Python之仿射密码算法的加解密实现

假设X，Y，a，b是Z52整数环中的元素，a和b为密钥，X是原文，Y是密文
加密函数：Y=（aX+b）%52

获取乘法逆元
通过扩展的欧几里得算法求a的乘法逆元

加密过程
加密函数：Y=（aX+b）%52

解密过程
解密函数：X=（a的逆元）*（Y-B）%52

#仿射密码
z=['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','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']
#通过列表来表示Z整数环中的52个元素
def exgcd(a,b,arr):#通过拓展欧几里得定理来求a对于整数环中的逆元
    if b == 0:
        arr[0] = 1
        arr[1] = 0 
        return a
    r = exgcd(b,a%b,arr)
    tmp = arr[0]
    arr[0] = arr[1]
    arr[1] = tmp - int(a/b)*arr[1]
    return r

def Get_ei(a,b):
    arr = [0,1,]
    r = exgcd(a,b,arr)
    if r == 1:
        return int ((arr[0] % b + b )%b)
    else :
        return -1


def encrypt(k1,k2,message):#加密过程
    a = str(message)
    t =""
    
    for i in a :
        
        if i in z :
            
            c = z.index(i)
            Y = (k1 * c + k2) %52
            t += z[Y]
        else:
            t += i
    return t
        #print(ord(i))
# *************begin************#

# **************end*************#   

def decrypt(k1,k2,message):#解密过程
     k1_ = Get_ei(k1,52)
     t = ""
     a = str(message)
     for i in a :
         if i in z:
             c = z.index(i)
             X = (k1_ * ( c - k2))%52
             t += z[X]
         else :
             t += i
     return t
  

def main():
    mode = int(input()) # 1代表加密，0代表解密
    message = input() #待加密或解密的消息
    key1 =int(input()) # key的范围0~51之间
    key2 = int(input()) # key的范围0~51之间
    if mode == 1:
        translated = encrypt(key1,key2,message)
    else:
        translated = decrypt(key1,key2,message)
    print(translated)

if __name__=='__main__':
    main()