PyCrypto 可以检查文件是否已经 AES 加密？

Question

  from Crypto.Cipher import AES

    def encrypt_file(key, in_filename, out_filename=None, chunksize=64*1024):
        """ Encrypts a file using AES (CBC mode) with the
            given key.

            key:
                The encryption key - a string that must be
                either 16, 24 or 32 bytes long. Longer keys
                are more secure.

            in_filename:
                Name of the input file

            out_filename:
                If None, '<in_filename>.enc' will be used.

            chunksize:
                Sets the size of the chunk which the function
                uses to read and encrypt the file. Larger chunk
                sizes can be faster for some files and machines.
                chunksize must be divisible by 16.
        """
        if not out_filename:
            out_filename = in_filename + '.enc'

        iv = ''.join(chr(random.randint(0, 0xFF)) for i in range(16))
        encryptor = AES.new(key, AES.MODE_CBC, iv)
        filesize = os.path.getsize(in_filename)

        with open(in_filename, 'rb') as infile:
            with open(out_filename, 'wb') as outfile:
                outfile.write(struct.pack('<Q', filesize))
                outfile.write(iv)

                while True:
                    chunk = infile.read(chunksize)
                    if len(chunk) == 0:
                        break
                    elif len(chunk) % 16 != 0:
                        chunk += ' ' * (16 - len(chunk) % 16)

                    outfile.write(encryptor.encrypt(chunk))

这就是我加密文件的方式，但是如果您在同一个文件上运行它两次或更多次，它将继续加密它，不会提出任何问题，我想添加某种 if 检查它是否尚未由 AES 加密？这可能吗？

Answer 1

最常用的解决方案是在加密文件的开头写入一些“神奇”字符串，后跟加密内容。如果在读取文件时找到该字符串，则拒绝进一步加密。对于描述来说，它被读取为非常确定这是我们加密的文件，但除此之外它被忽略。

想象一下您正在使用“MyCrYpT”作为魔法（尽管您使用什么并不重要，只要它相当独特即可。

magic = "MyCrYpT"
# writing the encrypted file
with open(out_filename, 'wb') as outfile:
    outfile.write(magic)  # write the identifier.
    outfile.write(struct.pack('<Q', filesize))  # file size
    outfile.write(iv)
    # et cetera

现在，在读取文件时，我们读取所有数据，然后检查它是否是我们的。然后我们丢弃魔法并处理剩下的部分。

with open(in_filename, 'rb') as infile:
    data = infile.read()
    if data[:len(magic)] != magic:
        raise ValueError('Not an encrypted file')
    filedata = data[len(magic):]
    # Proces the file data