Question

I am having some trouble getting a asp.net C# file encryption / decryption process to work. I can get the file uploaded and ecrypted, but cannot get the decryption to work.

I get the error: Exception Details: System.Security.Cryptography.CryptographicException: Bad Data. on the decryption line:

byte[] KeyDecrypted = rsa.Decrypt(KeyEncrypted, false);

Here is my encrypt function:

   private void EncryptFile(string inFile)
    {
        RijndaelManaged rjndl = new RijndaelManaged();
        rjndl.KeySize = 256;
        rjndl.BlockSize = 256;
        rjndl.Mode = CipherMode.CBC;
        ICryptoTransform transform = rjndl.CreateEncryptor();

        byte[] keyEncrypted = rsa.Encrypt(rjndl.Key, false);

        byte[] LenK = new byte[4];
        byte[] LenIV = new byte[4];

        int lKey = keyEncrypted.Length;
        LenK = BitConverter.GetBytes(lKey);
        int lIV = rjndl.IV.Length;
        LenIV = BitConverter.GetBytes(lIV);

        int startFileName = inFile.LastIndexOf("\\") + 1;
        // Change the file's extension to ".enc"
        string outFile = EncrFolder + inFile.Substring(startFileName, inFile.LastIndexOf(".") - startFileName) + ".enc";

        lblDecryptFileName.Text = outFile;

        using (FileStream outFs = new FileStream(outFile, FileMode.Create))
        {
            outFs.Write(LenK, 0, 4);
            outFs.Write(LenIV, 0, 4);
            outFs.Write(keyEncrypted, 0, lKey);
            outFs.Write(rjndl.IV, 0, lIV);

            using (CryptoStream outStreamEncrypted = new CryptoStream(outFs, transform, CryptoStreamMode.Write))
            {
                int count = 0;
                int offset = 0;
                int blockSizeBytes = rjndl.BlockSize / 8;
                byte[] data = new byte[blockSizeBytes];
                int bytesRead = 0;
                using (FileStream inFs = new FileStream(inFile, FileMode.Open))
                {
                    do
                    {
                        count = inFs.Read(data, 0, blockSizeBytes);
                        offset += count;
                        outStreamEncrypted.Write(data, 0, count);
                        bytesRead += blockSizeBytes;
                    }
                    while (count > 0);
                    inFs.Close();
                }
                outStreamEncrypted.FlushFinalBlock();
                outStreamEncrypted.Close();
            }
            outFs.Close();
        }

    }

And here is the decrypt function where the error occurs.

   private void DecryptFile(string inFile)
    {

        // Create instance of Rijndael for
        // symetric decryption of the data.
        RijndaelManaged rjndl = new RijndaelManaged();
        rjndl.KeySize = 256;
        rjndl.BlockSize = 256;
        rjndl.Mode = CipherMode.CBC;
        byte[] LenK = new byte[4];
        byte[] LenIV = new byte[4];
        string outFile = DecrFolder + inFile.Substring(0, inFile.LastIndexOf(".")) + ".txt";

        using (FileStream inFs = new FileStream(EncrFolder + inFile, FileMode.Open))
        {

            inFs.Seek(0, SeekOrigin.Begin);
            inFs.Seek(0, SeekOrigin.Begin);
            inFs.Read(LenK, 0, 3);
            inFs.Seek(4, SeekOrigin.Begin);
            inFs.Read(LenIV, 0, 3);

            int lenK = BitConverter.ToInt32(LenK, 0);
            int lenIV = BitConverter.ToInt32(LenIV, 0);
            int startC = lenK + lenIV + 8;
            int lenC = (int)inFs.Length - startC;

            // Create the byte arrays for
            // the encrypted Rijndael key,
            // the IV, and the cipher text.
            byte[] KeyEncrypted = new byte[lenK];
            byte[] IV = new byte[lenIV];

            // Extract the key and IV
            // starting from index 8
            // after the length values.
            inFs.Seek(8, SeekOrigin.Begin);
            inFs.Read(KeyEncrypted, 0, lenK);
            inFs.Seek(8 + lenK, SeekOrigin.Begin);
            inFs.Read(IV, 0, lenIV);
            Directory.CreateDirectory(DecrFolder);

            byte[] KeyDecrypted = rsa.Decrypt(KeyEncrypted, false);

            ICryptoTransform transform = rjndl.CreateDecryptor(KeyDecrypted, IV);

            using (FileStream outFs = new FileStream(outFile, FileMode.Create))
            {

                int count = 0;
                int offset = 0;

                int blockSizeBytes = rjndl.BlockSize / 8;
                byte[] data = new byte[blockSizeBytes];

                inFs.Seek(startC, SeekOrigin.Begin);
                using (CryptoStream outStreamDecrypted = new CryptoStream(outFs, transform, CryptoStreamMode.Write))
                {
                    do
                    {
                        count = inFs.Read(data, 0, blockSizeBytes);
                        offset += count;
                        outStreamDecrypted.Write(data, 0, count);

                    }
                    while (count > 0);

                    outStreamDecrypted.FlushFinalBlock();
                    outStreamDecrypted.Close();
                }
                outFs.Close();
            }
            inFs.Close();
        }

    }

Any help on this would be great! I am not an RSA encryption expert and have been reading a lot of posts but still not able to come up with a solution.

Answer 1

I have finally figured this out. The code worked well in a desktop application when I tried it there. It just didn’t work in the asp.net 4 web application I was trying to write. The issue was the RSA object wasn’t persisted through the session. So, the RSA object was created okay. The file was encrypted okay. But when I went to decrypt the file the RSA object was not there. The error message of System.Security.Cryptography.CryptographicException: Bad Data is misleading as that wasn’t really the issue, the data was fine.

So, when creating the key and the RSA object I used the following:

rsa = new RSACryptoServiceProvider(cspp);
Session["rsa"] = rsa;

Next, when the decryption function is called I added in:

if (rsa == null)
     rsa = (RSACryptoServiceProvider)Session["rsa"];

Of course, there is a little more code around this also so catch if there is no key for the RSA session, but this is the high level solution for the issue I was having.

If anyone is looking for this let me know and I can share more of the code.