自动秘钥密码(Autokey)

# write in 2021/7/13
# 自动秘钥密码
DIC = "abcdefghijklmnopqrstuvwxyz"
def encrypt_autokey(string, key):
    ciphertext = ""
    key += string
    key_index = 0
    for i in string:
        if i not in DIC:
            ciphertext += i
        ciphertext += DIC[(DIC.index(i)+DIC.index(key[key_index])) % 26]
        key_index += 1
    return ciphertext
def decrypt_autokey(string, key):
    plaintext = ""
    key_index = 0
    for i in string:
        if i not in DIC:
            plaintext += i
        char = DIC[(DIC.index(i) - DIC.index(key[key_index])) % 26]
        key += char
        plaintext += char
        key_index += 1
    return plaintext
if __name__ == '__main__':
    key_ = "linux"
    ciphertext_ = encrypt_autokey("autokey", key_)
    plaintext_ = decrypt_autokey(ciphertext_, key_)
    print(f"{plaintext_}: {ciphertext_}")