逆元
def extendEdeuclid():
    global a,m
    x1,x2,x3 = 1,0,m
    y1,y2,y3 = 0,1,a
    while True:
        if y3 == 0 :
            return 0
        if y3 == 1 :
            if y2<0:
                y2 = m+y2
            return y2
        q = int(x3/y3)
        t1,t2,t3 = x1-q*y1, x2-q*y2, x3-q*y3
        x1,x2,x3 = y1,y2,y3
        y1,y2,y3 = t1,t2,t3

# 解密
def decode():
    global PlainText,CryptoText,a,b,m
    tmp = extendEdeuclid()
    if tmp != 0:
        for i in CryptoText:
            PlainText += chr(tmp * (ord(i)-97-b) % m + 97)
        print("[+]解密得到----->",PlainText)
        return 0
    else:
        print("[-]解密失败，密钥",a,"与",m,"没有逆元，请重新输入密钥a")
        return 1

if __name__ == '__main__':
    print('''

   ▄████████    ▄████████    ▄████████  ▄█  ███▄▄▄▄      ▄████████  ▄████████  ▄█     ▄███████▄    ▄█    █▄       ▄████████    ▄████████ 
  ███    ███   ███    ███   ███    ███ ███  ███▀▀▀██▄   ███    ███ ███    ███ ███    ███    ███   ███    ███     ███    ███   ███    ███ 
  ███    ███   ███    █▀    ███    █▀  ███▌ ███   ███   ███    █▀  ███    █▀  ███▌   ███    ███   ███    ███     ███    █▀    ███    ███ 
  ███    ███  ▄███▄▄▄      ▄███▄▄▄     ███▌ ███   ███  ▄███▄▄▄     ███        ███▌   ███    ███  ▄███▄▄▄▄███▄▄  ▄███▄▄▄      ▄███▄▄▄▄██▀ 
▀███████████ ▀▀███▀▀▀     ▀▀███▀▀▀     ███▌ ███   ███ ▀▀███▀▀▀     ███        ███▌ ▀█████████▀  ▀▀███▀▀▀▀███▀  ▀▀███▀▀▀     ▀▀███▀▀▀▀▀   
  ███    ███   ███          ███        ███  ███   ███   ███    █▄  ███    █▄  ███    ███          ███    ███     ███    █▄  ▀███████████ 
  ███    ███   ███          ███        ███  ███   ███   ███    ███ ███    ███ ███    ███          ███    ███     ███    ███   ███    ███ 
  ███    █▀    ███          ███        █▀    ▀█   █▀    ██████████ ████████▀  █▀    ▄████▀        ███    █▀      ██████████   ███    ███ 
                                                                                                                              ███    ███                                                                                                
                                                         is insteresting !
                                                         编码系统为26小写字母
    ''')
    a,b,m=int(input("请输入密钥a: ")),int(input("请输入密钥b(任意正整数): ")),input("请输入编码数m[不输默认26]：")
    if m == '':
        m = 26
    else:
        m = int(m)
    txt=input("请输入明文:")
    while(encode()):
        a = int(input("请输入密钥a: "))
    while(decode()):
        a = int(input("请输入密钥a: "))