File Encryption and Decryption issue

Question

I've been playing around with encrypting and decrypting files in VC# Express 2010.

All the tutorials and documentation I've seen require two FileStream s in order to encrypt the file - one for reading the unencrypted version, and the other for encrypting. When I actually wrote the code it kept throwing an error telling me it could not open the file because it was opened by another process at the output filestream.

I'm assuming that's because the file is opened by the input filestream. So that means I have to specify a different filename? So even after the operation is successful I'll now have the original unencrypted file in the directory and a separate encrypted version? Doesn't that defeat the point? Or am I doing something wrong here? My code is similar to this...

public string filename = "test.xml";
using (FileStream input = new FileStream(filename, FileMode.Open, FileAccess.Read))
using (FileStream output = new FileStram(filename, FileMode.Open, FileAccess.Write))
using (....all the crypto stream and transform stuff...)
{
    ...do the encryption....
}

Answer 1

You're right but it's not defeating the point. The (streaming) crypto APIs are intended to encrypt from Src to Dst. Think encrypting output while sending/receiving over a network etc. This keeps them simple, as they should be.

You complicate the issue by using the same file for Src and Dst. That is not totally impossible but like Copying a File over itself it needs some extra care.

Consider that in general, encrypting will increase the File size. So it is not safe to Encrypt a file in place. Decrypting might be, but I wouldn't risk it.

What you need is a Temp file and a rename action after completion.

Answer 2

In your example, you can't create a separate filestream for both input and output on the same file, but you can create a handle that will read and write. The FileAccess enum has the flags attribute, so you'd just say var handle = new FileStream(filename, FileAccess.Read | FileAccess.Write); The obvious downside to this is you are going to have data lost if your encryption doesn't complete successfully.

I recommend having a separate file for the output though, atleast that way you won't lose data if your program breaks unexpectedly. If the encryption completes successfully, then delete the original and rename the encrypted file with the original file name.

Answer 3

Use File.ReadAllBytes . Then those bytes post to your encryptor, must work.

Answer 4

There is another parameter where you can specify whether or not to allow another process to read or write to the file.

openFile is a string that represents the file name.

using (FileStream fileIn = new FileStream(openFile, FileMode.Open, FileAccess.Read, FileShare.Write))
using (FileStream fileOut = new FileStream(openFile, FileMode.Open, FileAccess.Write, FileShare.Open))

This way, you can read and write to the same file.

while (myfileStream.Position < fileLength)
{   
    fileIn.Read(buffer, 0, 51200);    
    buffer = encrypt(buffer);    
    fileOut.Write(buffer, 0, 51200);                      
}

While this is easy and you don't have to write to a temporary file or have to move/rename etc, this can be really dangerous because if the encryption breaks suddenly for some reason, you will lose data!

Also, the encrypt function is something I implemented. AesCryptoServiceProvider along with CryptoStream can be used :)