使用C＃和RSA加密/解密文件

Question

I am trying to encrypt and then decrypt an XML file using RSA and C# and while I'm really close, there's a problem. Once it's decrypted, almost all of the file is there but there's a hiccup toward the end. It's either a gap toward the end of the file or more data is appended to the very end of the file.

Here is my encrypt method:

    public static bool Encrypt(ProcessingHolder ph)
    {
        FileInfo inFile = ph.encryptedFI;
        FileInfo outFile = ph.unEncryptedFI;

        X509Certificate2 daCert = new X509Certificate2(keyFP, daCertPassword);
        RSACryptoServiceProvider RSA = (RSACryptoServiceProvider)daCert.PrivateKey;

        bool done = false;
        FileStream fs = null;
        FileStream fso = null;

        try
        {
            //opens the file to encrypt into a filestream object
            fs = inFile.OpenRead();

            //240 is what the iOS side is using
            //algorithm that calculates max bytes ((KeySize - 384) / 8) + 37 
            //(returns 245)
            int chunkSize = 245;

            fso = outFile.OpenWrite();
            byte[] buffer = new byte[chunkSize];
            int totalRead = 0;


            while (totalRead < fs.Length)
            {
                int readBytes = fs.Read(buffer,0, chunkSize);

                totalRead += readBytes;

                //check to see if the final chunk of data is less than 245 so as not to write empty buffer
                if (readBytes < chunkSize) buffer = new byte[readBytes];
                //byte[] encr = new byte[readBytes];

                //actual encryption
                //encr = RSA.Encrypt(buffer, false);

                byte[] encr = RSA.Encrypt(buffer, false);
                fso.Write(encr, 0, encr.Length);
            }
            fso.Flush();
            fso.Close();
            fs.Close();
            done = true;
        }
        catch (Exception ex)
        {
            Debug.WriteLine("Decrypt failed with message " + ex.Message);
            done = false;
        }
        finally
        {
            if (fs != null) fs.Close();
            if (fso != null) fso.Close();
        }
        return done;
    }
}

and here is my decrypt method:

    public static bool Decrypt(ProcessingHolder ph)
    {
        FileInfo inFile = ph.encryptedFI;
        FileInfo outFile = ph.unEncryptedFI;

        X509Certificate2 daCert = new X509Certificate2(keyFP, daCertPassword);
        RSACryptoServiceProvider RSA = (RSACryptoServiceProvider)daCert.PrivateKey;

        bool done = false;
        FileStream fs = null;
        FileStream fso = null;

        try
        {
            fs = inFile.OpenRead();
            int chunkSize = 256;

            fso = outFile.OpenWrite();
            byte[] buffer = new byte[chunkSize];
            int totalRead = 0;

            while (totalRead < fs.Length)
            {
                int readBytes = fs.Read(buffer, 0, chunkSize);
                totalRead += readBytes;

                //check to see if the final chunk of data is less than 245 so as not to write empty buffer
                //if (readBytes < chunkSize) buffer = new byte[readBytes];

                byte[] decr = RSA.Decrypt(buffer, false);
                fso.Write(decr, 0, decr.Length);
            }
            fso.Flush();
            fso.Close();
            fs.Close();
            done = true;
        }
        catch (Exception ex)
        {
            Debug.WriteLine("Decrypt failed with message " + ex.Message);
            done = false;
        }
        finally
        {
            if (fs != null) fs.Close();
            if (fso != null) fso.Close();
        }
        return done;
    }

banging my head against the wall here, thanks in advance

Answer 1

What happens during encrypting if the file is not a multiple of the length of the chunk size? Ie. a file 500 bytes long would read two sets of 245, but they have 10 bytes left over? This might be loosing the last few bytes at the end or adding extra values?

Maybe you need to add a header to the file with the size in bytes of the decrypted file so that the decrypter knows where to stop and a way to pad out the final block during encryption