[SWPUCTF 2021 新生赛]crypto9 Seligman的WriteUp

# 根据输入的key生成key列表
def Get_KeyList(key):
    key_list = []
    for ch in key:
        key_list.append(ord(ch.upper()) - 65)
    return key_list
# 加密函数
def Encrypt(plaintext, key_list):
    ciphertext = ""
    i = 0
    for ch in plaintext:  # 遍历明文
        if 0 == i % len(key_list):
            i = 0
        if ch.isalpha():  # 明文是否为字母,如果是,则判断大小写,分别进行加密
            if ch.isupper():
                ciphertext += letter_list[(ord(ch) - 65 + key_list[i]) % 26]
                i += 1
            else:
                ciphertext += letter_list[(ord(ch) - 97 + key_list[i]) % 26].lower()
                i += 1
        else:  # 如果密文不为字母,直接添加到密文字符串里
            ciphertext += ch
    return ciphertext


# 解密函数
def Decrypt(ciphertext, key):
    plaintext = ""

    i = 0
    for ch in ciphertext:  # 遍历密文
        if 0 == i % len(key_list):
            i = 0
        if ch.isalpha():  # 密文为否为字母,如果是,则判断大小写,分别进行解密
            if ch.isupper():
                plaintext += letter_list[(ord(ch) - 65 - key_list[i]) % 26]
                i += 1
            else:
                plaintext += letter_list[(ord(ch) - 97 - key_list[i]) % 26].lower()
                i += 1
        else:  # 如果密文不为字母,直接添加到明文字符串里
            plaintext += ch
    return plaintext


if __name__ == '__main__':
    print("加密请按D,解密请按E:")
    user_input = input();
    while (user_input != 'D' and user_input != 'E'):  # 输入合法性判断
        print("输入有误!请重新输入:")
        user_input = input()

    print("请输入密钥:")
    key = []
    for i in letter_list:
        for j in letter_list:
            for k in letter_list:
                key.append('' +i + j + k)
     遍历key
    for i in range(len(key)):
        key_list = Get_KeyList(key[i])

        if user_input == 'D':
            # 加密
            print("请输入明文:")
            plaintext = input()
            ciphertext = Encrypt(plaintext, key_list)
            print("密文为:\n%s" % ciphertext)
        else:
            # 解密
            print("请输入密文:")
            ciphertext = 'AKKPLX{qv5x0021-7n8w-wr05-x25w-7882ntu5q984}'
            plaintext = Decrypt(ciphertext, key_list)
            print("明文为:\n%s" % plaintext)

遍历key，然后再控制台中搜索NSSCTF，找到flag