C# "Bad Data" exception when decrypting encrypted file

Question

Hey I'm very new to encryption and decryption, or even the c# language to be honest. Basically, I have a tcp chat server that "saves" logs and encrypts the text file. This is how I encrypt (based from the MSDN sample):

public static void EncryptFile(string strInputFileName, string strOutputFileName, string strKey)
{
    FileStream fsIn = new FileStream(strInputFileName, FileMode.Open, FileAccess.Read);
    FileStream fsOut = new FileStream(strOutputFileName, FileMode.Create, FileAccess.Write);

    DESCryptoServiceProvider des = new DESCryptoServiceProvider();
    des.Key = ASCIIEncoding.ASCII.GetBytes(strKey);
    des.IV = ASCIIEncoding.ASCII.GetBytes(strKey);


    ICryptoTransform desencrypt = des.CreateEncryptor();
    CryptoStream cryptostream = new CryptoStream(fsOut, desencrypt, CryptoStreamMode.Write);

    byte[] byteArrayInput = new byte[fsIn.Length - 1];
    fsIn.Read(byteArrayInput, 0, byteArrayInput.Length);
    cryptostream.Write(byteArrayInput, 0, byteArrayInput.Length);

    fsIn.Close();
    fsOut.Close();
}

The method success fully encrypts files. This is my decrypt method:

public static void DecryptFile(string strInputFileName, string strOutputFileName, string strKey)
{
    DESCryptoServiceProvider des = new DESCryptoServiceProvider();
    des.Key = ASCIIEncoding.ASCII.GetBytes(strKey);
    des.IV = ASCIIEncoding.ASCII.GetBytes(strKey);

    byte[] te = new byte[1024];
    FileStream fsRead = new FileStream(strInputFileName, FileMode.Open, FileAccess.Read);
    ICryptoTransform desdecrypt = des.CreateDecryptor();          

    CryptoStream cryptostream = new CryptoStream(fsRead, desdecrypt, CryptoStreamMode.Read);
    StreamWriter fsDecrypted = new StreamWriter(strOutputFileName);            

    fsDecrypted.Write(new StreamReader(cryptostream).ReadToEnd());//This is where the "Bad Data" occurs.
    fsDecrypted.Flush();
    fsDecrypted.Close();
    fsRead.Close();
}

And when I inspect the cryptostream object, it says that it has thrown an exception, "Stream does not support seeking".

Any help would be greatly appreciated!

Jon Skeet · Accepted Answer · 2011-04-08 06:44:37Z

5

Here:

    cryptostream.Write(byteArrayInput, 0, byteArrayInput.Length);
    fsIn.Close();
    fsOut.Close();

You're closing fsOut directly, without closing cryptostream. That means the crypto stream doesn't get the chance to flush any final blocks etc.

Additionally:

Use using statements instead of manually calling Close or Dispose
You're currently calling Read once, and assuming it will read all the data - you're not checking the return value. (You're also removing the last byte of the input file for some reason... why?) In general, you should loop round, reading into a buffer and then writing out however many bytes you read, until the Read method returns 0. If you're using .NET 4, Stream.CopyTo is your friend.

answered Apr 8, 2011 at 6:44

Jon Skeet

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1 Comment

Raphael Over a year ago

Thanks! I REALLY REALLY need to study this even more. I was experimenting with the code sample from MSDN without a clue to what they do exactly. I called the FlushFinalBlock() to cryptostream and removed the - 1 from the input file and that solved it. Thanks!

Raj Kumar - rajkrs · Accepted Answer · 2016-06-02 12:20:04Z

  objCryptStream.CopyTo(stream);

Worked for me, complete code is

 public static string DecryptString(string encriptedText, string key)
   {

       try
       {


           //Convert the text into bytest
           byte[] ecriptedBytes = System.Convert.FromBase64String(encriptedText);

           // Create a memory stream to the passed buffer
           MemoryStream objMemStream = new MemoryStream(ecriptedBytes);

           //Set the legal keys and initialization verctors
           objCrptoService.Key = GetLegalsecretKey(key);
           objCrptoService.IV = GetLegalIV();

           // Create a CryptoStream using the memory stream and the cryptographic service provider  version
           // of the Data Encryption stanadard algorithm key
           CryptoStream objCryptStream = new CryptoStream(objMemStream, objCrptoService.CreateDecryptor(), CryptoStreamMode.Read);

           // Create a StreamReader for reading the stream.
           //StreamReader objstreamReader = new StreamReader(objCryptStream);
           MemoryStream stream = new MemoryStream();
           objCryptStream.CopyTo(stream);
           stream.Position = 0;
           StreamReader R = new StreamReader(stream);
           string outputText = R.ReadToEnd();

           // Close the streams.
           R.Close();
           objCryptStream.Close();
           objMemStream.Close();

           return outputText;
       }
       catch (Exception exc)
       {
           return "";
       }
   }

Lord Darth Vader · Accepted Answer · 2018-07-12 08:27:47Z

0

What fixed my issue was calling FlushFinalBlock on cryptostream, When creating the file

                CryptoStream cryptostream = new CryptoStream(memoryStream, this._encryptionKeyHelper.Encryptor(), CryptoStreamMode.Write);

            xmlser.Serialize(cryptostream, builderObject);

            cryptostream.FlushFinalBlock();

answered Jul 12, 2018 at 8:27

Lord Darth Vader

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