为什么我不能用增量加密和解密照片文件？

Question

我制作了一个非常简单的加密和解密程序，通过将所有字节增加6来加密文件。然而，在测试中，只有文本文件才能工作。如果我使用它对照片进行加密和解密，操作系统将无法读取结果。

Python中的代码：

import os.path


class fileEncryptor:

    @staticmethod
    def encrypt(fileLocation, destination):
        if os.path.exists(fileLocation):
            file = open(fileLocation, "rb")
            fileContents = file.read()  # fileContents is a byte string
            file.close()

            btAr = bytearray(fileContents)  # Byte string needs to be changed to byte array to manipulate


            length = len(btAr)
            n = 0
            while n < length:
                increment = 6
                if btAr[n] <= 249:
                    btAr[n] = btAr[n] + increment
                if 249 < btAr[n] <= 255:
                    btAr[n] = btAr[n] - 250
                n = n + 1

            encryptedFile = open(destination, "wb")
            encryptedFile.write(btAr)
            encryptedFile.close()
        else:
            print("File does not exist")

    @staticmethod
    def decrypt(fileLocation, destination):
        if os.path.exists(fileLocation):
            file = open(fileLocation, "rb")
            fileContents = file.read()
            file.close()

            btAr = bytearray(fileContents)

            length = len(btAr)
            n = 0
            while n < length:
                increment = 6
                if 5 < btAr[n] <= 255:
                    btAr[n] = btAr[n] - increment
                if btAr[n] <= 5:
                    btAr[n] = btAr[n] + 250
                n = n + 1

            decryptedFile = open(destination, "wb")
            decryptedFile.write(btAr)
            decryptedFile.close()
        else:
            print("File does not exist")


if __name__ == "__main__":
    fileEncryptor.encrypt("D:\Python Projects\DesignerProject\ic.ico", "D:\Python Projects\DesignerProject\output\ic.ico")
    fileEncryptor.decrypt("D:\Python Projects\DesignerProject\output\ic.ico", "D:\Python Projects\DesignerProject\output\i.ico")

Answer 1

此部分需要更改为else：

if btAr[n] <= 249:
    btAr[n] = btAr[n] + increment
if 249 < btAr[n] <= 255:
    btAr[n] = btAr[n] - 250

就像这样：

if btAr[n] <= 249:
    btAr[n] = btAr[n] + increment
else:
    btAr[n] = btAr[n] - 250

否则，如果第一个if为真，则更改字节并运行第二个if，应用增量的两倍。

解密也一样。